Why Fresh is Best—when it comes to mulch?

Fresh wood chips!

One of the most misunderstood gardening practices is mulching. There is much mulch misinformation in horticulture books, web pages and even extension leaflets. First,what is Mulch? Mulch is any substance the covers the soil surface. Mulch can be inorganic (rock), hydrocarbon (plastic) or carbon based (chips, bark etc.) While any material applied to the soil surface could be considered mulch, the benefits of mulching especially to woody plants are greatest from fresh woody chippings of tree trimmings–so called “arborist chips” applied fresh—not composted. Annual plants such as vegetable plants are often mulched as well but usually with materials that rapidly break down such as straw or some mixtures of shavings and manures. These materials are easily incorporated later when the next crop is planted. For woody plants such as trees and shrubs, mulches that persist for a longer time are desirable. Plastic mulches used in agriculture are not suited to shade trees or other landscape uses nor are landscape fabrics. Each of these deteriorate into landscape trash rapidly and do not benefit soils under the mulch layer. Stone mulches while used extensively in the South west US are not as beneficial to soils as arborist chips.

Why use mulches anyway? Mulches support healthy tree and woody plant growth in landscapes around the world. They increase soil organic matter, the diversity and functionality of the soil food web (particularly saprophytic fungi), support mycorrhizal partners of woody plants, supply nutrients and suppress weeds. Thick mulch layers increase root development, and help to suppress soil borne plant pathogens. The breakdown of woody mulches on the soil surface encourages development of soil structure, increased water infiltration, water holding capacity, and nutrient holding capacity of underlying soil layers. Well mulched trees and shrubs grow healthfully without fertilization.

So why not mulch with compost? Compost is not suited for use as a mulch around trees and shrubs. Compost is often screened and is of fine texture. Fine texture presents a few problems. Fine compost will make hydraulic conductivity with soil and allow for water to evaporate through the compost/soil interface. Thus the moisture savings we see under arborist chips will not be the same under compost. Compost is also able to allow weeds to germinate in it so the weed suppression effects of a mulch will also be lost. Composts applied as mulch can make an interface between the soil surface and the mulch layer which should always be avoided as it will impede water movement through the interface.

Another important reason for not mulching with compost is that compost is poor nutritionally for soil microbes. Composts have most of their active or labile carbon burned away during the composting process by the rapid respiration of microbes. The compost is turned aerated and kept moist until the process stops at this point it has some level of maturity. It won’t reheat when turned. The microbes have consumed most of the available carbon for their own growth and respiration in the compost pile, none of this remains for microbes in the landscape. Fresh arborists chips are full of labile carbon. When laid over the soil surface spores of fungi invade and they begin to uses this carbon for their own growth as an energy sources. Placing fresh wood chips on the soil surface is feeding the soil microbiology at the soil-mulch interface. In time (a few years) these processes go deeper in the soil and begin to feed the soil food web beneath the mulch layer. The diversity of fungi increases, mycorrhizae begin to transfer mulch nutrients to their woody hosts and pathogens are destroyed by enzymes that leach from the fungi infested wood chips. While composts supply minerals (all that is left of the feedstock after composting) they can’t supply the labile carbon as a source for microbes. Fresh arborists chips do all this and are thus the best mulch for woody plants.

Fungi eventually invade fresh mulches releasing nutrients and enzymes to underlying soils

There has been some concern lately for using mulches that are recycled as yardwastes. This concerns me as well because gardeners may be disposing of dead plants in their greenwaste cans. In theory, pathogens could be coming through the greenwaste stream to gardeners. Getting tree chips is best because there is little likelihood for soil borne pathogens since the materials are chipped branches. There is some possibility of wilt diseases (Verticillium spp.) surviving in arborists chips but little research has established that the pathogen can infect especially if the chips are stockpiled for a short time. In my own research we showed that pathogens, weeds an insects had very short survival times in stockpiled (not turned) piles of greenwaste. There is very little chance of pathogens coming to your garden from arborist chips and the benefits to your soil and perennial plants are worth the effort to get a “chip drop” from your local tree care company.

