Maddening mulch myths

A good example of bad mulching

Long-time followers of this blog know that I’ve been researching, writing, and educating on the topic of landscape mulches for over 20 years. So whenever an article comes out in a newspaper or online that directly refutes our current understanding of mulch science, on-line and real-life colleagues quickly call it to my attention. Many times I choose to ignore the article, but when it’s from a highly regarded source with wide readership I feel the need to step in. Before I discuss the problematic statements, I want to explain part of my process in determining whether an expert is really an expert.

Here are two questions I ask:

  1. Is an expert regarded as an expert in the area of interest by other academic experts?
  2. Is there published research provided that supports statements that don’t agree with the current body of knowledge?

If the answer to both questions is no, then the source cannot be considered reliable.

This free, downloadable, peer-reviewed resource can help you learn how to differentiate bewteen credible and not-so-credible information.

To the writer’s credit, she seeks out academic sources for her information. Her source has stellar credentials in researching and educating about compost, but has no publications on mulching or mulch materials (Question #1 = no).  And there are source quotes and author statements throughout the article that are not supported with evidence (Question #2 = no).

I’ve identified the misleading or erroneous statements and quotes below with my rebuttals. I have included linked references at the end that address these points in more detail. And we have dozens of posts on mulches in this blog’s archives.

Just type “mulch” in the search box and find all kinds of good stuff!

1. “In a forest…there is no big heap, just a layer of an inch or two or three, breaking down and returning to the system.”
Observations of relatively undisturbed forest floors reveal deep layers of woody debris, leaves and needles, and other materials falling from the canopy. Research has shown that a minimum of 3 inches of a coarse textured mulch are needed to restrict sunlight from reaching the soil and prevent weed seed germination. Any less than this will enhance, not prevent, weed growth. Deep layers of wood chips have been repeatedly shown to suppress weeds and enhance the health of desirable plants.

This is what an inch or two or three will do for you.

2. “The process releases humates…described as ‘black, gooey liquid’…”
Humates, defined as recalcitrant materials that resist further decomposition, don’t exist in natural landscapes. The only place you find humates are in the lab, where analysis of organic material with an alkaline reagent (pH = 12) produces humus as a byproduct. And on garden center shelves, where heavily marketed humic acids, fulvic acids, and humates are located.

3. “The only difference in mulches, as long as you use organic materials, is the rate at which they decompose”
This needs clarification. Rapidly decomposing mulches release high levels of nutrients in a short period of time; slowly decomposing materials release low levels of nutrients over longer periods of time. Compost falls into the first category, and readily available nutrients from any source can lead to nutrient toxicity in soils and imbalances in plants.

Interveinal chlorosis is often associated with excessive soil phosphorus.

4. “In formal beds…fine- to medium-textured material”
For best oxygen and water movement, mulches should be coarse and chunky. Sawdust and compost, for example, are too finely textured to allow for gas transfer and water movement, plus weeds easily establish on top of compost.

Compost used as a mulch is a weed magnet.

5. “If a bed needs compost, spread an inch before mulching”
This statement needs clarification. The only way you know whether compost is needed is to have the results of a soil test showing an overall low level of nutrients. Then a layer of compost could be added before chips are applied.

If your nutrients are off scale, don’t use compost!

6. “Save…the chunks fresh out of the arborist’s chipper for pathways…Or at least pile them up to mellow before you use them.”
You don’t need to compost your arborist chips. They provide a burst of nutrients during the first month, when leaves are rapidly decomposed. Using older chips is fine, of course, but why waste that early nutrient boost to your soils?

There’s nothing better than fresh arborist mulch straight out of the chipper.

7. “If supplemental fertilizer isn’t applied when your piling on coarse, fresh, carbon-rich wood chips…it can cause some drawdown in soil nitrogen.”
Fertilizer should NEVER be applied unless there is a demonstrated nutrient deficiency, and wood chip mulches do not cause a drawdown in soil nitrogen. This myth has been dispelled by years of research showing no change to soil nitrogen covered with wood chips.

If wood chips cause a nitrogen deficiency, then plants apparently haven’t gotten the message.

8. “Generally, mulch is applied in ornamental beds at a depth of one to three inches”
See point #1. This is not a science-based recommendation.

9. On volcano mulching: “In addition to promoting bark decay, it causes the tree’s roots to grow up into the mulch layer, rather than down into the soil…the tree may eventually die, and even topple.”
This classic correlation-elevated-to-causation is getting tiresome. There is NO published evidence, anywhere, that proper mulches (i.e., coarse arborist chips) are going to injure bark. They do not cause bark decay. Furthermore, tree roots grow where they have water, nutrients, and oxygen. This might be in the mulch layer. Growing deep into the soil is unlikely (not enough oxygen) unless the soil is excessively sandy or otherwise well drained. Any toppling of trees can be directly correlated with poor planting techniques that prevent roots from contacting and establishing in the site soil.

10. “Keep the mulch at least several inches away from tree and shrub trunks.”
Why? Does this happen in nature? No. Per point #9, a natural woody mulch is not going to hurt trunks.

The soil of this lush landscape is completely covered with a thick layer of arborist chips.

11. “And don’t invite rot by smothering the crowns of perennials”
A good arborist chip mulch is not going to “smother” anything. Perennials are quite capable of growing through several inches of woody mulch, which also protects the crowns from freezing temperatures.

Our perennial rhubarb thrives in its deep arborist chip mulch.

If we are going to encourage gardeners to use nature as a guide (see point #1), then points 4-11 are, well, pointless.

Literature

Chalker-Scott, L. 2007. Impact of Mulches on Landscape Plants and the Environment — A review. Journal of Environmental Horticulture 25(4) 239-249.

Chalker-Scott, L., and A. J. Downer. 2020. Soil Myth Busting for Extension Educators: Reviewing the Literature on Soil Nutrition. Journal of the NACAA 13(2).

Chalker-Scott, L., and A.J. Downer. 2018. Garden myth busting for Extension educators: reviewing the literature on landscape trees. Journal of the NACAA 11(2).

Lehmann, J., Kleber, M. The contentious nature of soil organic matter. Nature 528, 60–68 (2015). https://doi.org/10.1038/nature16069

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

30 thoughts on “Maddening mulch myths”

  1. L C-S, I love these articles – once again you are our arborist mulch guru! One question – I still cringe when I see volcano mulching………is it really okay? Also – since I have been trying to replicate your advice – I have experienced weed pulling to be much easier – but weed whacking is dicier!
    Appreciate your work, thanks again.

    1. Volcano mulching is not a good practice and unnecessary unless installers are trying to hide a poor planting job. But arborist chip mulches touching the bark of properly installed, healthy trees is not going to hurt them.

      1. I appreciate all your efforts (web, books, Great Courses lectures) to help us improve our gardens. I’m most worried about blackspot on foliage of rose bushes – should the wood chips be separated a few inches away from the stem, or does it matter? From your 2017 paper on mulching, referenced in this blog post, I thought it would be risky to keep it too close. “Keep mulch away from trunks of trees and shrubs. Piling mulch against the trunks of shrubs and trees creates a dark, moist, low oxygen environment to which above-ground tissues are not adapted. Fungal diseases require a moist environment to grow and reproduce; piling mulch on the trunk provides exactly the right conditions for fungi to enter the plant.” Thanks for your help.

        1. The science has changed a bit since that 2017 fact sheet (which is currently in revision). To date, there is no evidence that arborist chips can cause this kind of problem. Fine mulches, such as compost and sawdust, can cause these problems.

          My own mulching supports this. I use arborist wood chips around or on top of everything. I have no disease issues with woody plants or herbaceous perennials that grow through the mulch every spring.

          For your roses, the wood chip mulch will reduce pathgoen splash from the ground, where spores from your black spot are located.

  2. I’ve used arborist chips for years with wonderful results. Lately neighbors have objected to wood chip piles on my property as unsightly — it can take me several weeks to get the chips to every corner of the yard. I’ve noticed compressed bricks of coconut husk marketed as mulch and have wondered about using it to supplement the chips. Any thoughts about its effectiveness against weeds or impact on the soil?

    1. I haven’t seen any research on coir used as a mulch. It’s pretty fine when it’s broken up and hydrated – that could be a problem as it could interfere with water and air movement.

      1. There’s now coco chips sold by the warehouse club from Kirkland. I purchased one thinking it’s the finer textured stuff. They’re pretty similar to wood chips in texture. I assume they take longer to break down than wood chips, from what I’ve heard about palm fibers. I won’t buy any more of it because of the cost.

  3. One of the statements I thought you’d flag: Is that really true that urban trees are planted on top of the soil and mulch piled around them? I find that hard to believe!

    1. Oh, yes they are. Sometimes the soil is thin and if the roots are left in the clay ball (another no-no as you know from this blog), the root ball won’t fit in the hole. Mulch is piled around to hide the fact.

  4. Hi Dr. Chalker-Scott, I really appreciate your evidence-based articles. I was wondering if you could discuss best practices for mulching when finely ground bark/leaf mulch is being used. I understand that arborist chips are best, but am not always able to use them. For example, this spring I observed finely textured mulch spread on top of Anyennia neglecta, which was unable to emerge through the mulch and looked quite stunted when I uncovered it.

  5. You say never to add fertilizer (including compost) unless soil tests indicate it is necessary, but you also suggest we not waste the burst of nutrients in the fresh arborist chips. How is this burst different from putting down compost?

    1. Very good question! The difference is that the nutrient release primarily feeds the microbes in the mulch. While some nutrients will make it through to the soil, it’s not the same volume as what compost will contribute.

  6. Marvellous mulch-myth manifesto! My question is what to do when mulch (3″ in garden setting) gets colonized by fungal growth, which seems to fuse (or glue?) the chips together? Do I break it up, leave it or top it up with fresh mulch? Most plants seems to be able to grow through it and it still inhibits weeds. Thx.

    1. Is it wood chips or bark? With wood chips, the wood still absorbs water so just hydrating that top layer will keep it managable (many of those hyphae are part of the mycorrhizal network). If it’s bark, it’s too hydrophobic to absorb water and it’s best to keep breaking it up.

  7. Hi Linda. Brilliant post as always. I have two questions if you have the time. (1) I live in a part of Ireland that gets a lot of rainfall ( averages around 1100 to 1200mm a year). We have large amounts of slugs and snails that wreck havoc. Would these mollusks enjoy the comforts of the wood chips as a home and would I be increasing my problem with them?

    (2) you say that arborist chip mulches touching healthy trees is not going to hurt them. How about if they were touching herbaceous perennials and annuals. Do you envisage any problems with these?

    Thank you.

    1. Thanks Will! In answer to your question, slugs and snails do not like coarse woody mulches. These mulches harbor a number of predators, such as predacious ground beetles, which feed on slugs. They don’t provide a safe harbor.

      Secondly, wood chips have not been shown to harm either herbaeous perennials or annuals. I use them in both situations. Look for my earlier posts under “set your roots free” and “raised beds.” You’ll see numerous photos of my pollinator gardens and veggie raised beds.

  8. Thank you for your reply. It’s just the answer I hoped for. I must try and track down some chips now. Nobody uses wood chips over here in a garden setting. I have seen them used as a playground surface. Shredded bark however is very popular but I know from earlier posts that it an inferior mulch.

  9. A few follow-up questions. Please feel free to refer me to prior content if I’ve missed the answers elsewhere.

    1. I was told in my extension gardener classes that the reason not to mulch against the trunk was because our local voles and other rodents would then have access to the tree bark without having to hazard being eaten by local predator birds. It is definitely a balancing act to put 4 inches of mulch everywhere except within a few inches of the trunk. I’d rather be able to put the mulch across all of the bare ground. Is the advice I’ve been given unreasonable?

    2. The only mulch I can get in quantity is chipped pine bark. It comes in sizes all the way from fines to 18 inch long chunks. Most of the volume is 1”-2” chunks. It breaks down over 2-3 years and I top it off to keep at ~4 inches, or whenever I note weeds starting to come up. It does seem to keep out the quack grass everywhere but the edges. Is there anything else I need to do differently since I’m not getting the benefit of arborist chips?

    3. I sometimes hand sort the pine bark to select for finer bits, and then use it to mulch my vegetable beds. It is still pretty easy to remove, it allows water through, and it regulates the ground temp and water evaporation. Is there any reason I should worry about the bark laying against the plant stems?

    Thank you.

    1. Hi Clint –

      1) Nothing can tunnel in wood chips. Tunnels would collapse. Rodents are much more likely to burrow under sheet mulches, which act as a ceiling for tunnels.
      2) Bark mulch is not a good choice. It does not absorb water. If you have tree services in your area, then you have access to arborist wood chip mulch.
      3) Ditto answer above. I use arborist chip mulch (the finer bits) for my vegetable garden. They work extremely well and have no negative effects on the vegetables once they are above the soil high enough to mulch underneath.

      1. I really don’t have access. The city/county manages tree trimming of power lines and public areas, but the refuse is dumped at a landfill compost site that is not open to the public. The nearest “arborist” operates over 60 miles from my area.

        1. That’s amazingly short sighted of the city and county. If nothing else, I’d send an email asking them to reconsider this practice, which will save them money in the long run.

          Try to find some wood based mulch. Bark just isn’t good material. Playground chips might be your best bet.

      2. So interesting as I was told by an extension agent NOT to mulch with wood chips in the vegetable garden, so I took all the wood chips I’d put on there out again! But you think it’s ok?

        1. Your agent is incorrect. Although the research is thin, there is no reason to avoid wood chips in the vegetable garden. You can see my own raised beds with arborist chips in an earlier post in the blog (just search for “rasied beds” in the search bar).

  10. Hi. If a one-year wood chip pile was highly suspected of having termites, how could the termites be organically removed and the wood chips be used close to the house? Can an amateur gardener do what you will recommend? Thank you.

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