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Question of the Week © 2017 Mike McGrath

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Yes, Virginia; There Is a Good Wood Mulch

Q: We had several large trees go down during hurricane Matthew and so we spread the resulting wood chips in an island in the middle of our driveway that contains palmetto trees and a large magnolia. Was that okay? And do we need to routinely treat the area with a fungicide and high nitrogen fertilizer to offset the potential nitrogen robbing and fungus producing qualities of the wood chips?

----Ted in Edisto Island, South Carolina

I recently came across a 2007 article from Washington State University that seems to refute much of what you've said about the dangers of using wood mulch. Now I'm confused. Care to comment?

---Tom in New Britain, PA

A. There's no need for confusion OR 'refutation', Tom! Now, that article was written by a good friend of the show, Dr. Linda Chalker Scott, author of the 2015 book "How Plants Work". When I interviewed her last year, we agreed on almost everything about mulch and were both denouncing the improper mulching of trees with equal volume. (We were practically finishing each other's sentences.)

So: What could possibly be in her article that "refutes" our wood mulch warnings?

Nothing. And it's important to note that her article is only about "the documented benefits and drawbacks of arborist wood chips as a landscape mulch." (And note that she even includes the word "drawbacks" in that introductory phrase.)

That's "arborist wood chips"; like from a tree trimming service. We have said many times in the past that such material is perfect to use in places like the lanes between raised beds and as a mulch around young trees and shrubs to prevent weeds and retain soil moisture—if you use it correctly. (More on what that means in a moment.)

My biggest 'mulch beef' is with those terrible dyed wood mulches, which can contain chipped-up pallets from overseas that were soaked in insecticides to try and prevent the spread of invasive insects and old-school arsenic pressure treated wood—especially after a tornado or hurricane rips through an area and the debris gets hauled to a chipping plant.

Our listeners in South Carolina would have been at risk of getting contaminated mulch if they had bought dyed stuff in bags or bulk after their hurricane, but they instead wisely used material from their own fallen trees. (Chips from your own property are doubly kosher because they don't need to be transported, thus making a carbon footprint the size of a baby shoe.)

But our listeners did unfortunately use this excellent material incorrectly, which we know because the photo they sent shows the mulch piled right up against the trunks of their trees. The dreaded 'volcano mulching'. And Linda's article does not disagree with us about the dangers of allowing mulch to cover the bark of a tree.

In fact, let's just quote her article: "Piling mulch against the trunks of shrubs and trees creates the dark, moist, low oxygen environment [that] fungal diseases require to grow and reproduce; piling mulch on the trunk provides exactly the right conditions for fungi to enter the plant. [And] opportunistic borers are more likely to invade a plant whose bark is wet due to excessive mulching. Rather than creating mulch volcanoes, taper the mulch down to nearly nothing as you approach the trunk."

Which is almost exactly what we say, except she forgot to add that volcanoes also make a great hiding place for mice and voles to nibble away at the bark undetected.

Now: There is one place in her article where we do disagree. She recommends mulching four to six inches deep (in some cases even deeper!); but she also specifies that this is just her personal suggestion. The University research I've seen on mulches warns that anything deeper than two to three inches can prevent rain water from getting through and cause oxygen deprivation at the root zone, especially in regions with adequate rainfall.

Now: What about the specific concerns of our listeners from South Carolina—that their wood mulch will steal nitrogen from the soil and breed nuisance molds?

Tilling or otherwise mixing high-carbon material like wood chips, leaves or sawdust INTO the soil will tie up Nitrogen, but it's generally not a serious problem when the material is on top of the soil. However, I do think they should add some coffee grounds or other high nitrogen material to their mulch—after they thin it out (if it's too deep, which it looks like it is) and move it away from the trunks of the trees (no matter what).

…Because any wood mulch can breed house-and-car staining artillery fungus and other nuisance molds and nitrogen-rich coffee grounds are one way to prevent such problems.

Curiously, Linda doesn't mention this problem in her article. But research conducted by (and numerous personal interviews over the years with) one of my other favorite plant scientists (the now retired) Doctor Harry Hoitink of Ohio State University, revealed that keeping wood mulch moist after application or mixing high nitrogen material like coffee grounds or composted manure into the chips can help prevent damaging fungal foes.

Keeping it moist?!

Yeah—how's THAT for counter-intuitive? It seems that moisture breeds bacteria that out-compete the fungal spores. And that's from actual studies, folks. Studies which also concluded, we note, that compost makes the best mulch of all!



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