Volcano Mulching

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We’re starting to see volcanoes!

It seems to be a yearly occurrence in many front yards. I’m talking about volcano mulching – piling large mounds of mulch around the trunks of trees and shrubs – a practice that slowly kills the very plants it was intended to help. It’s TOO MUCH of a good thing!

“Don’t think that volcano mulching is OK because everyone is doing it,” says Temple University Horticulture and Landscape Architecture Professor, Eva Monheim. “Just because something is done doesn’t make it right. It causes bark to rot and roots get smothered. You have an unstable tree.”

Volcano mulching is too much of a good thing.

Volcano mulching is too much of a good thing.

Mulch is used to cover soil around plantings for a number of reasons: It assists in reducing evaporation; it stops sunlight from hitting the soil so fewer weed seeds germinate; as the mulch decomposes, it breaks down and provides organic matter to the soil; and, mulch rings around trees can keep lawnmowers and string trimmers at a distance. Mulch is aesthetically pleasing and can probably be compared to the frosting on a cake.

So, what’s the problem?

The bark of a tree is its outermost protective layer. It needs to be exposed to the air in order to function properly. Moisture retained by the mulch softens the bark; as mulch decomposes it also decomposes the trunk of the tree – making it susceptible to fungus and insects.

Corey Cummings, Past President of the Maine Arborist Association, adds, “thick mulch provides great habitat for insects and rodents to crawl around in, most of the time undetected. We know what rodents can do to the base of trees in the winter. With the forest under a barage of attacks by native and invasive pests we don’t need to do anything that would lessen the vigor of a tree to help fight off pests. If done properly mulching can benefit the tree’s health and improve its habitat”

“Mulch cuts off the supply of oxygen to the roots, essentially suffocating them” says Mark Fuance, a Maine Certified Nursery and Landscape Professional (MCN/MCL). “It has the same effect as planting a tree too deeply – another reason for premature decline.”

After years of volcano mulching this tree is now in decline.

After years of volcano mulching this tree is now in decline.

So, what’s a homeowner to do?

How to make a tree mulch ring:

Prepare: Carefully remove grass and weeds from around the trunk. Do not simply pile mulch on top of existing grass and weeds in an attempt to smother them. You will also inadvertently smother the roots of your tree.

Size: A two to three foot radius of mulch around a tree will allow for plenty of space to keep damaging equipment away from a tree’s bark.

Depth: Two to three inches of mulch will keep most weed seeds from germinating in the soil beneath but will still allow water and air to penetrate. Mark Fuance, MCN/MCL, says, “At the base of every tree is the root flare or crown.  This flare should always be two inches or so above the soil or mulch surface.  That will allow sufficient air to reach the roots.” If yearly mulching is done, be aware that in time it can result in too much thickness. In this case, “Old mulch should be removed before installing new material,” according to Mark Fuance. “Landscapers need to take the time to expose the root flare.”

Material: My first choice is dark-colored bark mulch. Well-aged wood chips and compost are fine. I don’t recommend dyed mulches or chopped rubber mulch. There’s concern about chemical residue leaching into the soil. Fresh wood chips and sawdust should not be used because they can deplete the nitrogen in the soil as they decompose causing chlorosis (yellowing leaves).

Mature trees can be harmed by too much mulch.

Mature trees can be harmed by too much mulch.

Know The Facts: Large, well-established trees do not need to be mulched. If you feel compelled to mulch to match surrounding beds follow the steps above.

Well-intentioned people who don’t know what’s really happening to their trees often build the mulch volcanoes. Uninformed “landscapers” think it’s ok. It’s simply a lack of knowledge about proper horticultural practices.

Now you know better! Protect your trees.

Volcano mulchingproper mulching





Lisa Colburn is a Master Gardener in Penobscot County, Maine and Sarasota County, Florida.
She is the author of The Maine Garden Journal – Insider secrets from Maine people who love to put their hands in the dirt.
Contact Lisa at (207) 404-3494 or by email: