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Mulch Volcanoes: No Eruptions, But Plenty of Tree Disruptions

By Adrian Higgins
Thursday, September 10, 2009

Ah, fall is in the air, along with the stench of shredded hardwood mulch newly laid on top of soil. Mulch volcanoes won't be far behind.

Soon, landscapes across the fair metropolis will be marked by the mounds of mulch at the foot of hapless trees. They will rise 9, 12, even 18 inches above the base of each trunk. Some call these hills volcanoes, cones, even turtle mounds. I think of the tree as a fat candle stuck in a cupcake. Whatever you name them, the experts have been railing against mulch volcanoes for decades. They stress and kill trees in so many ways. Still they come.

"Monkey see, monkey do," said Frank Gouin, a retired professor of horticulture, explaining why they persist. He first raised the alarm about mulch volcanoes and America's addiction to excessive mulching as far back as the mid-1970s, when he announced that "over-mulching is a national disease."

It has certainly become a big part of the green industry. We lay an estimated 40 million cubic yards of mulch per year, either through the landscape maintenance companies that residential and commercial customers rely on, or ourselves. That's almost enough to encircle the globe.

Mulch has its value, but when it is misapplied, plants suffer and die. The tree volcanoes are the most egregious example of this. Gouin, formerly of the University of Maryland, finds the mounds to be "hideous." I have tried to understand the aesthetics behind this practice. When the drainpipe on your sink goes into the wall, it is surrounded by a metal flange, which gives the junction of pipe and wall a finished look. Could the same ambition be at work here?

The problem, of course, is that homeowners see landscaping companies fashioning the volcanoes in apartment grounds and commercial parks and think if that's how the "pros" do it, they must ape it.

Whether or not it works visually, the practice ignores the needs of the tree.

Mulch is supposed to achieve these main goals: retain soil moisture, suppress weeds and moderate soil temperatures. The root zone of an established tree extends beyond its drip line, so mounding the mulch against the trunk does little for the roots, except to cause the roots immediately around the trunk, especially in young trees, to grow into the volcano. Also, the piled mulch softens the bark of young trees and trees with smooth bark, such as maples, beeches and crape myrtles, and invites insects, rodents and diseases to invade. The lower trunk, unlike the roots, cannot survive long-term with the constant moisture trapped by the collar of mulch. It is the equivalent of planting a tree too deeply.

The roots of a tree or shrub find their own level in an attempt to balance their needs for water and air. If you pile mulch too thickly above the roots, the existing surface roots are suffocated and new ones grow into the mulch. Not only does that leave them at risk of drying and dying when the mulch decays, but the roots "grow across the stem, potentially strangling the tree to death," research scientist Jeff Gillman writes in his new book, "How Trees Die" (Westholme, 2009).

Gouin points out that certain shrubs are especially keen to grow roots in mulch, including azaleas, boxwoods and Japanese hollies. He rails against the notion that a garden needs mulching every spring or, even worse, every spring and fall. This essentially covers the whole garden in a mulch volcano, with the same drawbacks.

There is another potentially disastrous effect from over-mulching with a popular form of mulch called hardwood bark mulch. As it decays, it releases the metallic element manganese into the soil, where it stays put. If you repeatedly lay this mulch, the levels of manganese build to such a point that plants become robbed of the iron they need. This results in smaller leaves, leaf yellowing and branch dieback.

"It takes seven to eight years of repeated applications of hardwood mulch to get manganese levels above 200 pounds an acre, and that's when you start getting symptoms" of iron deficiency, said Gouin. Because you can't reduce the levels at that point, the only cure is to replace or heavily amend the soil.

How should we mulch? Gouin said that a thin layer of mulch is necessary only every two or three years, and that tired-looking mulch can be revived by scratching it with a cultivator. If you have a mulch volcano, push the mulch away from the base of the tree and excavate until you can see the point where the trunk flares into the roots.

I occasionally get calls from readers who are aware of the absurdity of mulch volcanoes but who live in garden apartment condos whose landscapers routinely practice the art form. Their efforts to get the condo board to fix it are often met with comments such as, "It looks so nice." This is all part of the mythology of mulch that crusaders such as Gouin have been unable to change.

When he lived in College Park and cycled to work, he came across a fellow spreading mulch at the base of a dead pine tree. "He said, 'This is great stuff; it will revitalize it.' "

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