Pathogens buried in fresh yardwaste do not survive for very long

Literature

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

Chalker-Scott, L., and A. J. Downer 2020. Soil Myth Busting for Extension Educators: Reviewing the Literature on Soil Nutrition. J. of the NACAA 13(2): https://www.nacaa.com/journal/index.php?jid=1134&fbclid=IwAR0cPfBl3V-3car-RPeEmlqzwW8bPEOPgND07xMTNgCOa5GkuSWtdD5WzF8

Downer, A.J., and B.A. Faber. 2019. Mulches for Landscapes UCANR publication #8672

Downer, A.J., D. Crohn, B. Faber, O. Daugovish, J.O. Becker, J.A. Menge, and M. J. Mochizuki. 2008. Survival of plant pathogens in static piles of ground green waste. Phytopathology 98: 574-554.

Downer, A.J., J.A. Menge, and E Pond. 2001. Association of cellulytic enzyme activities in eucalyptus mulches with biological control of Phytophthora cinnamomi Rands. Phytopathology: 91 847-855

Downer, J. and D. Hodel. 2001. The effect of mulching and turfgrass on growth and establishment of Syagrus romanzoffiana (Cham.) Becc., Washingtonia robusta H.Wendl. and Archontophoenix cunninhamiana (H.Wendl.)H. Wendl. & Drude in the landscape. Scientia Horticulturae: 87:85-92

Published by

Jim Downer

Dr. Downer has 34 years of experience as a horticulture and plant pathology Advisor with the University of California Cooperative Extension in Ventura County. Dr. Downer’s academic training is from California Polytechnic Univ., Pomona, (BSc. horticulture & botany, 1981; MSc. Biology, 1983;. In 1998 he earned a Ph.D. in plant pathology, from University of California, Riverside. Dr. Downer’s research is focused on mulch, soil microbiology and disease suppression in mulched soils, diseases of shade trees and cultural practices to maintain landscape plants. Dr. Downer is a member of the American Society of Horticultural Science, the American Phytopathological Society, The International Soc. of Arboriculture, and the Western Chapter of the ISA, and the International Society for Horticultural Science. Dr. Downer is an Adjunct professor at California Polytechnic University in Pomona. Dr. Downer serves on the Board of the John Britton Fund for tree Research as the chair of the research advisory committee, and currently chairs the regional conference committee for WCISA. Dr. Downer has a love of shade trees, Shinrin roku (forest bathing/walking) tree work, wood working, horses, gardening, horticulture and the study of plants and their biology.

19 thoughts on “Why Fresh is Best—when it comes to mulch?”

  1. Thanks for the article. I’ve been using fresh chips lately and have been happy with the results. Question: is there any risk of herbicide in typical tree chips? I don’t see folks spraying trees here in Florida, but generally speaking is there much risk of this?

    My main concern is with compost. Earlier this year I got a load of mushroom compost that clearly had horse manure as an original component. My pepper plants immediately started exhibiting curled, stunted growth after application. My chief suspicion is GrazeOn poisoning through the manure. So I’m getting a bit more paranoid about stuff like that sneaking in via compost and mulch.

    1. Usually there is not much risk of herbicide contamination in tree trimmings. Herbicides are not applied to shade trees in any regular way. Composts should be a concern because the process of composting is concentrating things. Usually just plant minerals but herbicides like clopyralid have been shown a few times to concentrate in compost. This is not always the case with herbicides but this one since it is used in alfalfa can pass through animals and be concentrated during composting of the manures. A simple bioassay of compost with seedlings of tomato will show weather the compost is toxic or not.

  2. Thank you for the information about the many benefits of fresh wood chip mulch. After reading one of the blog’s previous articles about this topic, I asked our local tree trimmer if we could keep the chips from our tree trimming, and he ended up bringing us 2 more loads from local jobs. It was enough to cover a large new area we’d just landscaped! The chips he brought were very nice without a lot of junk so I’m hoping I can continue to get all I need from him every year without having to join a site like ChipDrop. Thanks for your very informative blog!

  3. Thank you for this article. I have a question about sheet mulching with cardboard and then layering on the arborist chips. I usually follow this procedure when I’m looking to smother existing weeds or lawn in advance of a replant perhaps a year or two later. Could you let me know if the cardboard sheet mulching under the arborist chips is fine or exactly what your thoughts are about that. Thank you.

