I’m not a fan of using corrugated cardboard as a mulch, which like other sheet mulches creates problems for the underlying soil. Long-time readers of this blog may remember several previous posts (1, 2, 3 and 4) on this topic and I won’t belabor the points made in those posts. Instead, today I’m doing to focus on cardboard itself.
First, cardboard is a generic term that can refer to many types of manufactured paper. The box you see delivered to your front door is more properly called corrugated board or containerboard. It consists of two layers of linerboard sandwiching a layer of accordion-like fluting material. The linerboard is made from sheets of pulp that may be coated to improve smoothness (more about this later). The finished linerboard is laminated using adhesives to both sides of the fluting material.
These boxes are made to withstand rough handling and to protect the contents from the external environment. It’s tough stuff: while you might be able to bend a piece of corrugated board fairly easily, it’s more difficult to tear it in half. The more heavy duty the box, the more difficult it is to bend or tear its walls.
So let’s now consider using this tough material in your garden as a mulch. It may be coated as mentioned earlier to improve smoothness. That’s going to prevent it from absorbing moisture. The coating also reduces the ability for gases to move between the soil and the atmosphere. In fact, smoothness is measured using an air leak method – the smoothest materials have the least air leakage.
A garden or landscape mulched with cardboard (or heaven forbid several layers of cardboard as part of the science-free lasagna mulch method) is now covered with a tough, relatively gas- and water-impermeable material that will take some time to break down. It’s hardly a mulch that’s going to nurture soil life.
But cardboard mulch fans swear that they find more earthworms under cardboard than anywhere else in their garden. This is almost always the first response I get from gardeners who don’t believe that cardboard causes problems. And this is where it’s important to consider earthworm behavior.
We’ve all observed that earthworms crawl to the soil surface during heavy rains; this is due in part to water filling their burrows and reducing oxygen availability (Chuang and Chen demonstrated this nicely in 2008). Likewise, the reduction in oxygen movement from the atmosphere into cardboard-covered soil would cause worms to crawl upwards in an effort to find oxygen at the soil surface.
So don’t assume your lasagna mulching draws earthworms to your garden. It’s more likely that you’re smothering their habitat.
***An update on cardboard gas permeability. We’ve just published an article comparing diffusion rates of different mulches. You can find the article here but it is behind a paywall. Here is a graphic comparing diffusion rates of various mulches. This is a logarithmic scale.
Now, until cardboard proponents publish evidence to the contrary, it’s pretty obvious that cardboard mulch interferes with gas diffusion.
***And another update on how our blog works. This post, by far, is the most popular. It generates a lot of comments. All comments must be approved before they’re posted, and I don’t approve comments that are derogatory or promote a belief in the absense of supporting science. If you want your comment to be published, be polite and provide evidence to support your statements. Otherwise, you are wasting your time.
309 thoughts on “The cardboard controversy”
How about shredded cardboard?
Sure, Beth, shredded cardboard, paper etc. can be a great mulch. The trick is to take what’s essentially a two dimensional mulch and make it into something three dimensional.
Linda, how would explain success of Charles Dowding’s no-dig method using cardboard? He grows everything from vegetables and fruit trees to shrub roses and peonies, using only two ingredients: cardboard and compost. His plants are very healthy.
This is called anecdotal evidence and does not lend itself to any kind of objective analysis as there is no control, no replication of treatments, and no data collection. That’s what’s required to develop a credible, reliable practice of any sort.
Interesting article, Linda. Thank you. I don’t think the evidence Charles Dowding has published is entirely ‘anecdotal’. Dowding has run trials comparing ‘no-dig’ (cardboard and compost) with ‘dig’ and measured crop productivity (weight) in the two systems. He talks about it a little here https://www.youtube.com/watch?v=7WdM__pw7Sk .
In science, anything reported that is not published in a peer-reviewed journal is anecdotal. That applies to scientists as well as lay people. So I can report that using wood chip as a mulch in my vegetable garden keeps down weeds and enhances plant growth, but until that’s been researhced through rigorous and repeated experimentation, it’s only anecdotal.
Anecdotal research is more credible if it builds on other published research. So in my example, we know through multiple published papers that arborist wood chips are valuable landscape mulches that keep down weeds and enhance landscape plant growth. So my anecdotal observations are supported through related research, even though the plant material is different.
Right now there is NO published research, whatsoever, on cardboard mulches. So there is nothing to build on or exptrapolate from. Until proponents of cardboard mulch spend the time to run credible field epxeriments and publish those results in appropriate scientific journals, and then have other researchers confirm their results through independent trials, there is nothing to support the use of cardboard.
Hi Linda. Many thanks for replying to my comment. I think labelling as ‘anecdotal’ anything not published in a peer-reviewed, reputable journal suggests evidence other than that is irrelevant. You say there is ‘nothing’ to support the use of cardboard so I guess you may subscribe to that view.
Of course unpublished data is not irrelevant. It is frequently cited in peer-reviewed papers (personal communication etc.). I would agree with you if you said anecdotal evidence was less-reliable, but irrelevant, no. I’m sure we can all think of many instances from our daily lives where we can reliably predict the outcome of events whilst relying entirely on our experience without a peer-reviewed paper in sight. You may dismiss the experience on which we rely as ‘anecdotal’ and therefore irrelevant but if you do you may end up dismissively applying the word ‘anecdotal’ to such a vast range of useful knowledge and experience as to make the word useless.
I had the great privilege of visiting Dowding’s garden last year. I think he’s a very good gardener and careful about how he conducts his ‘tests’. He has a great deal of experience commercially and recreationally. He does not to claim his tests are ‘scientific’ although I thought he controlled most of the obvious variables well. My observations were that his cardboard method is no worse than a no-cardboard method if health and vigour of produce are the dependent variables of interest. The data he reports indicates there is no difference in wet-weight yield between the cardboard and no-cardboard plots. I think your thesis is that using cardboard is detrimental or less preferable than wood chip mulch. You publish a graph showing CO2 diffusion rates through various media. But you don’t explain the relevance of the observed rates of diffusion to subsequent plant growth or productivity. Is there a correlation with growth/productivity and, if so, is there causality? Do you not think that since there are no peer-reviewed papers on the subject of cardboard as a mulch then it would be difficult to draw any conclusion about its effectiveness? The answer seems to be we don’t know about cardboard one way or another. I’ve always found it useful to bear in mind the cautionary aphorism, “Absence of evidence is not evidence of absence”. One can be for wood chip but also be for cardboard. Both may be effective and/or more or less suitable than the other in different circumstances. One has been shown to be an effective mulch, empirically. For the other we have only ‘anecdotal’ evidence so we can say nothing definitively but what evidence we do have provides a guide in the development of an hypothesis. It surprises me that those interested in evaluating wood chip as a mulching product do not also include waste cardboard in their assessment (almost zero cost), if only to clear up the point.
Hi Moray –
As I’ve mentioned many times in this post’s discussion, and in many other posts, anecdotal evidence is not sufficient as a basis for a science-based recommendation of a product or practice. That’s what I mean by irrelevant. There is no theoretical plant or soil science that would support the use of cardboard, so the additional lack of any practical published evidence means there’s nothing that a scientist could use to support the use of cardboard, given the collateral damage imposed on the soil ecosystem (which is totally ignored by gardeners who are solely focused on crop yields). On the other hand, there is ample evidence, both theoretical and practical, that sheet mulches inhibit water and gas movement while coarse, chunky, three-dimensional mulches do not significnatly interfere with either gas or water water exchange. So that’s where we are, and that’s where we will stay until there is rigorous, peer-reviewed, published evidence that says otherwise.
Hi Linda, Thank you, once again, for taking the time to reply to me.
In the article you helpfully provide a link to your 2019 paper. As you say, it’s behind a paywall but the abstract states, ‘Despite the different diffusion coefficients of the different mulches, CO2 and O2 concentrations in the soil under the various mulches were not significantly different as compared to the control’.
I’m sure you make further comment on that in the paper but sadly I can’t see it so I’d be very pleased if you would mind giving me your interpretation of why that might be particularly when the CO2 diffusion coefficients are so different. What is going on? What do you think is the effect on the soil of the insignificant differences in CO2 and O2 concentrations associated with the various mulches tested? With regard to the possibility of damage to biologically rich, high-oxygen-demand landscapes mentioned, are these garden environments and how might such damage occur?
I hope you won’t mind me being specific. The diverse evidence on the issue of cardboard as a mulch is of great interest to me.
The reason the levels of CO2 were not very different among treatments (except under plastic) was because these are very simple mesocosms with only microbial life. No plant roots, no earthworms, no other oxgyen-requiring organisms. As the abstract also states, a more complex landscape with more life will have different results. That’s why the diffusion coefficient is so important. Soil oxygen is generally what limits the depth at which organisms can survive. Anything that reduces oxygen diffusion into the soil will negatively affect the soil ecosystem – and the more diverse and complex it is, the bigger the negative impact.
The great interest in cardboard as a mulch should be supported by going behind the paywall. Research takes time and time is not free.
I just wanted to find out if using cardboard as a weed barrier would work. People everywhere are buried in Amazon boxes. Every order means at least one or two oversized boxes that pile up. Amazon boxes that are easy to tear and fall apart when damp means okay for the soil but for keeping weeds out, not so much.
My brilliant unscientific conclusion? Put weed barrier fabric on top of biodegradable cardboard on top of rebar so my water saving artificial grass won’t sink.
Thank you for this helpful information.
Since the article is behind a paywall, it might contain information that I miss. My apologies in advance if some of the points are addressed in the article. How do we know what level of gas permeability is adequate for maintaining a healthy soil life? (assuming we have some metrics to define what a healthy soil life looks like) Has there been any scientific study to established the relationship between gas permeability and soil health in more detail?
Bare soil has excellent gas permeability at the same time it loses moisture. Wood chips DO reduce gas permeability (looking at the graph) but obviously all the other benefits that wood chips provide make it a good choice for garden.
From reading the abstract, the only thing we can conclude from the article is that wood chips are better than cardboard. A conclusion that cardboard harms soil health would require more extensive study.
This mesocosm study is limited, because lab experiments have to reduce variability. The only things using oxygen and releasing carbon dioxide are microbes. In a real world, soil is full not just of microbes, but roots and animals tlat all require oxygen. Oxygen is THE limting factor as you travel downwards in soil, so anything that slows gas exchange will limit oxygen availability.
Bare soil is not really a good standard in nature, because it becomes compacted, But again, we needed to limit variability so we could get good comparisons of the diffusion coefficients for different mulches.
You really don’t need another study to show that cardboard (or any sheet mulch) is a poor choice compared to wood chips, because we’ve established the basic science fo gas transfer and we know that oxygen is required in soil for there to be soil life. But anyone who was to conduct a field experiment comparing mulch types would now have one research-based reason (as opposed to a theoretical reason) why cardboard would not perform as well as wood chips. We can assume – though need to demonstrate through research – that reduction of water movement would likewise be affected.
Interesting debate about science here, which I’m not going to jump into. I stumbled onto this site looking for information about potential harmful effects of cardboard sheet mulching, especially on insects. I know many of our native bees nest in the ground and need bare soil for this. I work for the parks department of a large city and for the past few years they’ve been pushing sheet mulching to the point where I think it’s way overdone- it could be the right thing in certain situations, but I fear that all this sheet mulching will harm our insect populations. Any comments?
Sheet mulching, as compared to three-dimensional mulch (like arborist chips) is bad for soil life, period. There is absolutely no published evidence to the contrary.
Regardless of the type of proper mulch used, you can always leave bare areas that are not prone to weeds. Space under low shrubs is ideal as it is protected from foot traffic and can be easily colonized by ground nesters.
After reading both sides of the argument. It would be fascinating to compare the biodiversity of micro and macro arthropods across both mulch types. I believe this is the only valid way of proving your theory. Cardboard decomposes – therefore it is possible that it is not as harmful as suggested or only initially – it will ultimately be integrated into the soil. Have you checked your hypothesis at different intervals as the cardboard decomposes? Could be interesting. Thanks for a thought provoking article.
Our research is pretty conclusive: the bigger the air space in a mulch, the better the gas transfer. Wood chips are superior. Sheet mulches are not – and anything that forms a sheet can induce low-oxygen conditions. And sure, cardboard will break down, but consider a single sheet of wet newspaper over your face. Could you breathe well? Would you want to wait until that sheet naturally broke down? Thats the condition that cardboard covered soil is under. There is no evidence, theoretical or practical – that cardboard mulch is superior in the short term or the long term.
I’ve read that wood chips consume nitrogen as they decompose.
How does this affect plant growth?
I’ve attached a fact sheet here that addresses your question. Bottom line is that this is not an issue for mulches of any sort. https://www.researchgate.net/publication/315662938_Using_arborist_wood_chips_as_a_landscape_mulch_WSU_Extension_Fact_Sheet_FS160E
This may help the distinction of anecdotal vs personal communication. When an article refers to personal communication, it is almost always from a scientifically credible source that likely vetted the thought through decades of experience as a scientist. I get the same heart burn over most anecdotal “evidence”.
by that logic evidence of science didnt exist until peer-reviewed journals were created.
Science evolves. The scientific method evolved. We’re past the era of incantations, blood letting and humors.
I suggest you read some books on the history and philosophy of science.
If there is no published research on cardboard mulches, then your skepticism is purely anecdotal. Correct?
I suggest you read the post in full. You will see a link to published research at the bottom.
Linda: my brother shares your science-or-death philosophy (are you also triple vaxxed? 🙂 ) I am older than him and cut my teeth on scientific journals and history. While science is definitely paramount, labeling a technique as “death” and ‘anecdotal’ isn’t always correct simply because you have evidence that technique has flaws. There is logic and science behind the oxygen theory of earthworm travel, but in fact getting the earthworms to the surface is the goal (aerating the soil structure near the surface), and moistened cardboard breaks down just fine *while also suppressing weed roots*–now THAT is a win/win/win! If you know an easy way to shred cardboard, let me know, because the effort sounds like a loss overall.
Anecdotal is exactly the right term in this case. There is no published research demonstrating any beneficits to using cardboard sheets as a mulch – so any discussion of it is anecdotal. Since there now is published research comparing a single sheet of cardboard to 4″ of arborist chip mulch (among other treatments) – and finding a ten-fold decrease in gas exchange ability – we can say there is published research showing that sheet mulches, including cardbaord, are a poor choice if we are concerned about oxygen and carbon dioxide moving between the soil and the atmospher.
So what you are saying is if he collected all data on his work and had a control as well with the data showing better growth then it would be ACTUAL evidence?
No. That’s just anecdotal.
He would have to have a well designed experiment and collect the data correctly. Then submit it for publication in a peer-reviewed journal. That’s what constitutes actual evidence.
I think earlier you mentioned that there’s no funding for a more in-depth “cardboard mulch” study and I think that’s a real shame. Obviously the folklore on using cardboard as mulch is well ingrained, and lots of people haven’t yet been persuaded out of it. It would be great if someone could do a longer-term study where they compared a) how many weeds are killed long-term using cardboard-and-mulch versus just deep mulch; and b) how that affects insect life and other soil health measures. What happens after two weeks? Two months? Are they the same a year later?
Being able to present data like this, and have it then disseminated in garden magazines, books, etc., would go far towards changing this longstanding practice.
I agree completely – and proponents of the practice are best suited for providing funding for the research. University research is quite expensive to conduct and you won’t find faculty willing to do this work without support.
(Sorry for the late response – the pandemic has overwhelmed me with emails and information requests. Just now getting caught up.)
Dr. Chalker-Scott, the Master Composters at Clark County have build over 1,600 square feet of lasagna gardens using cardboard as a weed barrier. Our gardens are miraculous in their growth and require little to no water to sustain them even through this extensive drought. I have experienced first hand the rapid decomposition of cardboard as we build our gardens 12-24″ high and make sure the layers are well watered. My question is this. How would you advise us scientifically to show this success and that the decomposers prevail in nature. I am happy to continue the dialogue via email. email@example.com
Hi Pete –
What you have is a demonstration garden. It’s not a controlled experiment, which would require a completely different approach. It’s anecdotal evidence. You would need to show that it works better than some other method that has been researched, such as mowing and mulching with wood chips.
You can look at this publication for information on what would constitute a scientific experiment. Given that no one has done this yet, anything that was rigorously researched and publishable would be an improvement. https://www.researchgate.net/publication/315662987_Scientific_literacy_for_the_citizen_scientist_WSU_Extension_Manual_EM100E
As you know, WSU volunteers have an obligation to provide science-based information. Demonstration gardens are fine, but they should be labeled as such and without any suggestion that they are better than any other approach.
