Cardboard does not belong on your soil. Period.

In the quarter century that I’ve been researching, publishing, and educating on the topic of landscape mulches, one thing has become clear: cardboard should never be used as a mulch. This viewpoint has been of great interest to gardeners; in fact, my earlier post has been the most frequently viewed post since it was published in 2015. I occasionally appended new information to the original post as needed, but the topic deserves an update.

Landscape mulching with cardboard is wildly popular but has no published research to support it. Photo courtesy of Chris Martin on Flickr.

Rather than rehashing what’s been written earlier (which can be found here, here, here, here, and here in addition to the link above), I’m providing information in a Q&A format that might be helpful:

Q: Is there research on using cardboard mulch in home landscapes?

A: Not much. To date, the only peer-reviewed research relevant to landscape soil conditions is our own work published in 2019. The abstract explains the importance of the results to landscape soil health as stated in the abstract (below):

“The orders of magnitude differences in diffusion coefficients among the mulch materials, however, could negatively impact a diverse soil environment such as those found in biologically rich landscapes with higher oxygen demands. Among the mulches tested, wood chips are a preferred method of mulching in terms of providing best gas permeability, particularly in landscape conditions.”

This chart (derived from our 2019 study results) demonstrates the increased impairment of gas movement by different mulch types.

Q: Cardboard has been used as a mulch in agricultural production. Why doesn’t that research support using it in landscapes?

Sheet mulches, including black plastic, is frequently used in agricultural production where weed control and maximzing plant yield are the most important concerns. Photo courtesy of Wyoming BLM.

A: The goal in agricultural production is to maximize yield of an annual crop. In contrast, the goal in caring for a permanent landscape is to maintain a healthy soil ecosystem that will support plant life long term. The table below explains these differences in more detail.

Comparative criteria for intensive agricultural production, home vegetable gardens, and managed landscapes

Q: Okay, I understand that science doesn’t support using cardboard as a landscape mulch, but what about my vegetable garden? Isn’t the research on agricultural crops relevant there?

A: The research on agricultural production mulches is more relevant if maximizing yield is your most important goal. But your goals may include maintaining a healthy soil ecosystem, reducing the use of pesticides and fertilizers, and other criteria. Are you concerned about the established negative impacts that cardboard and other sheet mulches have on soil life? If so, then sheet mulches are not a good choice compared to chunky, three-dimensional mulches.

Q: I like reusing cardboard packaging as part of organic weed control. Isn’t that a good enough reason to use cardboard?

A: In addition to interfering with water and gas movement into the soil environment, corrugated cardboard has chemical contaminants that you really don’t want in your soil or even your compost pile. Corrugated cardboard contains environmental contaminants including dioxin and PFAs or “forever chemicals.” No gardener should want to introduce more of these widespread contaminants into their landscape or garden soils.

Recent peer-reviewed publication looking at hazardous chemicals contained in cardboard and other recycled materials.
Table from Fernandes et al. (2023). Compare the levels of contaminants between shredded cardboard and untreated wood shavings.

As I’ve been recommending for nearly a quarter century now, the very best mulch to use for treed landscapes is arborist wood chips. There is robust, peer-reviewed science establishing the benefits of arborist chip mulches in controlling weeds, enhancing growth and establishment of landscape plants, and maintaining a functional soil ecosystem. In contrast, sheet mulches such as plastic, weed fabric, and cardboard have demonstrated negative impacts on the long-term health of landscape soils. Any resource that says otherwise is not paying attention to the research-based facts.

Arborist wood chips protect exposed soil and suppress weeds while supporting desired landscape plants

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 a 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 - www.facebook.com/TheGardenProfessors "The Garden Professors" Facebook group - www.facebook.com/groups/GardenProfessors Books: http://www.sustainablelandscapesandgardens.com

42 thoughts on “Cardboard does not belong on your soil. Period.”

  1. Two questions…Has there been research, or do you know, how long after someone has sheet mulched an area that soil health begins to improve markedly? For instance, if it were covered with a tarp (as a neighbor has left in place for over a year!) How long would it take, if covered in arborist wood chips, for the soil to begin to host its macro and micro life?

