Mulch Myths with Dr. Linda Chalker-Scott

Last Friday, I had the pleasure of having Dr. Linda Chalker-Scott guest host on #GroundChat. She covered the many myths around mulch.  Many of the myths blew well-accepted mulch practices right out of the water!



Dr. Linda Chalker-Scott is an Extension Urban Horticulturist at Puyallup Research and Extension Center and Associate Professor and Horticulture and Landscape Architecture at Washington State University.

She is also a member of the highly acclaimed Garden Professors.


Here are some of the highlights…

Dr. Linda Chalker-Scott (L CS) “Mulch is anything that covers the soil – some are good, some not so good for lots of reasons. And lots of myths about them!”


Many people get confused about amendments and mulch. For the record, what is the difference between them?


A mulch is simply a topdressing. An amendment is worked into the soil. So compost can be either one.


And what are the benefits of mulching?


So many benefits! Moderates soil temperature, reduces erosion and evaporation, and organic mulches have more.

Organic mulches also increase soil nutrients and soil biodiversity. Love them!

And I like to consider wood chip mulches as “slow food” for soil. Much better than the fertilizer fast food.

Deep, coarse organic mulches are the BEST weed control mechanism out there.


Mulch Myths!

Wood chips


One of the most frustrating myths is that wood chip mulches rob your soil of nitrogen. Absolutely not true!

The microbes do use nitrogen right there at the soil surface – but not underneath where roots are.


And how what about the depth of mulch?


For chunky, woody mulches, the more the better in terms of weed control – up to 18″ have been used!

The tipping point: organic mulches of 3” or less will promote weed growth, while 4” or more will suppress it.

[p.s. compost used as a mulch will grow weeds, no matter the depth…coarse organic mulch is the best weed control]

And just to continue this thread, fine textured mulches can be damaging if they’re too deep.

Wood chips are the best! We try to avoid having them touch the trunks, although new research may change that.


But many gardeners worry about using wood chip mulch because of termites and mice. Do wood chips attract termites and mice?


Termites hate wood chips – in experimental taste tests they only eat them if nothing else is available!

Termites like nitrogen and phosphate-rich mulch. Wood chips have neither.

Given a choice, termites would rather eat nutrient-rich mulches; they’ll settle for cardboard but they avoid wood chips.

Mice don’t really care for chips either. They collapse into burrows. Rodents love tunneling under sheet mulch


And she tackled the garden media darling “lasagna layering” of mulch


Be careful of the “lasagna mulch” approach. Layers of paper restrict water and air movement to the soil.

Lasagna mulching may appeal to your emotions, but it doesn’t do much for the air and water content of your soil.

And I’d also caution people not to use too rich a mulch. It can contribute to nutrient pollution.


And addressed the issue of mulch transferring disease!


Research has shown that disease isn’t transferred from mulch to healthy trees. Unless there’s a splash issue


In choosing the right mulch…


In drier areas inorganic mulches are often a better choice. Rubber, not so much. I’ll elaborate.

If the EPA defines discarded tires as pollution, then why isn’t recycled rubber mulch in the same category?

Leaves are great [mulch], as long as you shred the big ones. Layers of big wet leaves restrict air movement to the soil.


And using mulch to change pH, much to everyone’s chagrin the answer was…


Wood chips, bark and pine needles will not acidify the soil.

Unless you’re working with container plants, mulch will not change the soil pH. The soil profile is HUGE outside

There’s been quite a bit of research looking at this. pH is determined by a lot of climatic factors.

Rainfall is very important in determining pH, because microbes need water for decomposition, which changes pH.


To wrap it up, I am giving you an opportunity to win two of Dr. Linda Chalker-Scott’s books. Great Xmas gifts!


To enter the competition, write a comment below and let me know what kind of mulch you use. One lucky gardener will win The Informed Gardener and The Informed Gardener Blooms Again, courtesy of THE REAL GARDENER.


Draw on November 30.

The lucky winner of the draw is Chris Sabbarese. Congratulations, Chris!




Written by Cristina da Silva
Wednesday, November 21, 2012 in Building Soil

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  1. Wayne "The Subversive Hippie" Johnson says:

    I use pretty much any organic mulch I can get my hands on. Sometimes that is leaves or hay, and sometimes it’s a top dressing of compost.

  2. I like to use either my own compost or I will purchase a compost soil blend to mulch my garden. With half an acre, not the whole garden gets done in a year so we save leaves, compost them and apply the leaf mold when it is ready. In our raised beds at our community garden we use straw on the beds to prevent soil erosion.

