I felt like a fraud last week. I was having dinner with Dr. Linda Chalker-Scott from Washington State University (WSU) and she was correcting my understanding of wood mulch depth around trees. For years I have been insisting on 4 inches (10 cms) of wood mulch at the base of trees in new landscape installations. She was disputing that.

Are you using wood mulch? How much mulch is the right amount for your trees? You may be surprised.

Coarse wood mulch is made up of variously-sized pieces and new information says we need more not less of this valuable yet often free product.

I feel like I am repeating myself endlessly but I insist on wood mulch for all landscape installations. Call me crazy but planting trees that will survive and thrive depends on the addition of mulch and my favourite material is the coarse free wood mulch you get from Arborists.  Imitating nature with a layer of mulch lets the natural system of organic decomposition protect and shelter tree roots while holding water and gradually adding nutrients to the soil.  Like a sippy-cup and slow food combined, mulch keeps trees and landscapes healthy and protected. But what I found out on Sunday is that more mulch  is better – I think.

I am talking to Dr. Chalker Scott and she is telling me I might not be using enough mulch so I am confused. I check in with the organization in charge of  training and certifying staff who look after our trees – the International Society of Arboriculture (ISA). It suggests less mulch is better. I read that the right amount of mulch is 5 -10 cm (that’s 2- 4 inches in the USA). One ISA study says “that the current recommendation for mulch depth of 7.5 cm is appropriate. Soil oxygen levels, temperatures, and moisture levels are all within acceptable ranges under a mulch of this depth.”

When wood mulch is screened so it is all one size it tends to blow away and leaves only a skim of woody material on the surface. This shrub bed with the mugo pine needs a lot more mulch on top of the soil.

Then I google Dr. Chalker-Scott’s new fact sheet on wood mulch. It says the correct amount of wood mulch on beds is 4-6 inches. Pretty close to what I suggest.

But at the dinner Chalker-Scott tells me her latest work shows that coarse wood mulch is better at depths of up to 18 inches…. and that is a crazy huge jump from what I normally suggest to gardeners. I quickly run home and search for references to confirm what Chalker-Scott has told me and I can’t find them. I speak with my daughter Chelsie who works every day in landscape installation and maintenance and we both shake our heads. This number seems so big it is sure to crush or kill all the smaller plants trying to get established under trees. The extra revelation Chalker-Scott makes is that wood mulch trials are stacking mulch against trees and this is also against everything I believe and have read. In fact it is the opposite of what Chalker-Scott has already published.

When rock is used as a mulch it can’t feed the trees because rocks don’t decompose that way. In fact rocks as mulch often become weedy patches. Can you see the weedy grass growing up in these rocks?

Garden consultants have been saying repeatedly that piling wood against trees will damage the bark because the sitting water and the action of the organisms living in the mulch on the living tissue damages the trunk. Chelsie has also noticed dark stains and damage on wood where it is piled with wood chips but this is anecdotal and not part of a scientific study. Chalker-Scott tells me  the new studies have deliberately sliced into bark and then buried the trunk with bark and the trunk has repaired itself. Like a band-aide on bark, wood chips have repaired injured tree trunks in recent (but still unpublished) studies.

So there is nothing left to do but email Dr. Chalker-Scott for more information and she is quick to reply by email.

“I don’t like or recommend bark mulch. I prefer fresh arborist wood chips as they actually absorb water, support mycorrhizae, and break down faster. That’s why you have to put a thicker layer down initially. Within a month an 8” layer of mulch will be 4”. It must be maintained at 4” to keep weeds from germinating.

Second, I don’t recommended 18” of mulch or piling it against the trunk. What I said is that research (not mine, and not yet published) has shown that thick layers of coarse wood chips will not interfere with water or gas movement. And that wounded trees with arborist chips piled around the trunk and covering those wounds seal faster than trees without the mulch. Until these research studies are published I can’t recommend doing either one. But there is absolutely no research demonstrating injury from either practice, either. Unfortunately erroneous online columns perpetuate that latter perception without providing evidence (I talked about that here last year.)

[Finally] I do have an article that looks at 10, 20 and 30 cm of wood chips mulches and tree health. I’m revising it again for publication and hopefully I can get it out next year. (Spoiler alert – the deeper the mulch the healthier the trees as long as the mulch was coarse. Fine textured mulches are the opposite.)”

So in fact I heard wrong – sort of – and in fact until the latest information is published we can keep adding fresh coarse wood chips in layers as deep as 8″, allowing them to  settle to 4″ and then maintain them there.  I agree with Chalker-Scott that trees and landscapes need wood chips and the ones you can get free from arborists are the best even if they are coarse and possibly diseased and erratic in size and source. The  mixed loads with leaves and bark and wood are ideal for the trees they are sheltering. At this time of year, just before the big snows of winter, start looking for and spreading this free mulch from arborists whenever you can find it.

So  I guess I am not a fraud. Until the new research is published  I will continue to suggest  tree and shrub beds need 10 cm of coarse loose mulch. You can always add more later to keep it to a depth of 4″. And seriously? Where are you going to get that much mulch anyway? It is getting tougher to get the really great free stuff because everyone wants it. In good news anything you do get at this time of year probably includes evergreens and the smell of evergreens chopped and dropped on the soil at your feet is delightful and special.

On the topic of tree care Dr. Chalker-Scott has some exciting news. “A professor from University of California and also with an Extension appointment and I are publishing a myth-busting article about landscape tree care at the end of the year in the NACAA Journal. I’ll be announcing when that comes out in our FB Garden Professors blog group.”

Tell me what you are mulching with by replying here now. I love your feedback and ideas.

This poor tree didn’t get any mulch at all and was planted in a whole where it died from sitting water. Dead by the first spring it is not an example to emulate.

PS Bark mulch or bark nuggets or even wood shavings are not the same as coarse wood mulch and neither Dr. Chalker-Scott or I like them, See her PDF about this here.

Bark “nuggets” never make a good substitute for wood chips and bark over landscape fabric is a shortcut to disaster because as the chips blow away and the fabric lifts it is ugly and of no help to the tree.