    1. Hi Kate –
      We’ve covered this topic before in our blog. Look for “cardboard” in the search box and you’ll find the discussion, plus a link to the recent research showing the impact of cardboard on gas movement. Bottom line: cardboard is not a good choice and there is no science behind its use for this purpose.

  4. I just watched Mulches the good the bad and the really ugly. It was excellent. My question is about using ground up leaves as a mulch. I’ve been doing it for years and now I’m afraid it isn’t such a good idea. Will it be hydrophobic? Also, I have a problem with voles so I don’t actually use mulch in the garden, only around trees etc. any thoughts?

    1. Ground up leaves are fine to use as part of a mulch layer. By themselves, they are too fine and compact too easily when used at the depths needed to make an effective mulch.

      Nothing can tunnel in wood chips. The tunnels collapse. Wood chips are good habitats for beneficial insects and microbe – not rodents.

  5. This is great information to know! I’ve been using arborist wood chips since they’re free, so it’s comforting to know they’re also the best choice.

    I’m wondering about fertilizing my roses using some sort of compost. I’ve used both organic fertlizer (rose-tone) and aged horse manure in the past. For the rose-tone I sprinkle it over the wood chips, for the manure I put it under wood chips. Which of these is better fertilizer for roses? I don’t bother fertilizing anything else in my ornamental garden.

    What about providing new nutrients for raised vegetable beds? I heard wood chips can spread disease in vegetable beds. Should we be using leaf mold instead? Do raised beds need a new layer of compost every year as the old soil compacts?

    Thank you!

    1. You shouldn’t add any nutrient-rich material unless your soil test indicates a deficiency. Just use the chips. There is no published evidence that wood chips spread disease in vegetable beds or any other system.
      If your raised beds are filled with actual topsoil, you won’t need to add much of anything other than a mulch layer each year. If it’s filled with amended soil, then yes, it will settle. Adding more compost, without knowing your current nutrient levels, is not advised.

  6. You’ve just blown my mind! Wow. I will have to rethink my whole approach to gardening, which primarily has been sheet mulching covered with fresh wood chips. The chips have been wonderful. But what should I do with my compost pile now? Can I still use it in raised beds?

    1. We’ve done a lot of posts over the years on the problems with sheet mulches, especially cardboard, and the benefits of wood chip mulches. You can use the search box on the blog to find them. In terms of your compost, you should add it only if a soil test indicates you have low or optimal levels of nutrients. I did a post on this very topic a year or so ago (again, use the search function for “rasied beds”). If yuo don’t need nutrients in your raised beds, you can still use your compost to make your own container mix. We are planning to do exactly that with our compost.

      1. Great! I’m excited to enjoy less time/work mulching and still use the compost in my big pots. Got my next chip load scheduled. Thanks!

  7. Could you address the use of pine straw vs wood chip mulch in the low country ( zone 8b)? I have neighbors on either side of me very opinionated as to which is best. We live near a pond and lowland woods and both feel their material is less likely to draw critters, especially snakes.

    1. There’s no evidence, at all, that wood chips attract critters other than beneficial, predatory insects and spiders. No, straw does not even come close to wood chips in terms of benefits. It’s hydrophobic, so it does not absorb or release water. It doesn’t support soil life or mycorrhizal networks.

  8. Thank you for the great website. What do you recommend as a 2nd best mulch if you can’t find the fresh wood chips? I am never able to find wood chips or arborists chips here in Vermont, so I am usually stuck getting the bagged “cedar mulch” stuff or pine bark chips. Straw is available and composted cow manure. So far I’m unsatisfied with all of these options.

    1. Have you tried going through ChipDrop? That’s how many people can link up with arborists looking for a place to dump their chips.

      If not, the second best would be chipped wood debris, like from reclaimed wood from demolition. Research shows that it’s almost as good as arborist chips. Nothing else you can get in bags comes close.

      1. Thank you! I’ll try harder to find the woods chips. I’m sure my trees would love it. Our local dump gives away some chipped yard waste and clean wood. It’s always very heavy on the pine— is pine a problem?

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