I need to transform a large section of lawn into vegetables and wildflowers. Using cardboard I understand would help smother the grass, and placing 4-6 inches of compost on top would allow me to plant almost immediately, in my case I could plant wild seeds requiring stratification now, and vegetables next spring, then apply wood chips at least in wildflower areas to keep weeds down. Anecdotal or not it has been proven to work, whether better or not to accomplish the intended purpose or comparative results to soil biology and productivity with and without may be debatable as you point out with no scientific peer reviewed study. I would love to eliminate the cardboard because it is an extra step not easy to accomplish in a large area. Dumping a load of wood chips and raking them out is certainly easier. How thick of wood chips do you think it would take to kill the lawn and make the area ready for planting, and how long would that take? Do you believe the trade off for expediency with a potential risk of setting back the soil biology temporarily (until the cardboard degrades) based on reduced vapor transmission is important enough to not do it? In other words, would the advantages of expediency outweigh the disadvantages of a temporary condition where soil biology may be impacted?
If you haven’t found it already, there is a post on how to get rid of lawn using wood chips: https://gardenprofessors.com/how-to-get-rid-of-your-lawn/. This method is based on two decades of practical research by my research group as well as that of others. It prevents light from reaching weed seeds or crowns but does not affect the roots of established, desireable woody plants.
Yes. Unfortunately science isn’t sufficiently affordable to spread to this kind of studies. And also, if someone obtains results with a practice, and can convince others without the need for science, it is likely that he won’t do the extra effort to make it also scientific. Cardboard for sure is not plastic, for sure it is easier to lay down than wood chips, and for sure it also brings in addition to blocking light a mechanical barrier to anything growing under it. In fact all is does, as you mentioned, is replacing in 3d layer by a thinner structure. It is not exactly the same as a layer of autumn leaves, but somewhat approaching the principle.
If you read the last part of the post carefully, you will see we have now done research on the gas permeability of different mulches. The diffusion coefficient graph, which remains constant regardless of other environmental factors, shows that one single layer of cardboard is ten times worse for gas movement than 4″ of arborist chips. Sheet mulches are not good choices, period.
His plants may be very healthy, but if using sheet cardboard, is the ground and earth dwellers underneath healthy?
Great article with some peer reviewed studies to back up the claims. Do you have any recommendations or a link to a book website for mulching guidelines? Is there such a bitch hing as best mulch? Cedar wooden chips laid two inch thick maybe?
I have two things for you: a review article on landscape mulching, and a fact sheet on using arborist wood chips in the landscape. Both are peer-reviewed. Bottom line: you need at least 4″ of a coarse woody mulch to keep weeds down – and the more you use, the better the suppression.
What if you are starting with a weed-infested area? Does the science support something like 4″ of cedar chips being effective after de-weeding the area? Are there any known differences in recommendations of depth of chips based on climate? I’m on the Oregon Coast for example and everyone I talk to swears landscaping fabric is the only way.
I’ve done a post on how to start this process – you can find it here: http://gardenprofessors.com/how-to-get-rid-of-your-lawn/
There is a lot of research correlating mulch depth with weed control – I’ve also written about this. https://meridian.allenpress.com/jeh/article/25/4/239/80254/Impact-of-Mulches-on-Landscape-Plants-and-the
If you are using wood chips (NOT bark), you will have good success with a deep (8-12 inches) layer over scalped weeds.
Hi Linda, I don’t know much about gardening. I have flower beds that were neglected for years, there are many weeds that are very overgrown (annual sow thistles,pokeweed,lots of others.). After pulling them (attempting to pull by the roots which worked for some but some also snapped), they seemed to all come back as strong as ever in 1-2 weeks. I am trying to figure out the best action plan to move toward weed-free flower beds that are lower maintenance going forward but am really unsure of the best strategy or step by step process? I reviewed your linked document and it says that mulch 3” and under did not suppress weeds while 4 and 6 inches does, but then I also noticed you referenced using 8-12 inches to another commenter above. My flower beds have other bushes and trees that I will need to preserve. Is 4” of cedar chips adequate to address the weeds after trying to pull them again or is the 8-12” needed? I couldn’t find anything that reviewed the results of 4” versus 12” for example. or which cases call for more than 4”. My other question is regarding the weed seeds that are currently in the soil of the flower beds. Is there a way I should try to kill the weed ‘seeds’ before mulching? It is alarming to find out the weed seeds can survive decades. I’ve heard a lot of gardeners like the pre-emergent Preen, but from the description it appears to create a barrier where the seeds can’t germinate but doesn’t actually kill the weed seeds off? Does that mean that as soon as you stop applying the product, seeds in the soil would just be waiting to come back with a vengeance? Long term I would prefer not to use chemicals in the flower beds, but I would be willing to use them for a year or two if it offered significant long-term benefit. Is there anything that would do that, in addition to the mulching, whether it product or other task, or really anything I can do to address the weed seeds in the soil or prevent myself from many years of nightmare weeds from the existing seeds? What strategy would you recommend to get the flower beds in a good place going forward? Just do my best to pull them and put mulch down or is there more I can do?
Great questions regarding mulch depths. Here are the details:
1) To prevent seed germination, you need at least 4″ of chips. Otherwise, light can filter in.
2) To keep grass and perennials from regrowing, you need much more. I would say no less than 8″. We’ve found that 12″ is ideal (that’s from actual experimental research) but I’ve used 18-24″ to really suppress aggressive weeds.
3) Once weeds are under control, then the level can be maintained at 4-6″.
Be sure you are using arborist wood chips, not bark or any other commercial wood product. None of them have the benefits that fresh wood chips do.
Do you arrange some kind of baffle around the periphery to achieve 8-12″ at the edges…or maybe extend mulching a foot beyond the periphery? I followed a link here from a discussion about replacing lawn in the strip our city cuts into sidewalks for street trees and for homeowners to plant other things. It just occurred to me you might advise a different strategy for this case, since wood chips couldn’t be piled to the depth you recommend all the way to the edge, unless you extended the mulching over the curb and into the street. Or if we should not fret about the edges, because the grass will regrow so slowly and right where we’ll know to look, making it easy enough to police?
I personally just let the chips slide and once a week go out and sweep/blow them back in. You still may get a few weeds but as you say they are easy enough to see and remove.
Hi Linda. I am currently doing my Master Gardener training in NY. I have degrees in both Physics and Biology and have always been interested in philosophy of science. I just read about 25% of the comments here, and I have to say you are one of the most patient people on the planet. Thank you for your defense of science and attempting to educate people on non-science.
Thanks, Mike! Keep up that MG training – I hope you find it rewarding both educationally and socially.
Ditto, to what Mike said. I went through three MG programs in three states, so appreciate your attention to detail as well as the patience you clearly have demonstrated.
Hi Linda. I just found this thread. Mike is totally correct. You are the most patient responder to some of the most inane questions. Science and the scientific method is bound by accurate data collection, no bias and control of variables. The control of variables is incRedibly difficult. Without it, too many confounding factors can be at play. People don’t get it! Thanks
Hi Linda really great and informative article thanks! Would you say cardboard has to be properly shredded to be a effective ‘3D mulch’? Can it be just torn up into small pieces and laid down?
And does landscape fabric / tarp mulch have the same negative effect on worms and soil life by limiting oxygen flow in the soil?
Even shredded, cardboard is still going to act in a sheet-like manner. The pieces aren’t three dimensional, unlike arborist wood chips or stones.
Fabric is much worse than cardboard, and plastic is the worst of all. Neither is a good option for maintaining healthy soil conditions.
I would believe that leaves mat down and not 3 dimensional. Would they not act the same as what you think the card board is doing.
Also when ice forms in lakes and ponds over winter gas is trapped under the ice but has the chance to escape around the edges and holes. This is why the fish don’t díe.
It depends on the species. Leaves that are soft and thin, like maples, will mat and create anaerobic conditions in the soil. Leaves that are tough and crinkly, like oaks, will not.
I don’t “think” cardboard does this. We demonstrated this with research, and published the results. The CO2 diffusion coefficient is 10 times smaller for one single layer of cardboard than for 4″ of wood chip mulch.
Fish don’t die over the winter because their metabolism slows dramatically. Roots, on the other hand, never go dormant and continue to grow as long as the ground isn’t frozen.
Ok that’s very helpful thank you. But cardboard is still a useful addition to a compost pile as a source of carbon no?
Also, I was wondering would hessian/burlap fabric be a good, more breathable alternative to cardboard as an organic material to smother vegetation and cut holes in to for new planting? I have used this on hugelkultur mounds before, but not really looked into if there’s any negative effects – seems good for preventing mounds collapsing a bit and soil running off.
Yes, cardboard (shredded or otherwise cut up) is a good brown material for compost piles.
The goal of weed control is not to “smother.” It’s to reduce light without interfering with water and gas movement. Burlap and other open weave materials don’t reduce light. The best thing to use is a thick layer of coarse material that excludes light but doesn’t interfere with water and gas movement. Arborist wood chips are organic materials that most closely mimics the natural process of debris accumulation on soil.
Thanks again for your helpful advice.
Surely burlap would partially exclude light and suppress some growth from plants you are trying to control? Just like how panes of glass in a greenhouse exclude a certain amount of light and more if they are dirty?
Could a layer of burlap topped with wood chip be a good solution, as a lot of more vigorous wild plants would eventually break through the wood chip layer?
Sorry, it doesn’t. It’s been tried extensively in restoration sites and just creates a weedy mat than requires removal. Seed germinaton requires very little light, and some require no light at all. But the seedlings require light after they use up the resources in the seed. That’s why a thick layer of coarse material works well – and has been demonstrated through years of published research to be the most effective approach without interfering with soil life.
There is no non-chemical way to selectively control weeds and weed seeds while attempting to germinate seeds of desirable species. That’s what pre-emergent herbicdes will do.
Thank You for that.
Can other then natural looking cardboard be used shredded also?
Such as pizza boxes? And some of the other type of coated boxes like butter is often displayed in?
I do take the time to remove any tape or plastic labels from the boxes.
Really, any paper product can be shredded for use in a compost pile. Its use as a mulch is limited as it really doesn’t support much microbial life. Coated papers (which are not corrugated cardboard) like butter boxes will take longer to decompose.
God bless your patience, professor!
Hi Dr. Chalker- Scott, have you found evidence that mulching with cardboard cardboard raises the PH in your soil; therefore, binding the nutrients in the soil? I used cardboard and now all my acid loving plants seem to be lacking iron.
There is no published research on cardboard as a mulch, except for what we did in 2019. But there is solid evidence that no mulch will change the pH of soil.
Your iron deficient plants may be collateral damage from excess phosphate. I recommend a soil test before you add anything else to your soil.
is the glue that is used to laminated normal cardboard safe for gardening in pots of night shades? Should I soak first and how long ?
We don’t recommend you use cardboard at all. Everything added during cardboard manufacture is designed to create a product that resists wetting and decomposition.
Interesting. As someone who gardens mostly with native plants, I am not interested in attracting earth worms due to the competition. Could using this method actually be beneficial for native plants if the cardboard was used as a bottom layer while a good leaf mulch based soil was placed on top as a planting medium? The plants could go directly into the top layer of soil without cutting into the cardboard. I assume I would end up with fewer worms in the area because they would be suppressed by the cardboard while the plants would have a chance to get established. Any thoughts?
You might want to check the links to the other posts – it’s a matter of soil and plant health as well as earthworms. Cardboard does not help anything because it restricts water and air exchange. And you want to plant your plant into the soil – not on top of cardboard. They need to establish into the site soil.
i am trying to smother crazy weeds like buttercup and morning glory. i want something that the underlying weeds cannot grow thru yet will dissolve in a few years. Any stragglers i can manually remove. “Just say no!” to roundup.
“Smother” is really not what you want to do to your soil community is it? As there is zero research showing the effect of cardboard as a mulch, and lots of research showing the effectiveness of deep coarse mulches, that’s the only reliable, science-based recommendation that can be made.
When I uncovered soil from super deep wood chips, it was dry and lifeless. Whereas a couple inches of woodchips over a layer of cardboard still allowed the soil to be wet.
Wood chips do not do this. Bark mulch does. Wood absorbs water. Bark does not.
If this were true, then every forest would be dead.
This reply is contradictory?
You’ll need to be more specific than this. I don’t know what you mean.
I’m not the original poster, but I believe the confusion is about the sentence, “Wood chips do not do this.” What is it that they don’t do?
To be more clear, wood chips do not create “dry and lifeless” conditions underneath. They absorb water like a sponge. Bark, which many people erroneously call wood chips, don’t absorb water and create dry soil conditions below.
In my area, in Massachusetts, “bark mulch” contains a lot of ground up wood.
Then it’s not bark. Bark mulch is left over from the lumber industry and has little to no wood. If it’s sold as bark mulch, it’s primarily bark. Wood chip mulch has very little bark as the mass of wood is so much greater.
If bark mulch doesn’t absorb water, it therefore lets it through to the soil beneath which should remain moist.. On the other hand wood chips particularly if containing fine particles undesirable absorb incoming moisture ,letting less through to the soil beneath.
Sorry, but no. Our current research (and any casual observation) confirms that the soil beneath bark is dry. ON the other hand, arborist chips maintain the highest soil moisture. And when it’s published I’ll post the link here.
As someone who also struggles with noxious weeds like morning glory and witch grass I would gladly exchange smothered soil for eradicated weeds. I would then be able to revive the soil. I did on time smother a bed with a heavy wood chip layer over plastic. After two years I removed the plastic and found happy healthy morning glory root systems! Perhaps they were being feed from the peripheral but that seems unlikely as the periphery was mowed turf with no obvious morning glory.
The best non chemical approach so far has been deep mulch with regular hand weeding of the weed as it becomes apparent. The weakness in the approach is me being consistent with a busy work load distracting me elsewhere.
You would be surprised how far perennial weeds can extend their root system. I’ll guarantee that’s what you were seeing under the plastic. Somewhere outside your property there is a vigorous population of morning glory.
Over time you will see your weeding become much reduced, with just some recalitrant patches to manage. It’s well worth the initial work to get rid of 90% of the problem permanently.
I hope I’m not asking a repetitive question. But, what’s your position on newspaper? Is it a better option or not at all…
Thanks in advance
There is research to support the limited use of shredded newspaper. But intact newspaper has not been studied in a comparative way, and given its sheet structure it is going to restrict wate and gas movement to some extent.
How do earthworms compete with your plants ?
Have you conducted any scientific experiments or used apparatus to confirm lower levels of DO under the cardboard or is this still just at the scientific hypothesis stage?
Neither I (nor anyone else as far as I can tell) has run experiments on this – which would be very useful. It’s not the type of project that funding agencies would care much about so it’s not too surprising that no one has done it yet. And unfortunately research can’t be done for free. However, the facts remain that cardboard is made to reduce both moisture and gas exchange, and that earthworms (as well as other soil organisms) need oxygen in the root zone. The evidence, indirect as it is, is stacked against using cardboard as a mulch. I would like to see proponents of the method fund independent research to generate some direct evidence.
We just bought a house that many of the plants were dying. When we dug a hole to plant some trees we noticed that the previous owners used cardboard in all the flower beds. In every hole we dug there were no earthworms and there ground was dry. Now I know why…. makes sense
This is anecdotal evidence. You claim to remove these types of posts but don’t do this when it so happens to support the argument you give. This was posted on July 22, 2018. Plenty of time to take it down, but it never happened.
Amy’s observations are supported by published science. Anecdotal evidence, by definition, is “based on personal accounts rather than facts or research.”
I have been using same pieces of lanscape cloth in vegetable garden for several years. Graph from above makes it seem even worse than cardboard, at least for gas exchange. Garden seems to do well enough and weed control is excellent. I was starting to amass cardboard, now I am not so sure what to do Don’t want to have to cultivate or invest in mulch to control weeds. Lanscape fabric is paid for and boxes are free.
Well, it depends on whether you want to follow science-based practices or not. I don’t tell people what to do – I present the current science and encourage people to follow it for a holistic approach to soil and plant system health.
If you fall into the latter camp, then I encourage you to get rid of fabric and cardboard and follow a management system that more closely repliates what happens in nature.
Linda, I have enjoyed reading through the Article and these comments. We have run into the problem of Herbicides leaching out of compost brought in from a Horse Farm, Pig farm and/or purchased cottonseed but compost. Currently affecting a bed of Potatoes. But may other things. The Wood chips are something we will utilize. However, they seem to attract pill bugs and may have other ground up wood, like 2×4 ect. Our Spoil is 19 feet sifting sugar sand that Mom has been Organic Gardening on for 56 plus years. So it is going to be interesting not using Cardboard/Newspaper (10 sheets folded) or manure compost. This soil doesn’t retain much. As it just goes down to the “Trinity River” down below.
As long as it’s not chemically treated, a little ground-up lumber is okay in your chip pile. The pill bugs are just an indicator of how biodiverse your wood chip mulch layer will be: most of the insects and other critters you find here will be beneficial (like spiders and predacious ground beetles). They will reduce your pest problems.
How about using it to kill your lawn? I started covering parts of my lawn with cardboard boxes, filling them with mulch. Bad idea?
I assume you want to keep your soil healthy – so I wouldn’t smother it with cardboard. A thick layer of mulch will kill your lawn. (One of the links in the first paragraph goes to a post explaining exactly how to do this.)
