Rubber mulch – the discussion continues

Almost a year ago I posted my complaints about rubber mulch (you can find the posting here).  This week I was contacted by Jesse, a purveyor of rubber mulches.  We’ve had a very civil discussion about the topic, and he asked me to review his fact sheet.

Which leads me to today’s assignment. I have no personal experience with rubber mulch, so I’d like to hear from you about your experiences with this product.  Specifically:

1) Have you seen fungi growing on rubber mulch?

2) Have you had issues with the heat captured by the product – either to your feet or to your plants?

3) Does the mulch continue to smell, especially when hot?

4) How quickly do you notice degradation of the product?

Obviously this is anecdotal, not scientific, evidence.  But the scientific literature regarding rubber mulch is thin, and anecdotal evidence can often indicate directions that science should explore.  Perhaps this can be the beginning of such a study.

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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 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 - "The Garden Professors" Facebook group - Books:

20 thoughts on “Rubber mulch – the discussion continues”

  1. On for 5 years (4 before we bought the place) No fungus. No smell. No visible degradation. We replaced a small area of wood chips with more, and the smell was rapidly gone, over one summer. Plants are doing fine, no apparent toxicity.

  2. Linda, I have no experience with rubber mulch (I’m with Deirdre on this one, though), but do appreciate your point that anecdote can point science in fruitful directions. I’ve been thinking about this issue quite a bit recently as it relates to bare-root tree transplanting with air tools, which several MA arborists are working on these days. Using air tools to move specimens is a technique really in its infancy, and what these guys are learning — at least about digging-time hazards — sometimes runs contrary to convention. The work is empirical in the loosest sense — learning is happening with each tree, but without scientific controls. It’d be great if the academic crowd could start studying this work and the best way(s) to do it, and it may well happen eventually, but right now, the way that arborists are learning is by doing and by passing along what they learn, tree by tree. Anecdote certainly does have its place in the spectrum of inquiry, even if it’s not the whole story.

  3. There is a recent article by Wik et al. (2009) that document some toxic effects from tire leachate in aquatic organisms. “Toxicity assessment of sequential leachates of tire powder using a battery of toxicity tests and toxicity identification evaluations” CHEMOSPHERE 77: 922-927. How these data relate to ground tires as mulch, I’m not sure. My objections to rubber mulch are mainly aesthetic – the early stuff I saw didn’t look very natural. I suppose they are making inroads in this area. Nevertheless, I stand by recommendation to support the local economy and buy local. In most locations around the country, ground bark mulch or ground wood mulch is available that is produced as by-product of local forest products operations. You’ll need to buy it in bulk rather than conveniently bagged, but you’ll keep your dollars local and you won’t support the destruction on sensitive cypress stands in the Southeast.

  4. There was some research done here at OSU about the flammability of these rubber mulches and guess what? They are actually MORE flammable than the regular ‘wood’ mulches.

  5. I don’t think there’s any doubt about many of the drawbacks of rubber mulches (flammability, leachate toxicity, etc.). I reviewed a lot of the science behind rubber mulches, crumb rubber, and other rubber products and am firmly convinced that they are not the most environ
    mentally friendly option. (Like Deirdre, I want my mulch to degrade and become part of the soil ecosystem.) But I’ve criticized other problems – like the smell and heat generation – from my own perceptions and those of others. There doesn’t seem to be any research on these drawbacks – and hence my query to you all.

  6. In Chicago, most of the playgrounds have rubber mats and rubber mulches instead of wood chips. The older parks and playlots still have wood, but all of the newer ones are rubber. The heat radiated by the mats made my 2-year old cry the other day when she took her shoes off to play in one of the water features. I decided to test it myself and removed my sandals. It was extremely uncomfortable, almost painful instantly-moreso than hot sand at the beach.

  7. Hello, I’m the “purveyor” in question (and I do appreciate the link, Dr. Chalker-Scott). There are many counter-intuitive things in life … apparently rubber mulch is one of them.

    I’m sorry, John K, that your daughter and you had negative experiences with mulch pads in Chicago. It may be that the pads were the problem…much smaller shreds of rubber are bonded into mats with polyurethane adhesives which may retard the unique circulation factor which keeps loose rubber mulches cool. (For the record, I don’t sell wear-pads just for this reason.) I live in Virginia which generates torrid heat in August. I tended my orange azaleas barefoot during those weeks… without pain. (And Hannes, with the beautiful temperate climate you enjoy in Austria, you are not well positioned to speculate on this matter…)

    Regarding fungus or bacteria consuming rubber mulch: I’ve not yet fielded such a complaint from any of my clients. Also, degradation of rubber mulch is measured in DECADES, not in months. And Diedre, if you want to improve your soil, why not use compost vs wood-based mulch (which often is of doubtful provenance: think treated wood PALLETS, shredded and dyed). Those poisons will really fix up your soil, your flowers, and your tomatoes.

