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.

Published by

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:

42 thoughts on “The cardboard controversy”

  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.

  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.)

  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.

  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.

  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.

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