???? How is layering w/carbon and nitrogen NOT better than other methods. What about those using brown material, like lawn bags?
What matters is the structure of the material. Three dimensional mulches are better than two dimensional mulches. We’ve now published our research on this topic: bottom line is cardboard is 10 times worse than wood chips in terms of gas diffusion. https://www.sciencedirect.com/science/article/pii/S016719871930580X
Would you please explain the results of this study in layman’s terms? In the study is the cardboard 2D or shredded? The link only points to a short synopsis of the study without a detailed conclusion.
I can’t post the entire article because of copyright. But it is just a single layer of cardboard.
Bottom line – all sheet mulches are worse than wood chips in terms of gas exchange.
Does this include layers of newspaper? 6-8” of wood chips seems like a lot. And if I only need 3-4” after planting, where would I put the extra? I am in a subdivision so I can’t just put it in the forest.
Newspapers will restrict gas mavement as well. The wood chip layer is not permanent – it’s broken down by fungi, which are necessary for healthy root growth. The volume of fresh wood chips shrinks by at leat 50% over the course of a year.
Have you considered comparing also with gas diffusion in condition similar to forest, with layers of dead leaves?
That’s been done in the ecological literature. It’s shown conclusively that leaf litter can create temporary anaerobic conditions in the soil.
A worm is a worm is a worm… Worm species’ behavior is as varied as humans’… I don’t buy your logic about worms… I lasagna garden with eisenia foetida, and the cardboard is eaten by the worms… Epigeic species are not the same as anecic species… And some of these plots have zero worms to begin with…
I suggest you read the article I linked. Yes, some species are better able to tolerate low oxygen levels in the soil. But others do not. And to be clear, it’s not my “logic” about worms – it’s what’s in the scientific literature. When and if someone publishes research on lasagna mulches, I’ll be more than happy to read it. But until then it is an unsubstantiated method with some significant potential drawbacks.
I reviewed the summary of the paper, and the first two species appear to be endogeic… This is consistent with my statement in that I assumed you were not talking about epigeic species…. I think the anecdotal evidence will support my position… I’ll research a little to see what the experts say… I think your article referenced is too narrow to draw a conclusion that lasagna mulching will not improve soil health and worm density… My experience is with red wigglers in the arid southwest Mojave Desert… Whether the scientific literature supports the observed results, I don’t know…. You have to be kind of crazy to be gardening in the desert anyway… Even crazier to research worm behavior there…
What you need to provide is evidence that lasagna mulching *does* improve soil health and worm density. Is there a published research paper somewhere that I’ve missed?
Absent that information, all we have is published evidence about worm behavior, comparative mulch characteristics, and cardboard manufacture. Anecdotal information is not equivalent to scientifically derived data.
I also recall a discussion about German research on anaerobic conditions being much better than generally considered… However, the literature has not been translated… This was mentioned by Allyson Jack who seems to be a monster when it comes to finding literature in the field…
This would need to be translated and scrutinized as it goes completely against the current body of published work.
Dr. Kevin R. Butt has the credibility concerning this issue…
He states an important point in that whatever approach is made, a gardener should consider two things – (1) providing material for the worms to eat – whether organic or mineral and (2) providing an environment that fosters ideal conditions – stabilization of temperature and moisture. These factors are interrelated and should be the goal.
Build the garden, and the worms will come!
Butt suggests that a combination of deep burrowers and surface dwellers may get the best results.
However, Butt emphasizes that further research is needed before trials and projects can be performed – a greater understanding of the various worm species needs to be pursued first. Then, the field work can be better analyzed.
Testing in the field has limitations (ie. the cost is prohibitive), so the “show me the money” argument is a relevant factor in why the research is not as strong as one would like – apparently, this is a major problem for you.
I believe that certain worm species will do well with lasagna gardening and that anecdotal evidence is relevant due to the absence of scientific data either way.
It makes sense that lasagna mulching can help create an environment (temp and moisture) that will increase earthworm carrying capacity.
I’ve only been working with worms for about six years, but the usefulness of cardboard is something I have experienced many times.
I don’t have much experience with the cardboard causing problems….
Even with lots of money, the scientific evidence is sometimes swayed toward a preferred outcome (ask Monsanto).
Personally, lasagna gardening makes sense to me, and I plan on continuing the practice…
You can, of course, do whatever you choose. But to make a convincing argument you need to provide published research. This is true of any practice in any field that appears to contradict the current state of the science. Lasagna gardening is not based on anything one would find in nature. Deep mulching with coarse organic materials, however, is similar to what one finds in forests and its utility has been borne out in published research.
Sorry, but the biological and medical sciences have been so thoroughly corrupted now by corporate interests that I would sooner trust anecdotal evidence from a bona fide experimenter with the interests of the planet at heart than I would any published paper.
We don’t have time to clean up science. Ecosystem restoration remains our best hope for reversing the damage we’ve done to this planet (much of it sanctioned and supported by published science) and we need to do it fast. The more methods we have for consideration and experimentation, the better.
Cardboard doesn’t last long once it’s wet. There may be a short-term downside to its use in some contexts and it won’t work in every situation, but it’s a tool we can use. There is some evidence that lining planting holes for trees with cardboard in dry environments increases water-holding capacity and promotes development of mycorrhizal fungi. Context is everything!
And please. don’t let’s get too precious about science or drive wedges between anecdotal evidence and scientific evidence. At this point, judging by the evidence all around us, science wins no prizes. And at the end of the day, “nothing exists except atoms and empty space. Everything else is opinion.” (Democritus of Abdera)
Then this is certainly not the blog for you.
This is insulting to the many hard-working researchers in the sciences everywhere. Most don’t make the $$$ working for Monsanto or similar, more like underpaid and on their second post-doc. I can’t emphasize how great it is to have found a site and author like this with a strict research focus in a field riddled with hearsay and culty “permaculture” gurus.
OK, Im calling BS. (Rather biased here, as I created my entire garden ((120 ft x 60 ft))with layers of cardboard.) But, you refer to how cardboard is *so* water impermeable. Really? Try leaving a cardboard box outside in the rain and – does it not become saturated? Ironically I have used this method because in fact I do want earthworms to help my garden. I have never used a tiller over the past ten years, and I’m quite pleased with the results. I do think it is crucial not to overload the area with organic materials, but all I know is that there is a crap ton of worms when I dig around. I used this method in a former pasture,and there were some pernicious grasses that I think were helped by the cardboard. I’m really not sure how I would have started a garden without tilling or herbicide. BTW, I have come to the conclusion that tilling is not good for worms.
Without some sort of comparative experimental study you can only guess what your cardboard is doing. What I’ve summarized here and in previous posts is based on published research and, in terms of cardboard, standard manufacturing processes. You may not like it or agree with it, but I fail to see how you can call it BS without some substantial evidence to the contrary. And anecdotal evidence isn’t comparable to scientific evidence.
These are good things to consider . . . however I’d be careful in dissuading diy gardeners from using any organic material. Part and parcel of the whole point of why cardboard makes so much sense is that it is readily available and the act of using it in a garden instead of letting it go to a landfill or through the very energy intensive recycling system decreases stress on the the entire cycle of production/consumption.
In terms of permeability: To be used effectively, cardboard should be placed over existent plants, like grass or other ‘weeds’ (usually useful wildflowers themselves). Yes, cardboard will suffocate these plants, that’s the point, in creating an anaerobic condition, certain soil bacteria not usually present will help to break down what had been the surface plants, releasing nutrients into a bio-available compost. These ‘greens’ combine with the ‘browns’ of the cardboard to create sheets of humus as they break down together. This entire process is very attractive to nematodes (earthworms and other soil animals), bacteria and fungi, which as they crawl toward and through the cardboard actually open up pockets where gasses and moisture can transfer, both from the atmosphere and from subsoil.
You are never doing only one thing in gardening (or any act for that matter), so it is very important to think about the short, medium and long term effects of sheet mulching; which by and large have many more advantages than perceived problems.
In good lasagna garden designs cardboard should be relatively close to the bottom, which, as I said above will ensure that it breaks down and seeds the layers above and below it with nutrients. Temporary ‘impermeability’ is actually a good thing in this case, and in fact this is how soil is created; by different materials forming an aggregate with is neither uniform in composition or structure.
Your own reasoning begs the question “so what?” In fact the corrugation in cardboard is ideal for leaving pockets of permeability. Especially, as you say ‘heaven forbid’ many layers of cardboard.
Have you ever actually tried this? Many layers of cardboard will form incredibly dense forms of mycelial mats which can seed the surrounding soil for years to come with beneficial fungi and encourage processing of all types of pollutants. I have experimented with even leaving giant ‘bales’ of cardboard out and just watching them break down. It is a beautiful sight as the wildflowers surrounding them explode with health and vigor.
Also, common earthworms are technically an exotic species in America. Certainly useful and helpful and at this point naturalized and deserving of habitat . . . but, they are not the be all and end all of gardening. There are many places like high elevation and arid landscapes that do not need or would be harmed by earthworm presence in competition with more efficient and adapted local detritivores (things that eat dead or decaying things). If you are really worried about worms, why not just make a few worm bins? Or better yet, set up your own experiment, one garden bed vermicompost/no cardboard, one without, one with both, etc.
Definitely use all the cardboard.
Also, as a side note: Dismissing gardener and folk wisdom (often gained through generations of painstaking trial and error and high attention to local microclimate detail) as simply ‘anecdotal’ information and overly relying on dubiously funded science as ‘fact’ is very dangerous, especially right now given corporate interest in trying to shut down grass roots food sovereignty movements. Don’t give up on experiential wisdom, it is what will save us as the toxic rivers flood our fields. It is also a remnant product of the patriarchy that has subjugated wise women and country folk for hundreds if not thousands of years. What side of that do you want to be on? Science is not something that is only accessible for lab coated technocrats. The garden is the perfect laboratory for experiencing science in real time.
Permaculture earthworks forever!
Let people use all the organic material!
Always remember, everything composts, it is our job to pile it up and plug some starts in it.
You were doing well (even if you have no science to back up your claims) until your last major paragraph.
Thumbs up, thank you for expressing this!
404 error linking to mulch literature review. Link goes to https://sharepoint.cahnrs.wsu.edu/blogs/urbanhort/Media/JEH%2025(4)%20239-249.pdf. Can you provide an updated link? Thanks.
Thanks for letting me know, Chris. We’ve migrated the blog a few times and sometimes the links don’t get fixed. It’s there now.
Gardens always look amazing that have been planted into a lasagne bed–happy plants=good soil. You plant the plans AFTER the cardboard and the materials have broken down. I have started mine in the fall and plant in the spring with awesome results.
I love to keep an eye on how the cardboard is doing, and it disintegrates pretty quickly so can’t be so airtight, and I find moving, living earthworms and other shredders. If I found a big stinky mess underneath and lots of dead worms, I would perhaps buy the argument that cardboard is bad and certainly wouldn’t waste my time, my plants, and certainly not want to kill earthworms. Earthworms are often found under objects that help them stay moist and cool while they find things to pull down into their burrows.
Where is the definite science that it’s bad? Soil looks great, smells good, etc., which should mean no anaerobic conditions.
If you’ve read the other posts linked at the beginning, you’ll see discussion about the problems with sheet mulches, which include newspaper, cardboard, and landscape fabric. All of these things, by their physical design, will restrict water and air movement. I reviewed the literature in an article in 2007 which is available by email as a pdf if you care to read it.
Earthworms don’t come to the surface to stay moist and cool. That’s what the soil does. Interior soil conditions are always more moderate than those at the surface: cooler in hot weather, moister in dry weather, and so on.
The point of this post – and all the others – is that you don’t *need* the cardboard or newspaper: there is no demonstrated, added benefit published in the literature. Wood chips are all you need, and their permeability is vastly superior to any sheet mulch.
Through observation you have confirmed what has been known for millenia; cover the soil with an impermeable barrier – such as solid wood or flat stones – and the vegetable matter below will die. WIth the increasing availability of waste cardboard in the second half of the twentieth century you have discovered a free and easy way of suppressing vegetation to create growing space. Old Wives, traditional wisdom and ‘remnants of the patriarchy’ have nothing much to do with the use of cardboard sheet mulches – the issue did not come up until the late 70s.
The barrier to natural gas exchange and moisture penetration created by layers of cardboard affects not only the troublesome roots of perennial grasses and wildflowers, or the visible soil life, earthworms, insects, arthropods and other soil invertebrates. Your sheet mulches have a fatal impact on the things that you *can’t* see. The nematodes, protozoa, bacteria and fungi that form the soild food web are even more vulnerable to changes in oxygen and moisture levels than the plants and critters you *can* see. As far as the soil biology is concerned you might as well drag a rotovator through the soil every day for six months.
Yes, cardboard sheet mulch kills off perennial roots, but that is only a symptom of the damage its use inflicts on *all* soil biology. In fact the invertebrates are able to recolonize the cardboard wastelands relatively quickly; it is the complex soil biology that takes years to restore itself.
Isn’t cardboard full of adhesives and recycled paper that has undergone many industrial processes to make it usable again? Why would anyone want that their garden, especially if it’s a veggie garden? I guess if you really can’t get mulch you might resort to it. I’d wager that if people tried a cardboard bed versus a mulched bed they’d see that the cardboard isn’t the miracle they are perceiving.
I agree Jay. Cardboard is meant to last – otherwise Amazon would have a tough time shipping things in inclement weather.
So what do you do if you already put down some cardboard or a few sheets of newspaper covered with soil and mulch a few months ago? It’s had both snow and water and is breaking down but still pieces. Do you just leave it at this point?
If it’s already breaking down I’d just leave it in place. You can speed up the process by poking holes in it with a pitchfork or some other tool.
How about poking holes in the cardboard? You might still get some weeds but probably no more than with wood chips. Another method: use cardboard for pathways and layers of newspaper for the garden. Just saying.
Why bother using it at all? You get NO weeds with wood chips if you use a deep enough layer, and they don’t interfere with water and gas movement like sheet mulches do. There is no science behind using either newspaper or cardboard in gardens or in agriculture – except as shredded material.
Hi Linda! I have very much enjoyed this post and the subsequent conversations. I’ve had experience with sheet mulching, and without it as well. My results were mixed.
What got me into it was the perception that it would be a good way to smother bermudagrass turf. Bermudagrass is the predominant turf grass where I live, and it is great…… as long as it is where you want it. It is ridiculously difficult to dig up and incredibly invasive. I thought sheet mulching would be a good way to get rid of it without trying to dig it up or using round-up (which is what most folks are told to do where I live). The grass nevertheless grew up through the layers.
I would love to just use wood chips, I’m just doubtful that it would be effective against the bermudagrass scourge.
First, I love this whole blog with a scientific bent. Second, I can state that for certain weeds, no amount of woodchips is going to impede them. I have built mounds of chips up to 50cm high and nettles, morning glory and some sort of trailing blackberry came up through it. That’s why I now use two layers of cardboard underneath. Problem solved. And I peeked under the cardboard at various stages including after full decomposition and the soil biota looks pretty rich to this biologist- and grows the best vegetables in our two gardens. I know, not a published, refereed article…
The only way deep layers chips will not keep weeds out is if they are invading from adjacent, unmulched areas. That is probably the issue. We will never recommend cardbaord, as it now is demonstrated through published research to be much worse for gas transfer than wood chips.
I’m really finding this exchange valuable. For about 15 years, I’ve been using cardboard plus 4 inches of chips when converting from lawn to landscape. Over time, we’ve converted over 8500 sq. ft of lawn to landscape. About 2500 sq ft. is vegetable garden and orchard. The rest is native plants (in California where we live).
You’ve started me thinking about whether to skip the cardboard. In places that have the morning glories (bindweed) and Himalayan blackberry that Earthdave brought up, however, I’d still consider whether to keep using cardboard. These are vigorously invasive plants that spread by underground rhizomes. As soon as the chips wear down below 4 inches just a bit, those plants come back. It’s hard to keep 4 or more inches of chips everywhere on our property at all times. I’m always pulling up bindweed from among the chips as it is. I guess the cardboard would work for a while, and then it decomposes, and you’d be left in this situation regardless.
I’ve published on the technique of deep chip mulches and have used as much as 18″. The only way that any plant will survive that is if it’s attached to an underground rhizome system that supplies energy. Without light, you will have no seedlings surviving to reach the surface of a deep mulch.
If you have adjacent rhizomatous plants, there is no mulch that will keep it out. You need to be able to isolate the area from invasion at least until your desirable plants have successfully colonized the area.
You have opened my mind to a new of thinking. i had been watching No Dig Gardening channel on Youtube with Charles Dowding all winter and was really just about to sheet mulch a large area of my yard with cardboard. My big problem is bermuda grass. It comes up through just about anything. As a Biology major in college, I do see your points scientifically.. however I do have a question. How do I stop Bermuda grass from coming up through the material if I can’t “smother it” with cardboard? Can’t it send it’s runners up through the thickest piles of park? Do I have to install a weed barrier to prevent the rhizomes from running underground from adjacent grass to the piles? I really dont want to dig but would flipping the soil help? I am willing to do whatever to get rid of the bermuda grass runners. I look forward to your reply. Thanks!