    Also, does your research on cardboard mulching as it affects soil health also apply to the rolls of paper mulch used for annual vegetable or flower gardening/production? Because of a lack of enough wood chips to kill off my grass in a large area, I am thinking of using that paper and then covering it with a lesser amount of chips, keeping the area mulched with chips over the long term as I plant into it. Thank you.

    1. As I stated in the blog, there is no research on sheet mulches used on landscape soils. There is substantial work on the benefits of arborist wood chips to soil health. All you need to do is search the blog.

      There is no sheet mulch that is appropriate for ladnscape soils. All sheet mulches will restrict gas movement more than a coarse, chunky mulch. Skip the paper – who knows what chemicals are used in processing? – and just use the chips.

      1. I see an important definition of cardboard in this discussion. Are you referring to plasterboard which is chemical laden with glues or ordinary cardboard like used to make shoe boxes? What about how it is used? Cardboard breaks down quickly especially when combined with worm and their castings. It certainly is correct to say there is inadequate research. Defining terms are lacking here.

      2. Check out chipdrop.com for Woodchips. That is a way to get enough Woodchips to kill off your grass area. It is $20 for up to a 15yrd. Load.

  2. The contamination is really quite disappointing. Do you know why all those persistent chemicals end up in it? I’ve always ignorantly pictured the manufacturing process as simple because of how raw the product seems.

    1. It’s the processing of wood pulp to make it resistant to water and/or other environmental degradation. Much of this information is not readily available. While I did a post doc in forest product chemistry, I was not involved with the pulping/processing side of things. I don’t know all the chemicals involved in making paper products.

    2. Greg, Old corrugated containers, also known as OCC, are used cardboard materials. Old corrugated containers include everything from cardboard shipping boxes to pizza boxes. Cardboard is the most recycled packaging material in the U.S.
      More than 93% of OCC was recycled in 2022 – and increase from 2021, and overall, a three-year average of 91.3%.https://www.afandpa.org/news/2023/how-does-afpa-calculate-paper-and-cardboard-recycling-rates#:~:text=The%20rate%20has%20followed%20a,or%20exceeded%2082%25%20since%202009.

  3. I’m down in Miami, where our soil is poor for veggie gardening, so EVERYBODY uses a layer of cardboard at the bottom of their raised beds before layering wood/compost and then soil. Even at our extension office they recommend cardboard or newspaper to line the bottom of a bed. If it’s under the growing surface, suppresses weeds, and helps with water retention, how bad is it? It breaks down within a year in our climate.

  4. Dioxins are formed when paper is bleached with chlorine. Since most cardboard is brown and unbleached there are no significant amount of dioxins in it. I got a degree in paper engineering from WMU

    1. The table I shared in this post reports levels of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs, dioxins) and polybrominated dioxins (PBDD/Fs) in bedding materials.
      I suggest you read the actual paper.

    1. If you search this blog for “arborist wood chips” or “arborist chips” you’ll find all the posts we’ve done about them in the past. They are ground up wood from branch and tree removal by arborists.

    2. Long time vegetable gardener (60 yrs) and also worked in the paper / cardboard industry for 10 years. Years ago when I first started seeing gardeners use cardboard in their gardens I cringed. Just seeing and smelling all of the chemicals that go into the production was more than enough reason to avoid it. Thanks for spreading the word.

  5. I am curious if the chemicals listed are from what I call “plastic cardboard” – rather than the typical plain brown cardboard? This is the stuff that has a shiny layer, often with colorful pictures.

      1. Do you know if the authors of the paper were using plain brown corrugated cardboard only or were they including this “plastic cardboard”?

        1. I don’t know what plastic cardboard is. If you read the article itself it tells you they used commercially available cardboard bedding, which is recycled, shredded, corrugated cardboard. You can easily find it on Google along with images.

      1. I want to add another pollinator garden in my yard. The last one I started by breaking up the sod where the garden was planned. I just loosened it up. Then I placed a layer of brown grocery bags over it and topped it with about 10″ of quality soil, and four inches of wood chip mulch.

        Is this safe, or do grocery bags have the same problems as cardboard? It worked fabulously, and the original garden is five years old with zero grass in the garden.

        1. Please read all this blog on sheet mulches. You are smothering the soil (and the life it contains) with this proccess. Imagine lying on the ground and trying to breathe through the bags and soil covering your face. Just use the wood chips.