    On the flammability issue: While I’m sure that the Garden (no matter what mulch) as well as the House would go up in flames if someone threw an accelerant on it, I can only quote from the USG study: “Crumb Rubber and Rubber mulch were tested using the method described in 16CFR 1500.44 of the Federal Hazardous Substances Act for Rigid and pliable solids (A material is considered “flammable” if it ignites and burns with a self-sustained flame at a rate greater than .01 inches per second along a major axis.) Summary: In each test reading, the Crumb Rubber and Rubber Mulch had a burn rate significantly less than that required for it to be considered “flammable”. Based on the testing results, the Crumb Rubber and Rubber Mulch is considered non-flammable according to the requirements of the Federal Hazardous Substances Act. The testing company also documented that “specimens were easily extinguished after the 60 second testing period”.

    I find it fundamentally dishonest to denigrate rubber mulch by mis-identifying it. Virtually all studies on this subject use the terms “tires,” “rubber”, or “crumb rubber” — carefully avoiding the correct term — RUBBER MULCH. Secret: Form can radically change Function. Consider first a giant block of steel, then an even larger ship. Which one floats? Scrapped rubber tires abandoned in swamps for years bear virtually no relationship to their derivative product, RUBBER MULCH. The shredding and freeze/hammering processes radically alter its behavior; whether water absorption or, more importantly, leachate charactistics.

    Finally, the irony is that many of my largest (if grudging) clients now are landscapers (traditional “enemies” of rubber mulch). Because their customers — tired of the continual mulch replacement contracts, dirt, and insect/fungal threats — are demanding it. While it’s clear that I have economic interest in these products, I researched them very thoroughly before coming into this business. Best regards to all, JESSE

  8. I bought over 40 tons of rubber mulch from a company about a year ago. They claim to have a 15 year color warranty and its already gone from red to black. The children come in from playtime and are covered with black residue that will not wash out of their cloths. The company claims that this has never happened before and I feel they are just brushing me off. Any opinion on this.


  9. Greg,if you’d like to send photos of the mulch (showing the color change) and even of your kids’ clothes, I’d be happy to do another update on this topic. We rely on readers to supply their experiences. Even though they are anecdotal, they supply useful information.

  10. Linda: I don’t know if you are still accepting reports of experience with rubber mulch but I’d like to offer my observations. I’m an elderly Master Gardener in the Niagara Region of southern Ontario. It’s a steamy long summer, 6a/b. Moved to this home 4 years ago. Thick layer of black rubber mulch had been applied to all beds probably not more than a year before by previous owner. I knew very well I would remove all of it as we began a major overhaul of the plantings. After 4 years there is little left but having lived with it, and knowing the concerns about leached zinc and chromium, I must confess, with a fair bit of embarrassment, that I liked it. As I said, we have hot, steamy summers (mid 30’s) and I spend a lot of time working in the garden. There has never been an odour, though if I put my face right down to it I suppose there would be some, but nothing noticeable as I worked in it. One would think it would be very hot but I never really noticed that. I have never walked on it barefoot but working on and around it was not a problem. My clothes and gloves do not seem to have been stained by it. Have never seen fungi on it.

    During the 4 years I’ve lived with it I have noticed no degradation in colour or deterioration in form. It was a bit of a love-hate relationship. I like black mulch and I like it even better when it doesn’t fade much, and this didn’t. It gave me the ‘look’ I wanted, all the while contaminating the soil, not adding to structure, etc. It did a reasonable job of weed suppression but the current 3″ black cedar mulch layer does a better job. Honestly, I can see why people fall for it. It’s a tidy look, texture is very nice. If you don’t know what it it and what it’s doing and not doing, it looks good. To date, over 3,000 lbs removed.

    1. The reports I’ve heard on smell tend to be in arid areas. Perhaps the humidity helps keep it in check?
      Good for you in removing it – if you ever plant to use the area for growing edibles please do have a soil test done first.

      1. Thank you for such a fast reply! We will not be using the beds for edibles and we have not yet had a soil test but I’ve been thinking I’d like the data. Do you think it would be wise to test the soil considering that some of that mulch has been on for 5 years? I want the soil tested simply for the knowledge but I suppose heavy metals testing may not be needed on all beds? I don’t know how long it would take for soil to become contaminated – I’m assuming it’s a process.

        1. I would definitely get a test, if for no other reason than personal knowledge. But I hope you will share it here!
          You can combine the samples from all of the mulched areas and then take a single sample from the consolidated samples.
          If you have other beds that did NOT have mulch, that would make for a nice comparison.

  11. I have a Type 1 anaphylactic latex allergy and I can’t touch it, inhale latex particles, be where it has been. It could cause death for me. Also my granddaughter’s preschool located in Shermansdale, Pa., 11 years ago covered their whole outside play area with black rubber mulch. My granddaughter who was 3 1/2 would be coated with black rubber soot. Her clothes, skin, sneakers and kids put their fingers in their mouths and a lot would have residue around their mouth. It was exposing her to dangerous chemicals and anyone like me with a latex allergy. I couldn’t go to that preschool and couldn’t be around her after having been there until she left permanently.

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