HI Chris –
Sorry I am late answering your question! Yes, you have to isolate your weed problem from adjacent weeds, so a root barrier is required. Once this is done, then you can mow and mulch with a population that has limited underground reserves. Hope it’s not too late for this to help!
Would the deep layer of wood chip thwart established weeds like morning glory and witch grass? These come readily through six inches of leaf mulch, and even through bales of straw.
As long as you can isolate the root zone from nearby populations then yes, it will work. It needs to be VERY deep (e.g. 12 inches) to keep the regrowth from ever reaching sunlight. Six inches isn’t enough for initial kill. And you need to have something else that you want to grow there to keep new weeds from becoming established.
I wonder why worms congregate under plastic that has been left on concrete? I have left plastic bags of mulch in my driveway more than once for a couple of weeks and when I go to move them, there are multiple worms under there. I’ve also left cardboard in my driveway and it got rained on and when I moved it, I found worms under the cardboard, on top of the pavement. I assumed they crawled out of the ground and under the wet cardboard.
In both of those cases, they are looking for moist but aerated shelter. There’s lots of oxygen above ground. But underground those same material will cause a decrease in soil oxygen, as the microbes and other organisms use up the available soil oxygen but little is replaced. That’s when the earthworms come to the surface.
I have gardened for over 10 years and have 3 active gardens going on now.
I am no till lasagna.
Cardboard is made with non-toxic (usually fish-based) adhesives. Cardboard is NOT meant to last. Its a ONE TIME USE and then recycled.
You get plenty of weeds with just wood mulch. Back to Eden methods have been proven to fail. Termites like wood chips just fine, its not just a cardboard thing.
In my experience, worms love cardboard as does fungus. We know that fungus breaks down cellulose material, not bacteria.
When you lasanga your garden, you build up layer upon layer of humus and cellulose. Mycorrhizae takes a long time to really establish and tilling breaks the fibers, which disrupts the networking.
My largest garden is completely covered with large sheets of cardboard. Anything that is covered with compost is pretty much decompsed, the stuff that is only covered with mulch is still intact, for the most part. There are no weeds, which saves time/money/energy, lots of worms, and everything under any cardboard is a bit damp, even when there is no rain for days.
You keep waiving the banner of science, but for whatever reason, you seem to think that direct observation by experienced people who have done experiments on their own are just plain wrong.
These are anecdotal observations. Not a scientific experiment. When and if you do an experiment and publish the results, then we can talk about it. But you’re not going to find any plant or soil scientists who will take this seriously before then.
In the meantime, you might find this manual useful: https://pubs.extension.wsu.edu/scientific-literacy-for-the-citizen-scientist
Linda, the link you provide here doesn’t work. Can you repost?
Thanks, Sonia, I got the link fixed!
Seems to be broken again.
Thanks, I updated it again.
For your consideration:
The Truth Wears Off;
Is there something wrong with the scientific method?
Eleven years old, and not relevant to applied plant and soil research. The body of published evidence relevant to gardens and landscapes is not controversial to those of us who conduct and publish research.
I would love to feel comfortable that cardboard is free of contaminants and fine on my gardens as mulch and compost. Kev Man’s comment that “Cardboard is made with non-toxic (usually fish-based) adhesives” sounds very encouraging, and I hope this is actually true. Maybe I am being too cautious, but I would really like to obtain more information from trustworthy sources about research to confirm this.
Given that a single sheet of cardboard has been demonstrated to depress gas exchange between the soil and the atmosphere (compared to arborist wood chips), there just aren’t any compelling reasons to continue to use it regardless of contaminants.
I have been using the process of sheet mulching for the past eight years in my food forest and garden installations and these growing systems today are all very healthy with greatly improved humus, soil porosity and microbial life.
I have worked on landscapes where the soil was dead ‘inert’ no life whatsoever and has sprung to life after sheet mulching. For many people in Alberta dealing with heavy clay, sheet mulching or ‘building up’ is the only way they are able to get anything growing. I’m sorry but I have seen soil and growing systems greatly improve following sheet mulching.
As I’ve told others on this post, you are providing anecdotal evidence. It’s great that it works for you, but it’s not scientific research. (How do you know it’s better than some other process? That’s what scientific experimentation could tell you. You have no basis for comparison.)
It’s silly to say that sheet mulching is the only way to get things growing in Alberta. You have no evidence to base this kind of statement on, and if it were true that would mean that NOTHING grew in Alberta until smart people came along and sheet mulched the province.
(Also – humus doesn’t exist in nature. It’s a laboratory artifact.)
Did you mean to type “humate” here? “Humus” is widely used to mean the dark organic material that comes from decomposing organic (once living, carbon based) matter found in soil. You can check such sources as wikipedia.org which has an extensive and scientifically documented description of “humus”.
There is no such thing as humus in nature; it’s made by extracting organic material with strong alkali reagents (around pH 12). This has been extensively discussed in the article here: https://www.nature.com/articles/nature16069
Hi Linda,are you aware of any scientific studies involving the use of cardboard for fungal growth? It is discussed a great deal in many of Paul Stamets’ books. Since mycellium breathe CO2 & produce oxygen beneath the soil it seems likely that if you inoculated cardboard to be used as a sheet mulch the fungi would take care of the oxygen needs. I still appreciated your article. I think most people use the cardboard to try to stop pervasive weeds like quack grass and creeping bellflower. I was excited to see a study showing weeds stopped with 12″ of mulch. It is far easier to ask an arborist to drop a mixed load than to gather cardboard, remove staples and tape, and try to layer and overlap it enough to hope against the weeds rat-mazing their way through it.
Hi Nolan –
The only research I’ve seen that uses cardboard is for termite studies (it’s their preferred food).
Unfortunately fungi do not produce oxygen – they are not photosynthetic. They use oxygen and release carbon dioxide just like other non-photosynthetic organisms.
Yes, @Nolan Archer, we are stopping weeds, and as surprising as it may seem, it works a treat. Some weeds I bury in several inches of compost and soil, but for the truly pernicious weeds in degraded soil – it’s the WAY – at least for me. After a couple of seasons, the cardboard is fully broken down and the soil underneath substantially loamier. Results may differ in drier climates. It may seem easier to you to get a load of wood chips from an arborist, but for me, 14 yards of wood chips (minimum delivery) in my driveway is unthinkable. It would take me all year to get them down. Plus I have no idea if the trees were sprayed or are diseased. Some local people lost a bunch of trees just that way, so it’s out of my comfort zone, even though I’m aware of the benefits. Plus I want a softer, more predictable walking path, and 12″ would be too high to be practical in my circumstance. I use a couple inches of bark on top of the cardboard and am pleased with the look at feel. We all have different limits and compromises to make. I’m with you on the fungi – one of the gifts of cardboard and bark has been morels!
This blog promotes science – and there is no published science behind using cardboard as a mulch. None. So we don’t promote it regardless of people’s anecdotes. In contrast, there are numerous research publications on the benefits of wood chip mulches.
What we do know (from our on-going lab work) is that one sheet of cardboard – one sheet! – reduces the rate of gas movement bewteen the soil and atmopshere 100 times more than no cardboard. That’s a problem. And just think how numerous sheets of cardboard with compost in between would affect gas exchange.
We will be publishing our research within the next year so everyone can see these data.
Linda, you say there is no science behind using cardboard, but then you say there is no science around it period! So why do you argue against it? You don’t have any science against it. Annoying arrogance dismissing many people’s useful observations.
Responsible educators don’t make recommendations based on anecdotal observations. We certainly don’t do medicine that way. Why would you consider plant and soil sciences less worthy of being guided by good information?
The function of science is not to disprove anything (though often it does). It is up to proponents of a practice to demonstrate, with peer-reviewed, experimental science, that a practice or product works. If not, then said practice or product has no credibility.
But since you asked, we have data from a 2018 experiment that show cardboard restricts gas transfer ten times more than wood chips. That will be published this year.
Interesting thoughts regarding cardboard and sheet mulching in general. I’ve been doing restoration work and community gardening for many years and I think there’s too much willingness to jump right in and mulch the weeds before planting (be it sheet mulching or non-sheet mulching which also has an effect on the soil).
Specific conditions and specific goals need to be considered before any technique is used.
For example, I participated in a wetland restoration project. The site was overrun with nightshade, reed canary grass, rannunculus, lotus and other weeds. We decided that a total kill was needed and went ahead with sheet mulching using cardboard and wood chips. Yes, it caused anaerobic conditions, but the soil was already anaerobic.
The kill was total and we ended up planting in the chips that were on top of the cardboard. The sedges, rushes and willows loved it and eventually sunk their roots thru the rotting cardboard. A total success with reasonable effort on our part.
On the other hand, in the “dampland” right next to it we continued the sheet mulch and it killed off the grasses that had made the soil tillable. We were left with heavy, packed clay. On the plus side, we eliminated the rannunculus and reed canary grass that was there, but on the minus side our initial plantings did not thrive and we had to replant with species that could handle the resulting heavier soils.
so yes, sheet mulching is bad. and yes, sheet mulching is good. it all depends on the specific conditions of the site and what you are trying to accomplish.
Cardboard restricts water and gas movement just like other sheet mulches. We’ve got data to demonstrate that which should be out in press this year. Cardboard is ten times worse than wood chips in this regard. So yes, where you have plants that are tolerant of low oxygen condidtions, like willows and other wetland plants, sheet mulches are tolerated. I wouldn’t say they are “good” because those same plants also would thrive under other mulches – which you would see in a replicated field experiment. That’s why rigorous experiments are needed to base our practices on.
Wow, I’ve read all of the comments here and the gardeners using the cardboard missed the whole point. You may have gotten the results you were seeking but at a cost to the natural way of things. Just because it worked for you doesn’t make it right. These are the same people who claim to be organic gardeners, yet use plastic cups and containers throughout the gardening process. Plastic is never organic nor is cardboard board. This isn’t rocket science. You all are getting bent out of shape because their beliefs were attacked and they can’t handle the facts. You’re wrong.
I am having a problem with weeds in several long and narrow sections at my home. I live in Michigan. Don’t want to use round up, cardboard or mulch. Any suggestions? Was thinking of planting mint. Mostly sunny areas. Thank you.
Weeds are called weeds for a reason. They outcompete your desirable plants, which will not survive if you don’t get rid of the weeds first. If you are not willing to use mulch or herbicide, then you either need to pull everything by hand or get goats.
Thanks for the reply. I’ll try mulch, although a goat sounds fun.
How about using it as a weed blocker under a brick walkway?
It will eventually decompose and possibly ruin the lay of the brick. Sand base is usually what’s used for laying brick.
Dear Dr. Chalker-Scott,
I echo So Cold Design and Photography’s contention regarding your contradictions. On this page you have said, among other things:
“The only research I’ve seen that uses cardboard is for termite studies (it’s their preferred food).”
“This blog promotes science – and there is no published science behind using cardboard as a mulch. None. So we don’t promote it regardless of people’s anecdotes.”
But on your linked pages you have said:
“In published comparison studies, other mulch choices generally outperform cardboard in terms of plant growth, weed control, etc.”
“Published research on sheet mulching in landscape settings confirms the drawbacks of sheet mulching.”
The latter is less contradictory as they did in fact study sheet mulching, but not with cardboard, rather plastics, particle board, etc. (comparable to cardboard? I doubt it). The problem is that the results of the studies you cite in your review (which I might add also makes a number of unreferenced claims that the peer reviewers overlooked—hey, we all have to do that sometimes) that actually use sheet mulches as independent variables find very mixed results (the FN/citation numbers are 40, 68, 73, 86, 112, and 113). For instance: Tamás and Bubán found that black polyprop maintained soil moisture the most successfully (this could obviously be a mixed blessing with regard to the gas and moisture movement you are rightly concerned about); Litzow & Pellett found that plastic was among three of the mulches that resulted in the largest percentages of tree growth; and in Siipilehto’s article sheet mulching (not sure what “plane waste” is but the others were plastic fibre and newspaper which may or may not resemble cardboard) was superior for weed suppression and sheet mulched aspen trees were significantly larger than the controls and those given the other treatments. The triangulation of an answer from these studies with respect to cardboard is tenuous at best; I read them to indicate that straw may actually be the best mulch. It is as though you are cherry picking. My guess is that you are doing so based on a hunch informed by a combination of your own anecdotal experiences and readings on related topics (see below).
I am glad that you are pursuing “on-going lab work” on the topic as it would indeed be nice to have more rigorous data on the subject. I was originally going to comment that performing research on this shouldn’t require grants of hundreds of thousands of dollars, but when I considered the intervening and confounding variables I thought otherwise. For instance, how are you dealing with things such as the soil, cardboard, and mulch moisture levels (and spatial variability within them) before and after the sheeting, variations in the organic content and parent material of the soils, the pre-existing ecosystems of flora and fauna at all scales, or the rather large range of agricultural plants that may or may not benefit from a mulch? The price clearly goes up with such controls. At what level of detail and investment does one deem the research rigorous then, particularly given the myriad different conditions extant in the field? David’s example of restoration work is a good illustration of the influence of field conditions that are difficult to replicate in a lab.
To be honest I found your responses to your commentators (and challengers) nauseatingly pompous. Yes their experiences are, on the continuum of experimentation, singular (i.e. anecdotal) and limited, but remember that science itself is always fallible and that some of the most impactful studies were inspired by researchers’ experientially-drawn hunches (see for example Michael Polanyi on this point with respect to scientific epistemology). Which brings me back to my earlier comment regarding your reasons for so vociferously dismissing cardboard: I think there is nothing wrong with you _feeling_ that cardboard is likely a bad choice, and that these debates have inspired you to undertake studies on this. That is noble. But do not frame that sense as some sort of truth upheld by a dearth of evidence. That is clearly problematic.
Put another way, the same way the anecdotal proponents cannot say with confidence that sheet-mulching with cardboard is a one-size-fits-all miracle cure for weeds and panacea for plants, you cannot say that it isn’t. You arrogantly declare to one commentator: “What you need to provide is evidence that lasagna mulching *does* improve soil health and worm density. Is there a published research paper somewhere that I’ve missed?” Yet you have no direct evidence yourself at the moment for why ‘lasagna mulching’ doesn’t.
Regardless, I am looking forward toward your publication on the topic. I came to this page seeking more confidence in applying cardboard sheet mulching and have come away almost as unclear as I arrived (which may be a good thing). Please do keep up your formal research, but be on guard for the influence of personal bias not only in reporting your results, but also in the way you construct your experiments.
Thank you for your time and my apologies for being so blunt.
PS: Like the ‘goat-mulching’ idea!
Our peer-reviewed paper is currently in press. In it are data that show cardboard is ten times worse in terms of gas permeability than wood chips. Hopefully that is good enough for you.
In the event that you have notifications turned on for this comment — thanks for the great post, thoughtful, specific, reasonable. Sometimes the atheist turns out to be as fanatic as the believer, and the skeptic a victim of his own ability to reason, i.e. rationalize. Having just started a garden, I have exactly no knowledge, anecdotal or experimental, about the cardboard question. But as a writer, I appreciate the time you spent, and the critique you made. Thanks!
I’m glad the site owners allow critical posts, and respond to most of the posts they receive.
Yes, we have comments turned on for all our posts – but they have to be approved. The only ones that don’t get approved are spammers and trolls. We trying to keep thoughtful, civilized discussion going on every topic.
That is fantastic! Thank you for letting us know. It will be good to have another reliable piece of the puzzle in hand.
What types of wood did you use?
Arborist wood chips. Unknown species, and it really doesn’t matter in that regard.
Hello Dr. Chalker-Scott,
First of all, thank you for posting all this information about cardboard mulching – I’m so happy I found it before attempting to use cardboard. I’m hoping you offer some suggestions for how to establish a small prairie yard for our western Washington location. Our south facing back yard was a young stand of mostly Douglas Fir and Alder, but was cleared for construction, leaving behind very rocky glacial till. We initially sowed pasture grass on it just to occupy it, but because it essentially has no topsoil or organic matter has very little growing on it, except for some nasty weeds (Himalayan blackberry, bull thistle, tansy) and some grass. I will not use heavy duty pesticides as we don’t want to contaminate the ground water & well. I have tried to remove the weeds by yanking them out, but of course they’ve re-sprouted from their roots. I see you recommend 12″ of wood chips – but that amount seems out of the question for 1000 + ft2. I’ve consulted with the local extension, and the recommendation is to cover the yard with plastic for a season to kill off the weeds & then rototill in some compost, and then do the planting/seed sowing.
Hate to wait that long with plastic on the back yard – really not practical with the amount of wildlife we have.
After reading your entries, I’ve come up with this – what do you think?
Using 20% acetic acid try to kill the really nasty weeds.