  3. My mulch is anything and everything recycled from my yard like leaves & grass clippings. I’ve been known to “dumpster dive” my neighbor’s green bins to use around roses and other non-edibles. I learned some helpful tips from Linda’s chat on mulch like the wood chips myth…I had avoided it for fear of inviting termites…! Thanks Cristina for presenting a great chat, topic and host 🙂 Chris

  4. I hope many people picked up on the importance of air movement and soil health when it comes to using mulch. If we can avoid the “pack it on, pile it up, layer it deep” approach, our soil (and plants) will be happier!

    • Good point, Sarah! In our rush to pile deeper mulch, we may forget that water and air has to penetrate to the soil. I think the only mulch you can pile to 18 inches are the coarse wood chips. Lots of space between them to allow air and water movement.

  5. Kate says:

    It was a great #gardenchat!

    For mulch, I use one of three things, or a mixture of all three:

    leaf mulch
    worm compost or backyard compost
    coconut coir chunk mulch

    I’d love to do some more learning from Linda’s books!

  6. Wayne Clingman says:

    Due to Groundchat I now know the right way to deal with some soil issues vs just dumping Chemicals

  7. Jeavonna Chapman says:

    I use different mulches for different situations – white rocks in some containers that the cats favor (discourages them); shredded bark in flower beds and aroudn some trees; larger bark pieces in other beds for decoration (never the dyed bark). I use leaf mulch in the vegetable beds (at my mom’s house). I guess, it depends is the answer to what kind of mulch do I use.
    Not enough pine needles, should probably plant my own pine trees so I don’t have to go scavenging.

  8. In my dahlia beds I have always waited until the first heavy freeze and then cut them down in pieces and left on top…. Shredded leaves mixed in with compost on the front beds

  9. gardengeri says:

    Free Woodchips are my favorite mulch– I even wrote a blog explaining that I am a Wood Chips Connoisseur
    Fingers crossed- Would love to be considered for book.

  10. Sonia mojica says:

    Shredded bark mulch. Any hardwood, or cedar. 🙂

  11. Sonia mojica says:

    And compost!

  12. I use a combo of wood chip mulch, depending on the garden bed. For my veggie bed, I use pine straw mulch (it’s free in the backyard). For the flowerbeds, I use wood chip mulch given to us by our tree guy or cypress mulch if it’s on sale. Thanks for posting the highlights! I needed some of that info.

  13. Thanks Cristina, I’ve always been concerned about using wood chips because of the nitrogen problem, thanks for setting this straight. I’ve always found them to be clean and attractive but was afraid I was doing my garden a diservice.

  14. tea says:

    I am curious to find out what blog platform you are working with?
    I’m experiencing some small security issues with my latest website and I’d like to find something more risk-free.
    Do you have any recommendations?

  15. Norm says:

    I hope someone is still active, here. I noticed most of this post was related to landscape plants/gardens. What about vegetable gardening and putting the garden (veggie) to bed for the winter?

    I.E…..ground covers vs mulch vs compost vs lasagna methods. Remember, I need to be able to plant and garden next spring so I wonder about the value of 4″ to 18″ thick wood mulches! What is the vetted/research based better approach?

    • Great suggestions, Norm! I would say that a cover crop would the number 1 method for overwintering if you’re growing vegetables. It will keep the soil from eroding and add nitrogen and/or improve soil structure for your veggie crop the following year.

      As for mulches… a 4 to 18″ thick wood mulch is too much. At 6″, wood mulches start to compromise water and air movement into the soil. Definite no-no. And adding wood mulches changes the soil microbes to predominantly fungal, which isn’t conducive to most vegetables. If want to use a mulch, use straw, which is used a lot in traditional vegetable plots.

      I hope that answered your questions, Norm. I will prepare a blog to cover your questions. ‘Tis the season for soilbed preparation.

    • Thanks for your good suggestions, Norm!

  16. Patricia says:

    I would like to know, based on scientific research, whether or not cedar mulch repels mosquitoes.

  17. Willetta Les says:

    Hi There,

    Great work on the article. I realize it’s a few years old but the info is bang on! I love the comment on setting the parameters for the depth of mulch. 3″ is not enough and 4″ and up is good. We like to tell people the depth is dependent on the condition and location of their garden but your “baselines” are awesome. Thanks for sharing!!

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