The cardboard controversy

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.

Cardboard mulch under wood chips
Cardboard mulch under wood chips

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.

Corrugated boxes are built to be tough.
Corrugated boxes are built tough

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.

Photo credit vizpix at Flickr
Photo credit: vizpix at Flickr

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.

Photo credit: Kurt B. on Flicker
Photo credit: Kurt B. on Flickr

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 logrithmic scale.
Now, until cardboard proponents publish evidence to the contrary, it’s pretty obvious that cardboard mulch interferes with gas diffusion.

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Linda Chalker-Scott

Dr. Linda Chalker-Scott has a Ph.D. in Horticulture from Oregon State University and is an ISA certified arborist and an ASCA consulting arborist. She is WSU’s Extension Urban Horticulturist and an Associate Professor in the Department of Horticulture, and holds two affiliate associate professor positions at University of Washington. She conducts research in applied plant and soil sciences, publishing the results in scientific articles and university Extension fact sheets. Linda also is the award-winning author of five books: the horticultural myth-busting The Informed Gardener (2008) and The Informed Gardener Blooms Again (2010) from the University of Washington Press and Sustainable Landscapes and Gardens: Good Science – Practical Application (2009) from GFG Publishing, Inc., and How Plants Work: The Science Behind the Amazing Things Plants Do from Timber Press (2015). Her latest effort is an update of Art Kruckeberg’s Gardening with Native Plants of the Pacific Northwest from UW Press (2019). In 2018 Linda was featured in a video series – The Science of Gardening – produced by The Great Courses. She also is one of the Garden Professors – a group of academic colleagues who educate and entertain through their blog and Facebook pages. Linda’s contribution to gardeners was recognized in 2017 by the Association for Garden Communicators as the first recipient of their Cynthia Westcott Scientific Writing Award. "The Garden Professors" Facebook page - "The Garden Professors" Facebook group - Books:

155 thoughts on “The cardboard controversy”

      1. 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.

        1. 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.

          1. 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 .

            1. 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.

              1. 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.

                1. 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 irreelevant. 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.

                  1. 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.

                    1. 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.

          2. 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?

            1. 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.

      2. 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?

          1. Hello Linda,

            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.

            Thank you!

            1. I’ve done a post on how to start this process – you can find it here:
              There is a lot of research correlating mulch depth with weed control – I’ve also written about this.
              If you are using wood chips (NOT bark), you will have good success with a deep (8-12 inches) layer over scalped weeds.

  1. 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?

    1. 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.

      1. 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.

        1. “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.

          1. 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.

                  1. 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?

                    1. 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.

                1. 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.

                  1. 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.

                    1. 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.

          2. 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.

            1. 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.

  2. 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?

    1. 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.

      1. 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

    1. 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.)

          1. 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.

  3. 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…

    1. 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.

      1. 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…

        1. 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.

          1. 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…

      2. 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…

        1. 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.

          1. 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)

            1. 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.

  4. 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.

    1. 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.

  5. 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.

  6. 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.

    1. 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.

  7. 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.

  8. 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.

  9. 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?

  10. 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.

    1. 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.

      1. 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.

      2. 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…

        1. 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.

          1. 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.

            1. 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.

              1. Linda,
                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!

                1. 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!

      3. 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.

        1. 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.

  11. 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.

    1. 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.

  12. 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.

  13. 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.

    1. 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.)

  14. 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.

    1. 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.

  15. 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!

    1. 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.

  16. 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.

    1. 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.

  17. 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.

    1. 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.

  18. 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.

  19. 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.

    1. 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.

  20. 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!

    1. 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.

      1. 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.

  21. That is fantastic! Thank you for letting us know. It will be good to have another reliable piece of the puzzle in hand.

  22. 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!

    1. 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.

  23. 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.

  24. 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!

    1. 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.

  25. 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?

    1. 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:

  26. 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.
    Thank you

    1. 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.

      1. Hi Linda,
        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.

        1. 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.

          1. 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.

  27. 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. 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?

    1. 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.
      Good luck!

  28. 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.

    1. 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.

      1. 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.

        1. 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.

  29. 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.

    1. 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.

  30. 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!

    1. 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.

  31. 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.

  32. 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?

  33. 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?

  34. 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.

  35. 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.

    Thank you,

    1. 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.

      Good luck!

  36. 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:

    Thank you very much!

    1. 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.

  37. 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.

    1. 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. (I updated the link to the public one).

      1. …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!!

        Best regards,

    2. 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.

  38. 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?

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