Spreading a thin layer of bark mulch ~ 2″
Top soil mixed with compost on top of that
Sow seeds (this fall)
Cover with burlap to prevent washing away since it’s a sloped yard.
Alternatively – hire someone to use a bobcat to level the soil & remove the top layer with weeds in it, then topsoil with compost & then sow seeds.
I’d really appreciate a response!
First off, ignore the “advice” from extension. Plastic mulch kills everything in the soil and that is not your goal. Furthermore, rototilling destroys soil structure. The advice you were giving may have been the best possible 30 years ago but it is certainly not today. No-till is preferable.
Don’t bother with the horticultural vinegar. It will not kill perennial weeds.
Mow everything to the ground.
Avoid bark mulch. It does not absorb water and will not contribute to soil building.
Cover the entire area with wood chips. Yes, it sounds like a lot, but your 2″ of bark mulch will cost far more than a foot of wood chips. (I’ve personally done a 3 acre site this way. I hired students to spread the chips.) The wood chips are cheap to free, and they are delivered by the cubic yard (usually around 10-15 at a time). This is the single best way to (1) kill weeds; (2) protect and nourish the soil; (3) introduced beneficial microbes; and (4) build soil slowly. You must keep it covered and light free until you are ready to plant.
When you want to plant, move aside the chips until you get to a layer that looks composty and is moist. This is your planting bed. Do not get rid of the chips, but keep them for later. I use them as a mulch for all my garden and landscape beds.
Sow the seeds into the planting area. You will need to sow densely because there will be weed seeds around as well. Don’t cover with burlap because you can’t remove it without damaging the new plantings. You can cover with straw (not hay!) when the seeds start to emerge. Wood chips do not wash away like bark does.
You should never have your soil removed. That’s actual, natural topsoil. Whatever would be brought in to replace it is basically a little topsoil mixed with organic material. It is not topsoil and it is not natural. Keep your soil no matter what.
Hope that helps.
Thank you!!! That clarified a lot for me. I had in my mind bark mulch when reading your posts, and am glad to know that’s an important difference. Also – you’ve saved us a lot money – burlap isn’t cheap, and neither is hiring a landscaper with a bobcat.
Linda, how do you recommend planting plants when planting in deep chips? I’ve been planting in chips 4 inches deep for quite a few years. I’ve been using a couple of techniques. In both cases, I plant the plant’s crown even with, or a litle above, the top of the chips. The first one, based on the recommendations of Deva Luna, who teaches lawn conversion in the San Francisco Bay Area, is to plant directly in the chips. This has worked well when I’ve used it. In recent years, I’ve wanted to provide more native soil to plants. So I’ve created a larger opening and placed extra native soil in which I’ve planted the plant. (I’ve often had to work hard to find a source for this additional native soil.)
Also, are you saying that cardboard has no effect on whether former lawn grass returns, that chip type and depth are the only variables? There’s a limit on chip depth when planting.
Are you recommending more than 4 inches deep? Thanks!
Do not ever plant into the chips. They are a mulch – not soil. The crown should be at grade with the soil. The mulch will be higher. If it makes you nervous to have chips next to the trunk, you can taper it so it looks like a donut.
Four inches is minimal for weed control and has to be replaced frequently. I use anywhere up to 12″ depending on the site and my ability to maintain it. There has been no depth found that’s been published as being harmful to plant health.
Cardboard restricts gas and water movement. That fact alone should steer you clear of ever using any sheet mulch.
Fellow Oregon State University graduate here (Master of Natural Resources) hailing from southern NJ.
We were thinking of employing a sheet mulch approach to reclaim a large shady garden area located between our driveway and our house that had become overrun with invasive English ivy and vinca before we purchased the house, but reading information about sheet mulching like you’ve presented here has me second guessing. We have tried physically removing the plants in the past, but they seem to come back with a vengeance, so we have obviously been unsuccessful at removing all root material. The garden area is quite large, so I am not sure how feasible it would be to obtain enough wood chips to employ the deep wood chip method that you normally recommend. Also, the proximity of the garden area to the house (directly adjacent) has me worried about termites being attracted to the wood mulch and/or the cardboard. What approach would you recommend for this large garden area filled with aggressive plants alongside the house without using chemical herbicides?
Hi Joseph –
Sorry to be so late in responding! First of all, you will have to isolate your weed problem from any adjacent populations that you can’t control (like a neighbor). Use a root barrier to do this. Next, mow that stuff right down to the ground and then heap on the wood chips as deep as you can – at least 6″ but you can go even deeper if you have the chips. Don’t worry about termites – they do not each chips. They eat cardboard.
The trick to keeping weeds that spread underground under control is to keep taking off the top growth so eventually the roots starve. Take a look at this recent post that discusses this: http://gardenprofessors.com/a-tale-of-two-weeders-lessons-in-managing-aggressive-perennial-weeds/
As a manufacturer of cardboard products, I feel I need to clear up some of your unsubstantiated claims about that substrate.
Corrugated is a combination of two exterior walls of thin Kraft paper (made from hardwood trees in NA) attached to a tissue like kraft corrugated interior of varying amplitude.
They are all adhered by corn startch, not glues.
The verticle fluting direction gives strength to the board… much like trying to crush a paper tube vertically.
Protective coatings are rare, and this stock shouldn’t be used for mulching. Stock with high quality print on white outer surfaces shouldn’t either.
Simple brown kraft shipping boxes or rolled one side faced corrugated is recommended.
Water breaks down these components to paper quickly. (Try moving a corrugated box after it rains)
So please address the nature of your opening statements about the nature of the material in this discussion.
Thanks for the clarification on different products. Unfortunately, people who use this as a mulch are not paying attention to details on what might be more permeable.
In any case, cardboard should NOT be used as a mulch, as there is absolutely no research demonstrating any benefit over any other method. And in fact we have now published research to show that a single layer of cardboard is 10 times worse in terms of diffusion rate as wood chips.
Interesting read and thank you. I have a home vegetable garden that I expanded last year, and the newest plot had a *ton* of uncontrollable weed growth in late spring/early summer.
I want to utilize the area but the weeds made it impossible to keep up. I thought about putting down a layer of cardboard, cutting out a 4-inch diameter circle in the center of each square-foot, and planting inside that area. The plants would still be planted in the soil proper, just using the cardboard to suppress weeds in this area.
At the end of the season I plan to take the cardboard up and let nature do its thing in the Fall–I let the leaves fall and don’t clean up until spring. I’m not trying to layer cardboard to decompose, just use it to temporarily keep the weeds to a minimum.
Thoughts? I would appreciate it.
Hi Will –
The only guarantee you will have is that the weeds will grow like crazy around the outside edge of the cardboard and any overhead irrigation will be reading them, not your new transplants. I really encourage you to try the arborist chip method instead. It will suppress the weeds without damaging the soil, and you can use those chips in your veggie garden between the rows after planting. Or you could put down the chips just like you were planning to put down the cardboard – a mulch donut.
Yes, this is what I did in previous years–I laid down cutout patterns and mulched the entire area, then picked up the piece and there would be a small area uncovered by mulch that I would plant in.
Now that I’m thinking more about it, in this particular area that I am referring to, the weeds were only out of control in sections where I could not mulch heavily because of what was being planted. In areas where it was 1 plant per sq foot I mulched heavily w straw and had minimal weeds.
Hi, Dr. Chalker Scott. As I read your articles and the great course, I changed my mind about sheet mulching and decided to do wood chip mulch to kill existing lawn. My question is 1) Do I need to stay away from certain species of trees like invasive ones? I have been fighting with tree of heaven in my yard in Virginia and I do not want further problem. Here is the article related to my concern. https://www.theoaklandpress.com/news/nation-world-news/oakland-county-discontinues-program-to-prevent-spreading-of-invasive-species/article_aac5f1f2-dbd6-5520-83dd-f56ab11f6743.html 2) Is there any contraindication about placing 12 inch deep against the foundation of a house? You mentioned wood chip won’t cause pest related problems, but how about interfering with yard grading/drainage?
Hi Ayumi –
Any species of wood, as long as it is chipped, will work regardless of its invasive nature. You don’t want intact branch pieces, which could root. It’s a great way to use invasive species for something useful.
Wood chips will not interfere with drainage. I would not put them next to wood siding, because they do hold moisture, but next to concrete they will be fine.
3 months ago, I put cardboard on hard dry ground on top of a hill, and covered it with 12 inches of leaves. After seeing your article, I just now attempted to remove it and nothing was there except a few edges of the cardboard. 90% of the cardboard completely rotted away to nothing.
The ground that was covered with cardboard is dark and moist, while the ground that was covered with just 12 inches of leaves without cardboard is still hard and dry. I never water because it is very expensive.
I believe you, but this situation with the cardboard completely rotting away and leaving the soil moist seems like a good thing.
Hi Betty –
This is anecdotal evidence (as I’m sure you know) and doesn’t compare the method to others that don’t interfere with water and gas movement. We’ve now published research that shows cardboard significantly reduces gas transfer compared to wood chips. While the soil may be moist, you don’t have any idea what the oxygen levels were during this time and whether your soil life was affected negatively. There’s way more to the issue than soil moisture.
Linda, you keep saying that you have a proven scientific study showing the reduction in gas transfer. Your study covers 16 days. What happens beyond the 16 days? When the cardboard has achieved its goal of killing the underlying weeds, and starts to break down, and the roots of your plantings begin to penetrate the cardboard, it allows the subsoil to be reconnected to the atmosphere. In particular, these areas are in the vicinity of your plantings, having your gas interchange exactly where you want it.
Your study is for sixteen days. Most gardeners I know tend to garden for longer than that.
Your study seems specifically limiting, designed to prove your gas interchange contention, but really studying that aspect in too short of a time frame to be of any use to a gardener.
It’s like you were trying to prove a point, and not really letting the science guide you. Your experiment seems specifically designed to circumvent the exact benefits of cardboard mulching.
“Initial barrier – eventually dissolving” You are only measuring the “initial barrier” period of the cardboard sheet, and completely ignoring the exact intrinsic characteristics that make cardboard superior to other methods. It is a sheet method that breaks down over time, returning the gas concentrations to their pre-sheet norms.
I would like to see the results of this experiment after 32 days, 64 days, 364 days, and beyond. You know, “real life” conditions, as the multitude of “anecdotal” studies are doing.
Sixteen days without oxygen will kill you. It also kills plant roots, earthworms, and so on.
It is up to proponents of a practice or product to demonstrate something works. Please let me know when you’ve carried and published research.
Hi I am late to the discussion. I am considering using cardboard as much. i have been using mostly grass clippings and pine needles and little leaves and wood chips.
Your chart on CO2 gas diffusion In my opinion is an argument for mulching. Yes bare soil passes more CO2 into the atmosphere and that is the reason bare soil ends up depleted in Carbon. Soil that is disturbed by tillage becomes depleted in carbon (organic matter) and the primary mechanism for that carbon loss is CO2 gas diffusion into the atmosphere.
Adding an organic mulch will build good rich black soil mostly because it encourages the type of microbial action that retains carbon in the soil while bare soil encourages microbial action that just consumes the carbon turning it into CO2 which ends up back in the atmosphere.
So its good to block CO2 gas diffusion to the atmosphere to some extent because a CO2 build up and lowering O2 in the soil suppresses the action of microbes that are just turning carbon into CO2 and encourages other soil microbes like Fungi that build soil organic matter that doesn’t just end up back in the atmosphere. Of course you don’t want to overdo it, but it sounds like the cardboard decomposes long before the soil can become anaerobic.
Real soil systems don’t just have microbes; there are all sorts of larger animals, as well as plant roots. And by turning off your aerobic microbes by using a sheet mulch you will create an anaerobic environment for pathogens. That is certainly not a desirable outcome for any garden or landscape. Healthy soils are oxygenated, period, and the best mulch for them is a coarse organic one that breaks down slowly and doesn’t interfere with either water or gas movement.
First I should say that deep wood chip mulch has been amazing to enable gardening in the hot and arid desert southwest. Also, I’m not a proponent of sheet mulching as is typically described. But, and I’m surprised this hasn’t been brought up previously, is the idea that the rate of CO2 diffusion is in any way related to the level of CO2 or oxygen in the soil. In fact, your recent publication on this subject seems to state that the levels of CO2 and O2 did not depend on the mulch, cardboard or otherwise. Here is the quote from your abstract:
“Despite the different diffusion coefficients of the different mulches, CO2 and O2 concentrations in the soil under the various mulches were not significantly different as compared to the control, except for the polyethylene treatment.”
So, a few questions on my mind. First, under what context is the rate of diffusion critical to soil or plant life? In other words, is there a real world context where the rate of gas exchange limits soil life? I’m not asking about an extreme condition where diffusion is zero. Is the necessary rate of gas exchange typically 100x higher than necessary and so even a 10x decrease doesn’t limit life in some way? My speculation is that in clay soils that tend to hold significant moisture and an environment that gets significant moisture this may be more of a concern. Any scientific paper stating that diffusion rate limits soil health should share some insight into what diffusion rate is necessary.
Second question, you’ve stated several times that wood chips absorb and hold water and I’m wondering how this is completely counter to my experiences. In my environment, once wood chips are dry they become hydrophobic and repel water.
Lastly, you make several comments about soil health being adversely affected by cardboard but I haven’t seen anything you provided to substantiate that from a scientific perspective. Are you aware of anything scientific that shows cardboard is detrimental to plants or soil life? Diffusion rate needs far more correlation to soil life before I can make that connection.
I’ve been experimenting with using cardboard in a different context. Around perennial plants I take a sheet of cardboard that is roughly a similar area as the dripline of the plant and cut a hole in the middle for the plant. I place 2″ of woodchips down, then the cardboard, then a 2″ layer of woodchips so I can’t see the cardboard. Drip irrigation is beneath the cardboard. There is no comparison to apparent plant and soil health with and without cardboard. The cardboard appears to drastically reduce plant stress between waterings and increase life below. I’ve seen this difference between two plants from cuttings from the same parent plant only several feet apart with drippers that were within ~2% output of eachother. In my context in the desert, I believe soil moisture is typically the limiting factor for health, not O2 exchange. I should also point out that this is not the same as sheet mulching, I’m using smaller sections of cardboard with spacing to the next cardboard. In addition, I have mulch beneath the cardboard as well which likely helps considerably with O2 exchange. The other significant benefits of cardboard is that it allows the limited rainfall we get to be directed towards the plants, greatly increasing moisture at the roots. For anyone considering sheet mulching in a hot, arid environment I would urge caution as the cardboard doesn’t readily break down.
Thanks for your time and interesting website!
Hi Brian –
If you read the end of the abstract, and the end of the paper, it discusses how a simple mescocosm that has only microbes in its soil is not representative of what you will find in a landscape. The oxygen demand in a natural system is quite high underground – from roots, microbes, and lots of other soil organisms. The oxygen demand in the mesocosm was not high, so it didn’t decrease much with just microbes. Plus, microbes can go dormant when there’s not enough oxygen. Everything else dies.
The important point, as I’ve made elsewhere, is to look at the diffusion coefficients. Those are constants for each treatment that will not change regardless of environmental conditions or soil life. And a single layer of cardboard slows gas transfer 10 times more than 4″ of wood chips. Many people use multiple layers of cardboard and compost. That will only exacerbate the problem.
So we can theorize that a healthy soil biota, rich in plant, animal, and microbial life, will become less so when covered with cardboard compared to being covered with wood chips. And that’s why I dont recommend the use of ANY sheet mulch if you want to conserve soil ecosystem health.
Cardboard and weed-control fabric damage the soil and it’s ecosystem. Also the cardboard and fabric eventually will raise to the top making your lawn look ugly. And it also gets caught when securing decorations/lights into the ground.
Thanks for including some practical drawbacks. Having never used any of these materials myself it’s good to have warning from those that have.
I am concerned about using wood chips because I have read that they can deplete the nitrogen in your soil. How do you address this? Also we can get a good compost from our sewer district that has a good rating and you can get a trailer load for ten dollars. Is it okay to use this as a mulch?
Compost does not make a good mulch. It’s too rich to suppress weeds.
There are lots of unsupported myths about wood chips. They will not affect nitrogen. Feel free to download this fact sheet. https://pubs.extension.wsu.edu/using-arborist-wood-chips-as-a-landscape-mulch-home-garden-series
I asked our local arborist about getting some wood chips from him, and he discouraged me, saying that there were many diseases that might be brought into our yard and infect our trees from them. Is there any way to be certain you won’t be introducing disease into your yard?
