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


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.

38 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?

  9. Hi Linda! Thank you for this informative article.

    Re: “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.”

    Your article “Wood chip mulch: Landscape boon or bane?” reads:
    “Before installing wood chips, create a thin underlying layer of a more nutrient- rich mulch (like compost) if there are concerns about nutrient deficiencies. This “mulch sandwich” approach is a logical one that mimics what you would see in the mulch layer of a forest ecosystem. It’s not required, though, and over time a wood chip mulch will develop this same structure as the lower layers break down.”

    Is this latter point still accurate in view of the first point? Will compost under a layer of wood chips “impede water movement through the interface”?

    If this is the case, would you be able to update your fact sheet at WSU?

    Always with much admiration at the quality of your work and your generosity in making these valuable resources and materials available to gardeners!

    1. I agree with what Linda wrote. I think the difference is applying a thick compost layer as opposed to coarse mulch breaking down into something that resembles compost on its own. When compost is applied it may be texturally finer (if it is a screened commercial product) than the soil under it causing interface issues. There are other problems as well that go with commercial composts such as incomplete composts with chemical issues that harm roots, etc. I have often wondered how compost would work as a light sprinkling over a coarse mulch. Of course the fine particles would work their way down to the interface but it may not matter so much. We need actual research on this…..


  10. Hello, I have a few questions about arborist mulch:
    1. Does it matter at what time of the year that mulch is applied? Should the ground be water first?
    2. Does arborist mulch collected from fall trees have the same benefit as green trees?
    3. Is the mulch beneficial to perennial beds that contain bulbs? If a mulch layer is added to existing bulbs (like narcissus and allium) would contribute to the below-ground depth (for example, 6 inches becomes 10 inches deep)?
    4. For northern gardens (for example, Winnipeg, Manitoba – Canada climate zone 3), how mulch is required to protect less hardy perennials? In my zone, the freeze days index (Celsius) is 1750.

    1. 1. Mulch can usually be applied at any time but be carefully around seedlings or young emerging plants. Don’t cover them with mulch, apply it around them and then move into place once adequate top growth has been achieved.
      2. Mulch from fallen trees is fine. Ideally it will still have green foliage, etc attached.
      3. Mulch is great for bulb bed and a mulch layer won’t contribute to soil depth, it breaks down. Be sure to not cover young emerging bulbs with mulch. Mulch around them and then move into place once the foliage is taller.
      4.A 6-12″ layer of mulch will provide good protection.

  11. Hi I am in the tropics of Australia. I have previously used fresh wood chip and the long term result (one year on) was even better than expectations. However – in the short term I had nitrogen loss within weeks with plants that had been thriving, turning yellow. The local garden centre told me I was foolish to use fresh wood chips. I fixed the problem by watering in urea and it seemed to do the trick. I have read through many comments but see nothing on how to treat fresh woodchip to ensure it does not reduce the nitrogen in the soil and compromise existing plants.

    1. Fresh wood chips will not deplete soil nitrogen. This has been shown conclusively by several independent researchers. So the yellowing was due to some other factor. I can assure you it is not from the wood chips unless they were incorporated into the soil.

  12. Hi! I’m an avid reader of your blog. Thank you for well researched info on various topics!

    I am still wondering though: Why do you prefer not to use bark as mulch? Here in Norway, bark is easy to come by and much used in garden beds and landscaping. Is it something to do with it’s possibly hydrophobic quality?

    1. Hi Tuva,
      It is an interesting question. Bark is realcitrant so in that way it is a good mulch because it does not break down so fast. But it is also lacking in cellulose so does not provide a good source of energy for microbes in/on soil. The best mulch feeds the microbiology of the soil while performing its other functions. Wood chip mulches break down and need to be replenished but build soil quality rapidly. Bark mulches depending on climate can stay in the same place for 25 years and not change at all. I assume in your location there is more moisture so even barks will break down eventually. Its not bad, its just not as good.

      1. Thank you so much for the quick and informative reply!
        I take great delight in finding advice backed by knowledge and science instead of the latest trends, as it seems quite hard to find out what will actually strengthen and support a natural and healthy garden (I say this as I was about to cardboard sheet mulch most of my garden before I found your blog. Not anymore, though).

        I’ll be hunting for arborist chips then, thanks again! And keep up the good work!

  13. You mention the use of straw and compost in mulching annual vegetable gardens, but is that optimal, or would fresh or aged wood chips or pine straw (that is, long-leaf pine needles) be preferable? And whatever the optimal veggie mulch is, how thickly should it be applied? Thanks in advance.

    1. We all have our preferred mulches – I use arborist wood chips, fresh or aged. They are perfect for getting beds ready for winter (no weeds). Then you just move them aside from where you plant your seeds. Let the seeds germinated (and pull any weeds that come up there), then move the chips back into place afterwards. Your weeds are suppressed and water retention in the beds is vastly improved. Compost as a mulch will promote weed growth and straw of any sort doesn’t hold water, so that’s why I choose arborist chips.

  14. Hi! Is this the best place to ask mulch questions, or should I put them on a newer topic?

    How do I calculate how much mulch to buy? F.e., for a 10′ by 20′ space, if I want – is it 8 inches that’s best? – by 8″, how much is that?

    I asked the internet and it said “3 775.57955 liters” but that didn’t make sense.

    Or do i put it all into inches and just multiply that way? 120 x 240 x 8= 230400 /12 = 19200 This doesn’t seem right at all.

    I will go ask other people too, but I’d love to know how to do this.

    1. Arborist wood chips come by the truckload, and cost little to nothing; you don’t need to figure out how much you need per se. I would just contact a tree service or chipdrop.com and order a load to be delivered. It’s likely to be around 11 cubic yards.

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