  6. The table from Fernandes et al. (2023) comparing the levels of contaminants between shredded cardboard and untreated wood shavings is indeed distressing. Level of PFAS in shredded cardboard is more than twice the level in untreated wood chips, and the difference is many times greater for all the other toxins shown.

    What is most striking to me, however, is that all these toxins are found in untreated wood chips. This leads me to conclude that while we may take steps to minimize exposure to such poisons, they are inescapable. They are everywhere and are persistent.

    Recycling corrugated and other cardboards seems the best way to keep the toxins they contain out of our landscapes. Better yet would be to require producers to not use these chemicals in the first place.

  7. Many vemiculture casting producers use cardboard. Does your research include their products as well? If so, what referces would you cite?

    1. I did a quick literature search and found no research on this question. However, vermicomposting uses cardboard and there’s no reason to think that the PFAs and other toxic chemicals taken up would not be bioaccumulated in earthworms just as they are in chickens.

      1. This was stated in an article on bioaccumulation of PFAs in earthworms. “The results indicate that PFAA bioaccumulation into earthworms depends on soil concentrations, soil characteristics, analyte, and duration of exposure, and that accumulation into earthworms may be a potential route of entry of PFAAs into terrestrial foodwebs.” Reference at https://pubs.acs.org/doi/abs/10.1021/es504152d

      2. I’ve been a certified composter in Texas for over 20 years and vermicompost at home. I do NOT use cardboard, I use shredded paper with soy inks. I hav red wrigglers and brown nose compost worms.

  8. Great data, it makes more sense to me now. Thanks for putting science to it.
    So what do I do now if I’ve used cardboard already and put mulch on top of it? It’s been several months, do I dig it up? If it’s already decomposed, is my soil ruined? Will it recover if I just use wood chips from now on?

    1. I would just leave it there and only use wood chips from now on. Soil is not going to recover quickly from forever chemicals but they will disperse through the soil systems for a lower overall concentration. Not great, but now you know.

  9. I get not using cardboard as a base to mulch, but what about in situations of killing off grass and heavy weed pressure. Much of what I’ve seen puts cardboard over remanent grasses then covered in arborist mulch to kill the grass and weeds more so than feed the soil.

    1. This topic has been discussed many times on the blog. A layer of 12″ of arborist wood chips on top of mown grass or weeds will restrict light but not water and oxygen. That’s what will kill weeds without negatively affecting the soil underneath.

      1. Is there a soil-friendly way remove grass that requires less mulch, short of renting a sod cutter? I’m sure you’re right about the cardboard being a bad option, but you say 12″ of wood chips like it’s something you can just pop down to the store and get.

        1. You can find arborist wood chips either from a tree service or from chipdrop.com. That is the most effective way of reducing light (and weeds) without impairing soil and root health. There is no similar product you can buy from a store.

  10. Does this apply to newspaper as well?
    What about those soft, brown, cardboard-ish containers that much fast food comes in? I’ve been ripping those up & putting them in my compost….

    1. PFAS are frequently found in “grease-proofed” cardboard and papers used for restaurant take out containers (and lots of food packaging in general). They don’t even need to feel waxy, like in the case of paper fries and cookie packages (think kids’ happy meal fries). Many restaurant chains, once aware, are taking steps to phase them out, but you can bet that they are in the cardboard you’ve put in your compost.

      Here’s an article by Consumer Report that’s on point: https://www.consumerreports.org/health/food-contaminants/dangerous-pfas-chemicals-are-in-your-food-packaging-a3786252074/

  11. Wow! I have a horrible invasive Bermuda grass problem everywhere a drop of water hits. There is no way to remove it. Local experts say only Roundup will kill it, but that is not an option for me. I thought I was being a better steward with the cardboard. Disappointing. I don’t think any amount of mulch alone will stop this stuff.

    1. Yeah, anecdotally, in the South the weeds come regardless of mulch or even cardboard. Even 4-6″ of mulch in beds is insufficient in my area. And I don’t think 12″ of mulch is possible cost wise, plus it will run off in first first southern downpour. Weeding frequently is the only things that works. Don’t do chemicals, chemicals just destroys so much, is only temporarily effective, and the more hardy nasty weeds come back quickly and first.

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