It’s really not an issue! Here is a fact sheet that talks about arborist chips and those concerns. https://pubs.extension.wsu.edu/using-arborist-wood-chips-as-a-landscape-mulch-home-garden-series
newly relocated, housebound & isolated at the moment due to vehicle problems, I’m always seeking ways to save money, and, at same time am oh so desperately wanting to simply plant a vegie garden. Literally after months of frustration due to being un-able to transport FREE flattened cardboard boxes in order to lay no-dig garden beds (I’m 60 years young), and prior to reading this, I had a breakthrough today after swallowing pride and seeking help from local recycler who will deliver a ute load, in. next. few. days. (gulp) AND THEN TONIGHT, AN UNUSUAL VISITOR, a single solitary (mole) CRICKET. Let me explain haha. My heritage is 200+ years of farming people, originating in northern hemisphere, yet, being relatively isolated on an island, off the mainland Australia. Actually, am now appreciating a lot of adaptive methods, but save that for more captive audience (aka grandchildren). I digress (often) but tonight being a blustery, wild & wet winter’s night (perfect conditions in coming days to lay the board down), typical here on the coast. . Nary a sound (apart from the gale) not even the crickets! So, maybe due to spending copious amounts of glorious time on my own due to pandemic, and being in later years I NOTICED when one cricket randomly came indoors. (trying to tell me something?) It got me thinking, and I googled, and here we are 🙂 Thankyou Linda (and everyone/discussion). I did wonder about the ”suffocation” and upset to insect habitat (thankyou cricket) — it is a big area. I now plan to go lightly, lots of holes (aeration, nutrients, water) plus excess will be used for many more compost. Never enough compost. Keep smiling.
Thanks for being open to the science behind garden practices! You’ll have to let us know how it goes.
As a mainlander, the cacophony of mole crickets here in summer is deafening.good luck with the vege patch..
Hi Dr. Chalked-Scott,
I have a brief question to ensure I understand about planting and arborist chips.
I intend to sow seeds of genetically native plants custom blended for my site’s conditions (from a local restoration nursery) in place of my lawn. If I layer arborist chips down, when it is time to sow the seeds, do I remove the chips? How do I know when the chips have suppressed the lawn and it is time to sow?
Thank you so much for your dedication. I have found this post and thread to be fascinating and informative.
Hi Ariel –
Yes, you will need to pull the chips back to seed. You need to wait until the turf is actually dead, which is dependent on temperature and moisture. You can check by pulling it back and if the turf simply falls apart, it’s ready. If it still has structure, it’s still alive.
I have a follow up question. I’ve had chips down for about a month now (Victoria BC). When I dig through the chips I find they soil is very soft, I can easily push my finger down 2 inches. Per your previous reply, it sounds ready to plant as the turf structure is gone. The issue is that there is still mostly white grass still trying to grow. Do I just need to wait longer?
I haven’t been able to source additional chips to top up what I have. I’m probably sitting at 6” right now.
If the grass is white, it’s dead. It’s not growing. If there is any tinge of green, it’s not dead yet. But you said the turf structure is gone, so it’s likely that your grass is completely dead.
Be aware that there will still be weed seeds in the soil, so if you are planting anything yuo should retain mulch wherever you aren’t planting directly.
Wow, I am super impressed by this thread and its responses! I love that it’s still going strong after so many years. Thank you for everything I’ve learned today!
I have three questions if you wouldn’t mind answering:
1) I have a very crowded garden of ornamental perennials, annuals, and shrubs. I can only put down a couple of inches of much every year between plants without drowning them. Is it beneficial to put a few layers of newspaper under the mulch to smother weeds more effectively? Or does newspaper harm soil organisms like cardboard does?
2) In years past (about four-five years ago) I put down a tarp on top of weed-filled area for a whole season. Everything under it was dead dead dead by the time I bothered removing it. Then I tilled in a yard of compost and now it’s a thriving garden. Did I do permanent damage with my tarp? How long until the soil recovers?
3) Is there any research (or even anecdotes, haha) on using carpet in the garden? I use a yard of carpet at the boundary between garden and weeds to keep the weeds from invading. I have to replace the carpet every year as eventually weeds grow on top of it. The weeds I’m dealing with are japanese grass, creeping charlie, and crabgrass mostly. (Obviously a vertical weed barrier would be most effective but there’s a path leading through the area to get to the backyard and shed. Eventually I’ll expand the garden into the backyard.)
Whoops I have a fourth question! I listened to an interview Margaret Roach did with some researcher about solarization and obscuration. Do you have a good article on those? The interview was here: https://awaytogarden.com/reducing-weeds-a-101-on-soil-solarization-with-sonja-birthisel/
Thank you very much!
Hi Amy –
Using words like “smother” should alert you to the fact that you aren’t just affecting weeds. So no, I don’t recommend using newspaper. Your goal is to eliminate light, not oxygen. You can easily put down 4″ of wood chip mulch. It’s not going to hurt your plants.
Your soil system will recover from the tarp. Soil life moves in horizontally from unaffected areas. I really recommend you don’t till annything into your soil. It destroys structure and functionality. Just lay it on top and let it incorporate like nature does.
Wow. Carpet belongs indoors, not on top of soil. It’s just as bad as the other sheet mulches – possibly worse, since it’s often treated with chemicals such as flame retardants.
Solarization is useful for monocultural agricultural production because they aren’t terribly concerned with killing everything in the soil. You should be. There are beneficial bacteria and fungi that make your plants stronger and more able to resist disease.
“Your soil system will recover from the tarp. Soil life moves in horizontally from unaffected areas.”
Wait, what? After reading through 5 years worth of comments to the effect of “Don’t use cardboard b/c it reduces oxygen,” now I find out that tarps are okay because soil life just moves in? Wouldn’t this also be true of a one-time application of a single layer of cardboard?
I’m so confused.
…oh wait. Sorry. You say “recover,” meaning that the tarp did real harm (some intended, some unintended), but upon removal, new soil life moves in b/c oxygen flow is restored. Do I have that right?
And if so, then I suppose that single-layer, one-time cardboard would similarly do harm, but then once it is removed (or degraded), new soil life would move back in. This would be preferable to on-going “lasagna” methods, which continue to add oxygen-suppressing material over the soil. But any cardboard at all is still more risky than wood chips. Is that correct as well?
Finally, I’m new to this and I don’t fully understand the risks involved. You mentioned in another reply the potential for pathogens to develop in an anaerobic environment. Is that the primary risk here? What kinds of pathogens, and what harm might they do? How “anaerobic” does it have to be? Is the lower-oxygen environment created by tarp/cardboard sufficient to produce these pathogens? Or does the gas exchange have to be even more restricted? Is the tarp-covered soil really going to recover? Or is there a chance that it is now pathogen-rich and dangerous? (Sorry that’s a lot of questions at once. Maybe I just need a source that would give me a basic understanding of this kind of risk.)
Thank you! (And many thanks for your research and replies over all these years!)
The difference is probably my choice of words. One situation is making an honest mistake and asking how to make it better. I reassured the writer that yes, your soil will recover. The other is a deliberate use of a material that will depresses water and air movement between the soil and atmosphere. When people continue to promote a bad practice, knowing or not caring there is no science behind it, my patience wears thin. After all, this is a science-based blog!
There is no on-off switch for aerobic/anaerobic conditions. It’s a continuum. As oxygen content decreases, stress on living soil components increases as well, making them more susceptible to pests and disease. Many disease organisms proliferate in low oxygen conditions, so they become more active at the same time.
Hopefully that helps answer your questions?
Very helpful. Thank you!
I’m so glad I’m not the only one who read all the comments! It was actually kind of fun.
We are all learning as we go, and reading stuff like this teaches me a lot! Now I know not to cardboard, newspaper, or tarp my garden. Live and learn.
This year my project is building a retaining wall and filling in four feet high with fill dirt behind it. I now know that I should put a bunch of compost on top–and I think it’s OK to till that in, since there is no existing soil structure or beneficial organisms to protect. Then I’ll dump a bunch of mulch on top–with no cardboard! Also no carpet or tarp. ^_~
It all sounds good…but I would encourage you to do a soil test before you add compost. Nutrient toxicity is a real thing in gardens and landscapes, even in fill dirt. It’s much easier to add a deficient nutrient than to remove a toxic one.
Good idea! Thank you!
Hello Linda Chalker-Scott –
I have an 100 tree orchard in the desert and I wish to use an abundance of chipped and shredded cardboard to improve the soil. I will drill 30 inch deep holes 3 feet and beyond from the 12 ft tall trees, nut trees. I will fill the 8 inch diameter holes with the cardboard chips/shred. The soil is clay, only clay, lacking nutrients in it. All fertilization is pumped through the water system. I would like to actually improve the soil. Your thoughts on this course of action with the cardboard would be appreciated. Thank you.
Hi Ricky –
There are a number of misconceptions in this plan and I hope I can dissuade you of them.
First of all, clay soils are nutrient rich. Clay is the only component of soil that binds nutrients. Sandy and silty soils are nutrient poor.
Amending the soil is never a good idea, as it creates discontinuities, meaning that water, air, and roots won’t move through those textural differences you’ve created. Plus, cardboard has a high C to N ratio, and you will create a nitrogen deficiency in your soil.
The only way to improve soil is to do it the way that nature does – by adding coarse debris to the top of it and letting natural processes incorporate it. You might be interested in reading this peer-reviewed article, which will further address the problems with soil amendment, and offers science-based alternatives to improving soil. https://www.nacaa.com/journal/index.php?jid=1024 (I updated the link to the public one).
…unfortunately to access the link, one has to be a member…
I’m curious to read how this last commenter will sort out his clay dilemma…
Would it be possible to provide title, authors, publication date so perhaps I can access through my library’s database collection?
Thank you Linda for taking the time to explain things, adding extra information and providing great links! Truly appreciated!!
Sonia, let’s try this one instead. It’s open access. https://www.nacaa.com/journal/index.php?jid=1024
It works! Thank you for that link!
Hi Ricky. We all must stop digging to improve any soil. Digging releases CO2, adding to the burning issue of the global warming. It also disturbs soil life. Mimic the nature. Nature doesn’t dig with spades or rototillers, it “digs” with earthworms by attracting them with a mulch (fallen leaves, pine needles, wind-blown green and brown debris, and so on) or ground-covers. I’m talking from my non-scientific but practical experience. We need to eradicate this destructive instinct to digging. It’s bad for the planet, it’s bad for your soil. For any soil.
Hi, I appreciate your blog and professional approach to research and conclusions of the issues discussed. I have a question to you as scientist. Are there toxic chemicals in shredded cardboard, in glue, ink pigments or paper itself? What will happen when it’s composted, will they break down? Or will they be absorbed by garden plants and present danger to our health? What is your understanding of this issues? Ir may be you could recommend some article on this?
Thank you so much!
Hi Yana –
Sorry for the late reply – I’m just now pulling through a pandemic-induced flood of email questions and information requests.
There is nothing really bad in the breakdown products of cardboard. Microbes degrade the glues, sealants and dyes pretty quickly – that’s why it is a great addition to a compost pile.
Do you know of any sources that speak to the potential toxicities of corrugated cardboard. I started shredding corrugated with little to no ink for mulch and it looks like it will be helpful but then I paused because of literature that cites toxicity. The toxicity papers were citing paperboard not corrugated from my meager search. Wondering if you have specific info for corrugated?
Unfortunately no one has ever published cardboard use as a mulch, shredded or otherwise (except for the 2019 paper we published on mulches and oxygen movement). So I can’t address your question – sorry.
Hi Linda — Such a fascinating discussion! I am trying to kill the goutweed currently growing in the parkway (between the street and sidewalk) in front of my house. Once under control I intend to plant natives. This parkway also has 2 well established and very tall catalpa trees growing on it. Based on the discussion above, I will forego a cardboard layer! In either case, if I lay 4″ of mulch over the whole parkway to suppress the goutweed, will it harm the trees? Maybe I leave a mulch-free ring around the base, but if I do that, does it negate the smothering of the goutweed (it’s a rhizome) elsewhere? Is it a Sophie’s Choice of the plant world?
Hi Gail –
The trees will benefit from a wood chip mulch. Use it liberally and deeply.
Thanks for the research and accessible explanations.
Do you know of any research on sheets of coconut coir? It is a fairly coarse, 3 dimensional, fibrous mat. I don’t use it for weed suppression, nor do I believe it would be very effective since it is so permeable. You can easily work it apart with a bit of tension, and water will soak through it, not run off the sides.
It is very handy for providing footing temporarily on steep slopes and preventing the soil from slipping when walking on it. Some sections of slope are too steep for mulch alone to do this, and it is also much easier to carry several mats down, stake it out, and then mulch over it. I’ve found coarse chicken wire to be good for this too.
My intuition is that it will be less restrictive for gas and water transfer than cardboard, but I am curious if you have anything to add.
Hi Matt –
I too have used coir in restoration, but primarily to hold wood chips in place until they hydtrate and become stabilized by roots and mycorrhizae. Burlap and jute work too in this regard. I don’t believe they pose any threat to gas and water movement given their loose nature, but I’ve not seen any research on any of these materials specficaly for this purpose.
Would layering a lawn totally with wood chips, and then building mounds of soil, for a vegetable garden be problematic?
After you scalp the lawn to the ground and then mulch it, yes, you will get rid of the lawn. But putting soil on top of the chips will create a nutrient deficiency in that soil. I would wait until the mulch has done its work, pull the chips back, then build your mounds or raised beds. Don’t toss the chips but reuse elsewhere.
This begs the question, if it’s lawn, why can’t one just scalp the lawn, put mounds of soil – skip the mulching part?
Mounds of soil will grow mounds of weeds.
The point of mulching is to suppress weeds and prepare the soil for planting.
Ah, ok. Thanks!
Thanks so much for the information. I was toying with the idea of using cardboard between the rows and plants in my garden since I have so much from online orders and have heard good things about it. These articles have convinced me to go with hardwood mulch and the extra weeding necessary. Here in North Carolina, weeds and insects are a constant struggle. I am now using beneficial nematodes and other insect predators and mulch and avoiding all pesticides and herbicides. Doug Culver
Hi Doug –
Glad you fouund our information! I do know that several of the members of the Garden Professors blog group on Facebook are from NC and do use wood chips, so that might be a good place to compare notes if you are so inclined.
Hello! How would you then suggest prepping the ground for a pollinator meadow. Can one sow wild flower seeds in arborist chips?
Unfortunately a garden from seed is difficult to do without getting weeds as well. You can’t use arborist chips – they suppress all seeds by restricting light. They are not nutritious enough to serve as a base for seeding over, either. You will need to use a bare soil, ideally prepared as shown here: https://gardenprofessors.com/how-to-get-rid-of-your-lawn/. After all the vegetation underneath is dead, move the chips aside and sow your seeds. Cover them with a thin layer of straw (not hay!) just as you would do if you were seeding a lawn. You will get weeds from any seeds that are there and you will have to pull them once you recognize them. If you are planting annuals you will have to repeat this process every year. I strongly suggest you use herbaceous perennials.
Perhaps not peer reviewed, but this study seems to contradict what you have put forth.
The article is about vermicomposting. Not using cardboard as a mulch. The only time sheet mulches are mentioned is in an off-hand “oh, here’s how you can create habitat for worms.” IAnd there are no references, so who knows where this came from?
We’ve published a peer-reviewed article on the impact of cardboard (and other mulches) on gas movement between the soil and atmosphere. That’s the state of the science, and it’s up to proponents of cardboard mulches and/or lasagna mulching to provide evidence that our study was incorrect.
That’s the state of the science, and it’s up to proponents of cardboard mulches and/or lasagna mulching to provide evidence that our study was incorrect.
You are repeatedly making a strawman argument. Its not a question of anybody disputing the claim that cardboard inhibits gas diffusion more than wood chips. You don’t even need a study to arrive at that obvious conclusion. The question is whether that higher rate of gas diffusion is good for the soil in the long run?
For thousands of years man has been stimulating plant growth by oxygenating the soil. That may seem like a good thing but the fact is that in the long run it has resulted in enormous damage to agricultural soils. In the US the soil organic matter content of virgin soils was often in the 10%-12% range. After many decades of promoting gas diffusion through tillage and bare soil planting most US farm soils have organic matter content in the 1%-2% range. So it seems to me that your assumption that more gas diffusion is better than less is flawed to say the least.
Yes indeed cardboard mulch reduces gas diffusion more than wood chips do, anybody should be able to figure that out on their own, but so what? Where is the evidence that wood chips are better for the soil health in the long run?
Oxygen is needed for soil life; the more oxygen there is, the deeper life can exist. Seems pretty obvious conclusion.
There are many studies on the benefits that arborist wood chips provide to soil. There are none on cardboard.
I have very persistent poison ivy and am trying to make areas to grow vegetables. I used cardboard in the bottom of my beds simply to kill off any PI that might be present and suppress other weeds. My question is, will the cardboard not decompose quickly under the soil? I now realize it’s not the best for soil health (didn’t know that at the time), but will that soil health not improve once the cardboard softens and decomposes? Or is the soil below the cardboard forever damaged? Also, mulch works well at suppressing weeds and preventing many new seeds from germinating, but in my experience, there are weeds and grasses that don’t care how thick your mulch is, they will still grow up through it. Is wood chip mulch the only option? Is it possible to first smother with cardboard and then remove? Will the soil health in that area recover?
Cardboard will decompose, but oxygen is needed for the process. So the deeper the cardboard the slower the process. I’m sure you’ve seen photos of newspapers that are decades old that were buried in landfills? It’s the same phenomenon.
No seed can germinate and grow through a foot of wood chips. However, weeds that spread underground continue to receive resources from the population outside the mulch, so yes they will grow through. That’s why it’s necessary to either treat the entire area or to sever root connections.
Anytime you use the word “smother” you should remember it’s an entire ecosystem you are killing. Your focus should be targeted towards the weed and removing what it needs (light). There is no sheet mulch that I would ever recommend for weed control given the lack of efficacy and the damage incurred. Wood chip mulches are the best mulches, without compare, for soil and plant health.
My apologies, I posted the incorrect link:
This report is over 10 years old. The conclusions are not borne out by the results, which are all over the place. The researchers state their bias right up front: “Our farm believes strongly in our use of cardboard mulch…” That is going to skew their perceptions, which is seen in their unsupported claims of causation (i.e., all these great things happened because of carboard mulch), when at this point it’s merely correlation.
If you read far enough down, you’ll find these paragraphs:
“The two growing seasons during which this study was conducted presented many challenges. 2010 brought an extremely wet spring, periods of unusually high temperatures in the early part of the season, followed by long very dry periods and unusually high temperatures for longer than usual duration. The research plot experienced severe repeated predation by herbivores destroying most of the crops. This limited the plant-based testing to qualitative results, as we did not have enough plants in reproducible conditions to generate sample sizes for statistically significant data.
2011 was another season of unfavorable conditions for conducting the research. We had planned to focus more on the monitoring of the earthworm population, but the record setting rainfall added complicating factors of earthworm behavior into the mix. The resulting data was useful descriptively, but would not be statistically significant.”
So no statistically significant data, yet cardbaord mulch is great. This doesn’t meet the standard of reliable evidence.
Mike McGrath in one of his “You Bet Your Garden” podcasts mentioned a fungus that grows on/in/under wood mulch. I think he called it Aspergillius. Its spores can cause death in chickens. I’m sorry I can’t be more specific because I working in the garden while listening to it and didn’t make a written note… My question is: Is shredded soft/hardwood from my native hedge clippings save to use as a 3D mulch on: 1) lawn to suppress the grass in preparations of a no-dig bed; 2) on a previously tilled fruit and vegetable beds to keep weeds away? Thank you in advance for your feedback.
Wood chips are full of all kinds of fungi, meaning there are lots of spores there too. None of them should be inhaled, so if you are working around your chip pile it’s a good idea to wear a mask.
Yes, you should use any woody clippings you have – just make sure they are not too fine or they will compact. If you use them along with wood chips so you have a nice coarse mulch, you can create a very thick weed barrier that will not harm your soil. You can find a post on this if you search for getting rid of your lawn.
Linda, your replies made the comments section even more informative that the article itself. Kudos to you for the thought and effort you put in to all your replies!
I ditto that! Thank you so much Linda – you are my horticulture guru
Could you comment on https://www.groasis.com/en – wouldn’t these kill the soil around the plants creating anaerobic conditions? But plants seem to love it.
I did a blog post about these contraptions several years ago. It really is nothing like cardboard mulch as it covers a very small soil area. And there is yet to be any reputable science published on this since I blogged about it 11 years ago. (There have been a few publications supporting the use, but their quality is questionable at best and the publishing companies, like “American Scientific Publishers,” tend to be single suites located in strip malls.)
Thanks for your thoughts. Allow me to apologize for the tone of my kneejerk cardboard loving response.
I’m wondering your anecdotal thoughts on a 3d sheet mulch scheme I have considered. First, take arborist chips and lay them down. Second, lay cardboard sheets in a “chess board” pattern with alternating areas of sheets and no sheets. Third, lay down another layer of arborist chips. Fourth, lay down another chess board pattern, with the opposing layout to step 2 (that is, if there was a sheet there, now do not place a sheet, and vice versa). 5th, lay down a final layer of chips. I assert without evidence that this could have benefits of gas and water exchange while also getting benefits of light blocking and physical weed barrier. Now, is this better than a simple thick layer of chips—that is for the scientists to figure out. Any way I can get a grant to study this properly?
That’s pretty complicated and I wonder why you wouldn’t just use the thick layer of chips? What could cardboard add in terms of blocking light that the wood chips would not?
I worry about unfriendly chemicals on some cardboards. Might you say something about this. I have to overwater my garden cause it full of salts n it gets hot here. I live above San Diego at 1500’.
All cardboard is manufactured with chemicals to improve stability and resist water. Just don’t use it. Arborist wood chips are all natural.
I just want to say thank you so much for considering all the soil life (worms and other invertebrates, microbes, fungi, etc) and doing so with science. I also want to thank you for having the patience, tenacity and clearly, the passion, to respond to so many people’s questions. If I had only looked at the first few links in my search, I would have thought it relatively benign to put cardboard out but I too have tenacity and deep curiousity so I’m happy to have found your info. I am a terrible gardener b/c I don’t like to kill anything but also a kind human. Healthy soil = healthy planet. Thank you!
Thanks, Tanya, for taking the time to read the information with an open mind and think about it objectively. We appreciate your kind comments!
I was wondering if the chemicals that are found in cardboard would also be a reason not to use cardboard for mulch. Thanks for an interesting discussion!
That’s only one reason that proponents of cardboard mulch need to start funding and publishing research!
What would you recommend be put underneath 24″ diameter open bottom plastic drums to keep root growth from nearby plants out of the compost contained within?
I would put these onto a solid surface, such as a concrete pad. That’s what we have done, and that’s how commercial composting companies keep their materials free of roots.
Thanks for the reply. I didn’t mean to hijack the thread. I was originally considering cardboard as a base material, which led me to this posting. After researching the chemicals used in making cardboard I quickly steered away from that thought.
I am looking at options for a non-permanent base. I’ve switched to what I call “nomadic composting”. For a while I simply used heavy weight plastic bags that I would fill and refill as the contents broke down. This worked surprisingly well and resulted in bags of compost that could be easily moved to its final use point.
Unfortunately, the bags weren’t a long term solution, quite the opposite when you consider the negative environmental impact of non-recyclable bags.
Then I picked up several dozen heavy duty plastic 45 gallon open bottomed bins tapered from 21″ at the top to 24″ at the bottom. These were surplus, formerly used as outdoor garbage receptacles. These will replace the bags literally size for size as the bins were designed for large bags to be placed inside. However, being open bottomed creates root intrusion issues in some areas.
To that end, I found some 29″ aluminum pizza pans, heavily discounted on Amazon, that are scheduled to arrive tomorrow. Not enough for my total needs, but plenty to test with. I would prefer stainless steel, but that’s pricey and harder to find size appropriate.
Another thought is to repurpose vintage metal roofing sheets. The standard panel width is perfect, it can be found cheap, will last for years and it’s easy to cut. I used this under a (formerly) permanent 8’x8′ bin with reasonable success. Although over years the location I chose proved too tempting for the roots of nearby vegetation and trees. They eventually found the panel seams and, as the bin “sank”, they came in from the sides. This shouldn’t be a problem for the migratory bins.
My apologies for the rambling. Unless you feel there is any value in the content for others it’s not necessary to post this.
You seem to have found a reasonable alternative to the problem of roots invading your compost. I would be curious to hear back from you about whether this works over the years.
I’ve much enjoyed reading all the comments and responses to this post over the last year or two. Thank you for continuing to respond to them! It’s taught me a lot–I previously commented about having used carpet as a weed-suppressant and you strongly advised against doing so again. You’ll be happy to know that I recently got a load of woodchips (free, from Chip Drop!) and am spreading them 12″ thick to prepare a new garden space.
I’ve been thinking about citizen science and just read the article on it that you linked to a recent commenter. I’m wondering if I could design an experiment to test the hypothesis: “After two months, using 12″ of woodchip mulch to suppress weeds results in more soil organisms under the area than using cardboard covered with 3″ of mulch.”
The null hypothesis would be, what, something like: “After two months, using 12″ of woodchip mulch to suppress weeds does not result in more soil organisms under the area than using cardboard covered with 3″ of mulch?”
My experimental setup would be three experimental units: nothing, 12″ of mulch, and cardboard with 3″ or mulch. I think I’d build a grid out of wood so it would contain the mulch and separate the units. I’d have four sets of this in a grid, for a total of 12 boxes. Randomly assign each box… etc.
Then leave everything for two months without touching it. Take a soil sample from the middle of each plot and have some sort of biologist or entomologist at UNC help me count the soil organisms in each one.
I’m not sure how big each EU would have to be though. And I’d have to get buy-in from someone at UNC who knew something about identifying and counting soil organisms.
You said that the experiment would need to be repeated three times before being published. I have two friends in other parts of my city (Durham-Chapel Hill, NC) who want to kill grass and would be willing to do this with me. Or would it be better to reach out to gardeners in other parts of the country?
And could I actually submit a paper to a scientific journal? Or would I need to get someone at a university to do it for me?
…I don’t suppose you’re interested in helping me organize this? It seems like a lot of people would be interested in the results! You’ve been saying “proponents of cardboard mulch should fund a study to prove it works” and this would be putting the labor on us citizens.
Anyway, thanks again for continuing to read our comments and for any advice you might have.
I’m glad you’re thinking along the lines of experimental design. However, it’s a little more complicated than the setup you describe.
First, you have to keep variable separate. Having a combined treatment of cardboard plus chips is two variables. You would need the folloing treatments:
Wood chips (you don’t need different levels as there is already published research showing that deeper mulches are better at weed control). I would suggest 12″ as that has been done previously.
Cardboard – 1 piece per unit, and all pieces identical in size and thickness
Wood chips + cardboard (the same as used above)
Furthermore, you need to have more than 3 replicates for field trials. There is too much environmental variability for only 3 replicates. It’s recommended to use a minimum of 10 replicates and 20 is ideal. That would mean 40-80 plots. And they can’t be touching one another – there needs to be separation among them so there is no influence.
Then there’s the issue of what you are going to look at underground, and how. Having any professional look at soil samples to enumerate organisms (microbes? worms? nematodes? insects) will cost you money because it is slow, painstaking work. You would also need multiple samples from each treatment (again to control for variability) and you would probably need to do this over a period of time, not just once. Why would 2 months be better than 1 or 3? What about immediate results, like in the first week?
This is why these experiments are rarely done. They take time, space, and money to do correctly. If you look at the photo at the bottom of this post (https://gardenprofessors.com/mulch-murder-misinformation/) you will see what such experimental setups look like.
I understand asking for peer reviewed sources to back up claims cardboard is beneficial when used this same way, but shouldn’t peer reviewed sources also be used to make claims that its harmful? Otherwise like the article said, this is all anecdotal at best.
What is benecial in one soil/climate but be detrimental in another.
Please read the post carefully. I have noted that we published just such a study in 2019.
Thank you for your reply! What I’m hearing is that I should look at this idea when I have either: won the lottery and can pay off a university or grad student to do this for me; or have my closest 19 friends on board.
I’m going to put this idea in my back pocket just in case.
Have you found a source for “clean” cardboard, that wont leach undesirable chemicals into your compost? I gave up on my plan to use cardboard mainly because there was no way to determine what chemicals were used in it’s manufacture. Even food grade cardboard containers (like pizza boxes) have had issues with chemicals used in their manufacture that led to the FDA banning the use of some chemicals. Then there is the transfer of chemicals through the recycling process to consider as well.
Fritz, if I was going to use cardboard I would grab it from work–I work in a bakery and there are big pieces of plain cardboard that are used with pallets. I figure if there’s any cardboard without extra stuff added, it would be those. They have the added benefit of being really big.
At the moment I’m finishing off a big ChipDrop load of wood chips so I’m putting those 12″ thick instead of using cardboard though! Are you in an area with ChipDrop?
Dear Linda, from these comments I can see that many people take your advice and as you are a revered scientist they trust your expertise and credentials. I am grateful for the debate and scrutiny that this whole subject has provided, over so many years, by people from a wealth of backgrounds. I am an amateur gardener, with an Environmental Biology degree, who practices ‘no dig’ in my garden (via Charles Dowding and similar professional gardeners’ advice). I initially use cardboard mulching to suppress ‘weeds’ (unwanted plants in my veg plot), with compost/well rotted manure placed on top, but generally I only use the cardboard in the first season, if the weeds are particularly pervasive. So, I am keen to understand if I should shift to using just thick layers of woodchip instead.
In your introduction you state:
‘A garden or landscape mulched with cardboard (or heaven forbid several layers of cardboard as part of the science-free lasagna mulch method) is now covered with a tough, relatively gas- and water-impermeable material that will take some time to break down. It’s hardly a mulch that’s going to nurture soil life.’
Your emotive language, right at the start, concerns me rather – especially the ‘heaven forbid’, from a scientist trying to promote science! (I appreciate it is just a turn of phrase, but to me, on the off, it detracts from the science.) Leaves can also be tough and ‘take time to break down’. Similarly wood chips are tough and ‘take time to break down’. So, if we are to take your statement at face value, do they not nurture soil life? Of course they do. You then go on to say that the woodchips are three dimensional. But, leaves are less so aren’t they? They are more laminate in morphology, like lots of pieces of paper. When stuck together, as a damp mass, they could have fewer air pockets and are certainly less three dimensional than woodchip.
Biodegradability – apparently leaves can take up to 1 year to break down, whereas paper can take 2-5 months (source: https://www.sciencelearn.org.nz/resources/1543-measuring-biodegradability). Of course, the rate at which they break down probably depends on a whole range of variables including the exact composition of the leaves, the soil organisms present, how damp the substrate is above and below.
August 5 2020: You state: ‘There is no theoretical plant or soil science that would support the use of cardboard, so the additional lack of any practical published evidence means there’s nothing that a scientist could use to support the use of cardboard, given the collateral damage imposed on the soil ecosystem (which is totally ignored by gardeners who are solely focused on crop yields). On the other hand, there is ample evidence, both theoretical and practical, that sheet mulches inhibit water and gas movement while coarse, chunky, three-dimensional mulches do not significnatly interfere with either gas or water water exchange.’
From my ecological (also a science) point of view, the lack of published evidence does not negate the use of cardboard, as nothing has been proven either way – you state: ‘given the collateral damage imposed on the soil ecosystem’ – so has this been scientifically proven that there is collateral damage (apart from your limited study on gas exchange)? How was this measured? What organisms died? For how long did the damage continue? (I am going somewhere with this, in relation to how using woodchip could actually be MORE harmful to the environment than using a limited amount of cardboard.)
You also state: ‘which is totally ignored by gardeners who are solely focused on crop yields’. Not true. I object to this sweeping statement and I’m sure many other gardeners would too! Please do not generalise. Millions of gardeners are not just concerned about crop yields, but also wish to boost the diversity of wildlife in their soil, gardens and the wider environment. I am one of them. Which is why I am significantly concerned about using woodchip on my garden in greater quantities.
August 6 2020: You state: ‘Anything that reduces oxygen diffusion into the soil will negatively affect the soil ecosystem – and the more diverse and complex it is, the bigger the negative impact.’
Did you think of testing the effect of a layer of dead deciduous leaves on gas diffusion, which reflects the conditions of a deciduous woodland floor in autumn / fall? Leaves are extremely ‘diverse and complex’ in structure (moreso than cardboard). When laying on top of each other, as they compress and if damp, they are not as three dimensional as woodchip (i.e. not as chunky) – so may present a more laminate structure, with fewer air pockets than woodchip. This variable could have also been tested on a mesocosm scale, alongside the other mulches you used. Of course, I appreciate that non-composted leaves aren’t often used as a mulch layer in gardens. There is a common sense reason for this. Gardeners generally compost them first for a year or two, as they take that long to break down to become a more beneficial soil amendment, i.e. longer than cardboard. So, why leave this variable out of your experiment? This is a mulch that would mimic nature more acurately than a 12″ layer of woodchip.
In your experiment, what about also testing a layer of compost over damp cardboard, which is generally what is used as a mulch by many gardeners? Could this have been included on the mesocosm scale you used? I don’t think you’ll find many gardeners reading this post who use cardboard alone. That wouldn’t be very effective as a mulch. I place at least a 3″ layer of compost or well rotted manure on top of the cardboard and many gardeners water the cardboard first, to aid the breakdown of the material. This use of damp cardboard may also permit easier gas diffusion (i.e. through the wet material). I’m not sure – did you test damp cardboard vs. dry cardboard? Also how thick was the cardboard you tested? I’m sorry, I can’t find the link to your paper to check this and I’m not sure if its stated in the abstract.
I found this article you recommend extremely helpful about use and application of woodchips and it seems to make a lot of sense to use woodchip in our gardens:
But, then I start questioning the much wider scale use of very thick layers of wood chips, if many more gardeners were to adopt this practice. What might be the implications?
Wood chips look like a good ‘natural’ intervention to address the gardener’s weed problem and to enrich the soil life, but firstly we have to remember that this is not truly reflecting the local natural ecosystems. Where do we see thick layers of woodchip occurring in nature? If you introduce woodchip sourced from conifer woodlands to a soil that hasn’t been exposed to this substrate before (e.g. to a chalk grassland or deciduous woodland where someone wishes to create a garden), then this is also bound to change the biology of the soil profile and affect the existing soil life. It may even be detrimental for a while to some of the soil life, whilst the local soil ecosystem finds a balance. Maybe not for a long period of time, but as it will introduce changes to the soil biology, there could be some casualties, or ‘collateral damage’. Can you quote any published papers that have specifically tested for this kind of event? You say that much has been published about using woodchips as mulches, so I’d be interested to know if this has also been studied.
Then I was wondering about supply and demand. Let’s say that most gardeners (i.e. millions and millions of us) decided to follow your advice, i.e. to lay woodchip to the level you suggest (4″-12”) in our gardens. This would potentially be great for our individual garden soil biodiversity. But, looking at the wider picture, considering the climate and global biodiversity emergency we are collectively facing, would this be a sustainable practice, where widespread REGENERATIVE tree management isn’t currently given high priority? As responsible gardeners, we must consider where all this woodchip is going to come from and what this ultimately costs.
With this in mind, and please correct me if you have evidence to the contrary:
To support your suggestion that many more gardeners follow your advice, I believe we would need many more local, carefully managed and coppiced woodlands to be created, which takes both land, time (to grow the trees), money and political will. Failing that, then the woodchip would have to be supplied from elsewhere. By importing woodchip from further afield I’d be concerned about the ‘embodied energy’ used in its supply. Fossil fuels would be used to transport the relatively heavy woodchip vast distances. Do I know where it has come from and how far? If local arborists could not meet local demand then it might need to be sourced from woodlands of which I have no knowledge, which may not be managed sustainably, or perhaps the woodchip is derived from ancient woodlands being cut down (which is happening here in the UK on a dreadful scale, killing so much precious and irreplaceable diversity).
Alongside this, If there was a substantial rising demand for woodchip we would find that it would no longer be a free resource, especially if arborists realised they could make a good living out of it. If local woodchip wasn’t available at a reasonable price, then I’d have to look at using other sources of locally available material. I think this is why cardboard (usually a waste product from deliveries) is used instead by many gardeners (especially those of us who don’t have access to a plentiful supply of free woodchip). As a careful gardener I always check that the cardboard isn’t white, doesn’t have a plastic coating, the plastic tape and staples are removed and it is of the variety that is biodegradable (as per a comment made previously by one of your readers, who manufactures cardboard.)
So, as an eco-conscious gardener, I’d prefer to use a wider, diverse range of more locally sourced materials (e.g. community and home-made compost, well-rotted local farmyard manure, local woodchip, waste cardboard), In anecdotal trials, which you seem to frown upon, this mixed approach has shown that over many years both plants and soil life thrive. There is the initial ‘smothering’ of the soil by using cardboard, which could be detrimental to the soil life directly below for a limited period of time, as your experiment illustrated. This is for a relatively short period (likely to be a few months) whilst the cardboard breaks down. The cardboard is generally only used in the first season to suppress weeds, thereafter (at least in the UK) the ‘no dig’ method uses compost (not cardboard) as the main mulch each year, which is rich in life and biodiversity. We use woodchip too, as it’s also a valuable mulch. But, I have not seen any evidence put forward that cardboard, used in this way, kills the soil life IN THE LONG TERM. If you know otherwise, please let me know.
As you said to Amy on, September 12, 2020 in relation to smothering the soil, as cardboard apparently does: ‘Your soil system will recover from the tarp. Soil life moves in horizontally from unaffected areas.’ Of course, this is not ideal, that I would be potentially killing life under the cardboard for a period of time, but as I hope I’ve explained, I’m trying to find a balance that does least harm to our world as a whole. I am trying to consider the bigger ecological picture, rather than the experimental scientific minutiae. Sourcing woodchip for my garden veg plot (cut and chipped by machinery and transported by lorries powered by fossil fuels) from many miles away, may actually do much more harm, than if I covered it with damp cardboard and compost for one season.
I’m sorry, but this is just way too long for me to respond to. If you have one or two specific questions to discuss, that’s fine. But my time is divided among many online sites and emails. If you have published evidence that disagrees with the research to date on mulches of any stripe, feel free to cite it. That will give me something tangible to address.
Summercloud, with the friendliest of intentions I would recommend that you re”figure” your assumption on cardboard that isn’t rated for direct food contact. If it’s not in direct food contact then it is regulated in an entirely different fashion than material that has direct food contact. Thus it can contain a wide range of undesirable chemicals – used during its manufacture, carried over from recycled content, obtained during its storage, and transferred during its use. What chemicals was that pallet wood treated with? What was on that pallet when it was previously used? There is practically no way to know and thus a possibility additional undesirable chemicals may have leached into the cardboard you mentioned.
Humanity has a very long history of “figuring” things are OK, until it becomes painfully obvious that they aren’t. We’ve knowingly and willingly introduced so many chemicals into our food and water supply, some at levels we thought were safe – some we didn’t even consider unsafe. All too often we discover the errors of those assumptions and the cumulative effect of some is proving tragic.
I’ll look into chip drop. It isn’t an option openly promoted in my area. The local landfill offers free compost, but it’s created from the yard waste they collect and I have concerns that it likely includes a wide range of pesticides and herbicides that people use on their lawns.
I love your science-backed blog! Thank you! This info is very helpful.
Linda, I have been told by garden “experts” that your principles, as explained here do not apply to gardening in the tropics. Although I don’t agree, I wonder what your response is?
Hi Gordon –
If they are gardening principles (i.e., benefits and drawbacks of various mulches, importance of water and gas movement in and out of the soil, etc.), they apply anywhere. If they are discussions about specific plants (i.e., temperate vs. tropical), then there could be regional differences. I am guessing since it’s this thread it is the former and not the latter. When your “experts” provide some peer-reviewed, published research to discuss, I can certainly do that. However, the disagreement appears solely to be between published science and anecdotal experience. This blog is science-based, so that’s what we focus on.
I have enjoyed reading through the article and all of the responses. I have used cardboard in the past and it seems to have worked fine, but I’ll avoid it in the future. I know that wood chips are preferred because they’re 3 dimensional. Here’s my question: I have a ton of pine trees and plenty of pine cones of course. I have shredded some of them in the past and used them for mulch. Is that a good idea? Seems very close to wood chips. Thanks for your response.
Pine cones are like bark in that they don’t absorb water. Wood chips absorb and release water like a sponge. So a few pine cones in a wood chip mix are fine, but they are not a replacement for wood chips.
Hi! I was thinking about doing this for a while to start a garden but I dont exactly have cardboard. After reading your article I understand why cardboard isn’t the best option. Would it be possible to use those paper yard waste bags or is that basically the same thing? Thank you!
Hi Chandanie – all sheet mulches are going to restrict water and air movement. Our research was conclusive that coarse chunky mulches (i.e. arborist wood chips) are superior to all sheet mulches we tested in this regard.
In your recent post, you stated, “all sheet mulches are going to restrict water and air movement. Our research was conclusive that coarse chunky mulches (i.e. arborist wood chips) are superior to all sheet mulches we tested in this regard.” Could you please share links to your research results. We’d like to be able to have these to back up this point of view in conversations with gardeners and when doing presentations. Thanks!
I am not allowed to post a link to this article due to publisher restrictions. You can email me for a pdf file.
Hello. Did the study also compare weed control performance?
Weed control comparisons have been done with mulches in other studies. By far the most efficient are deep layers of coarse materal, which allows water and air movement but restricts light.
This has been blowing my mind & changing my paradigm. I’ve been saving every plain brown corrugated box & laying it out over the section of lawn that I want to transform & covering with my own homemade ramial woodchip & some straw to fill in gaps. Not to kill weeds ~ I actually want to grow some weeds that are a part of my diet ~ but to get rid of the grass & instead create an herb & nutritious weed garden.
I was psyched for your delawning process of putting on 8″ of chip, waiting, & then pulling aside to sow seed. But then I read here that I’d want to wear a mask when working around the chip pile because of fungal spores. I’ve already started planting in one of the areas juxtaposed to where I want to chip. And the other place is right alongside our house, so I would be going out there all the time. Besides the unpleasantness of masking up while doing garden work (seems very much to contrast with the idea of getting fresh air & sunshine on your face), I can’t personally tolerate a mask for more than a moment. I am crushed to hear this, because I fear this solution may not be viable for me.
Is there any more information that you know of on how much of a concern this is? When you chipped your lawn, did you avoid hanging out around it or wear a mask all the time or what? This is the biggest obstacle of this system to me. Thank you for sharing your knowledge.
To be honest, I never wear a mask. In the 20+ years I’ve worked with wood chips (both and home and with university classes) I have never had a problem. Fresh chips are not a problem, anyway. Chips that have been sitting a while can develop fungal mats on top, primarily from “dog vomit fungus.” When disturbed, these harmless fungi release a lot of spores. I wouldn’t breathe these in just like I wouldn’t breathe in dust.
If you have allergies, or are immunocompromised, I would wear a mask when you can see clouds of spores when the pile is disturbed.
Fantastic. I am excited & inspired! Now to find a source of copious chip. And to figure out what to do about the area I have already sheet mulched, some of which I have already planted into. I really appreciate your responding to my inquiry & am sooo glad I ran into this article before doing anymore sheet mulching!
Hi Linda! Long time no see. Hope you are doing well. I have a question about the impact of using landscape fabric to suppress weeds on the vitality of soil in adjacent areas. Some gardeners in my community-ish garden have taken on the practice of laying landscape fabric (or some very thick brick color tarp) on paths between raised beds to suppress weeds, on the recommendation of our gardening teacher (a master gardener). The plots are 5 feet wide and the paths are about 5 feet wide too. Plots have wooden frames made of planks about 6 inch wide; that means 6 inch below the walking surface there is no divider between soil in the plots and soil underneath the paths. I understand this practice would be bad for the soil right underneath the fabric mulch. But what does/might research say about the impact on soil in the plot?
The only problem I would see with this situation is that the anaerobic conditions created underneath these barriers will favor the proliferation of microbes that do well under low oxygen conditions. Such microbes include pathogens that are found in poorly drained, low oxygen soil. Whether or not these microbes would affect anything in the raised beds is unknown but probably not likely given the wooden frames surrounding the beds.
Thanks for all this great work. It is amazing how much our knowledge has changed in the last 20 years. I love reading gardening memoirs, and boy, there was a lot of crazy technique that was supposedly ‘environmentally friendly’ even just 20 years ago! Anyway, a couple years ago, I had a natural experiment play out where part of my garden had 6-10 inches of chips and, right next to that part, another section had one layer of cardboard plus the chips. No difference in weed suppression, just like your research has found!
One thing that I continue to use cardboard for, though, is suppression and IDENTIFICATION of poison ivy in places where I actively garden (we have a house in the woods). My practice is (a) mostly ignore poison ivy, as it was here first and has an ecological role to play. When I cannot ignore it, (b) I use a really high concentration of RoundUp; let photosynthesis do its thing a few days; and then cover the plant with cardboard and wood chips (if the vine is long, I wind it up into a clump to minimize the size of the cardboard). Then I can walk around freely and let others do the same. I have a 15-18 inch pile of woodchips that the poison ivy continues to grow through (no matter how many times I spray it), but this process seems to work. It makes no sense to me that it works better than 18 inches of woodchips, but maybe it doesn’t and it is just feels like it does because the solution is more immediate since I can layer it up and not worry anymore. The bonus is that, when I dig and find cardboard, I know the dead vines are there and I should just be careful (the oil on the vines lasts at least 18 months after the vine dies).
If you have a non-cardboard alternative for this process, I would love to know what it is. I just don’t want to don a hazmat suit to pull vines out, bag them in plastic, and send them to a landfill! The cardboard seems a little gentler on the earth, and so much less stressful.
Thanks for your great work! Love this blog.
If you can weed whack or mow the poison ivy safely, I would do that first. Then spray the cut ends. Then bury in wood chips. You don’t need the cardboard. You need to make sure there is not a population on adjacent land that is sending roots into your property. If so, it will be very difficult to control.
The cardboard serves to reduce oxygen and water to the vegetative and to the microbes that will be breaking down the leaves and the toxin. Wood chips will promote this activity.
Thanks! I will try that!
Your argument against cardboard mulching is based on the scientific method as you deftly explain so there’s no arguing with that. But research seems to consider only the initial steps of cardboard mulching. You cite evidence that cardboard inhibits moisture and gas movement between the air and the soil and affects soil quality and normal earthworm activity. In my experience, having used a very large number of disassembled boxes to eliminate the grass in my 25’ x 70’ yard, the cardboard breaks down in about 4 months. There is no trace of it whatsoever after one year. May I ask whether its negative effects are permanent or whether soil conditions improve once the cardboard has transformed from a one-dimensional smooth surface from which it cannot recover to the preferable “coarse, chunky, three-dimensional mulch” that are preferred? Once the cardboard is no longer cardboard but has completely disintegrated, does it become integrated into the soil? At this point, has the soil gone back to the CO2 diffusion coefficient levels you show in the diagram? I hope that soil scientists have published research that measure the effects of cardboard mulching at this end stage.
Good questions that would require more research to answer. Unfortunately there are few funding resources that really care about the fate of mulch materials – you could find relevant information in composting literature, however, that could tell you how long it takes corrugated cardboard to disintegrate. Of course, that rate will vary depending on water, oxygen, and temperature – the higher these are, the faster the rate.
Thank you so much for providing this fascinating content. I wonder if you could advise me with a small tree planting i’m planning…300 trees…on an island in the north of Scotland. We have no access to wood chip, but we can get seaweed, what are your thoughts on using this instead please?
Hi – Sorry for the late reply. Blog comments pile up quickly!
The best mulch is always a coarse, three-dimensional material. You could try an inorganic matrix, like pebbles or larger stone, and supplement with your seaweed for organic material, but it will not provide the matris that mycorrhizae require in mulch material. But it’s better than a sheet mulch, or leaving the soil bare.
Hello neighbor! This thread has been so informative. I am a new homeowner and “land” (1/3 acre) owner in Clark County WA, battling super invasive buttercups and blackberries and you-name-it over the last 6 months of being here. I put in cardboard mulch on a small section of it as an experiment a month ago to see if it helps prepare the soil for planting next spring. I will be getting in an order of plugs and bare root native species next February, and I need to do SOMETHING for the ground to be ready to take these precious transplants.
I will definitely call an arborist who I know will be happy to bring us wood chips. I sure wish I knew about this sooner so we didn’t have to work out in the freezing rain to get it all put down!
My question: Is there a better or worse time of year to put down wood chip mulch? If I put them down now, will there be a larger risk of weed seeds germinating in the spring especially during the aggressive growth cycle of some of these weeds? And in the wet cold PNW winter, do you anticipate I’ll be able to plant my native plant starts by mid February?
If you put down chips NOW after clearing an area of existing weeds, and you put down 8-12″ of chips, there will be no weed seed germination between now and when you want to plant.
The best time to put down mulch, in general, is whenever you can get a load of chips.
Hi, love your blog. Thanks for sharing your knowledge. Like many, I have a hard time giving up methods I believe to be effective simply because they have not been tested by others. Another way of describing anecdotal evidence is “case studies” or “case histories”. ;^)
Regarding cardboard mulch. I agree it reduces air impermeability but so do natural mulches, like deciduous leaves, which were not mentioned. I think the real question is, “to what end”? And, “At what time are we measuring the relative cost versus benefit?”
There will be a period of time when cardboard is impermeable. That could be a benefit if the goal is to smother plants trying to grow underneath.
However, eventually, under the Oregon winter rains, it loses its structure and its impermeability. Then what? Is the soil helped or harmed six months after the cardboard has broken down?
What has a person achieved by using cardboard that could not have been achieved with leaf or other organic mulch vs barrier fabrics?
From my point of view, it’s just a labor saving device. It helps keep aggressive grasses down for a year, during which time I can start bunchgrasses and flowers from seed to go into that spot. When I am ready to plant, I don’t need to remove it. As your chart points out, it’s less harmful than other barrier fabrics.
Hi – Sorry for the late reply. Blog comments pile up quickly!
Our research is pretty conclusive: the bigger the air space in a mulch, the better the gas transfer. Wood chips are superior. Sheet mulches are not – and anything that forms a sheet can induce low-oxygen conditions. In fact, leaf layers in natural forest systems do exactly that.
“Smother” is not a good verb to use when you consider a soil ecosystem. What we need to do is reduce light – not oxygen. Consider a single sheet of wet newspaper over your face. Could you breathe well? Would you want to wait until that sheet naturally broke down?
There is no published research to demonstrate that cardboard – or any sheet mulch – is superior for weed suppression and soil health compared to arborist wood chips. You can believe what you like, but unless it’s grounded in science it’s not a discussion point in this forum.