Mulch Volcanos:  Tree Killers

By Lamar James, U of A Cooperative Extension

First posted on 11-04-2008

Mulch volcanoes are killing trees, says Sherri Sanders, White County extension agent with the University of Arkansas Division of Agriculture.

Sanders says mulch volcanoes are one of her pet peeves.

Never heard of a mulch volcano? It’s a term applied to the pile of mulch around the base of a tree to form what looks like a volcano as it angles up the tree trunk, says Sanders.

“The only thing oozing as a result of these volcanoes perhaps is sap from the trees once borers get into them or disease takes over. Applying mulch too thickly will invite invasion by insects, disease, fungi and small rodents,” says Sanders.

She sees mulch volcanoes everywhere, including subdivisions and businesses.

Unfortunately, she says, landscapes are falling victim to a plague of over-mulching.

“It’s fall and gardeners everywhere are sprucing up their properties and protecting tender plants, adding trees and/or shrubs and, hopefully, mulching the leaves,” Sanders says. “This is a good time to warn against creating mulch volcanoes in your landscape.”

There is a way to mulch a tree or shrub, she notes. More than 2 to 4 inches is over-mulching. Piling up mulch up around the trunk or stem will also kill the tree. Continuous moisture trapped in the trunk area by a wall of mulch will cause cankers and splits, and allow disease and pests to attack.

Excess moisture in the root zone will stress the plant and cause root rot.

Thick blankets of mulch can become matted and prevent water and air from getting to the roots. Roots need oxygen to breathe and without excess mulch and excess water, the trees are actually suffocating, Sanders says.

“Mulch applied correctly around young or newly planted trees is good for the plants,” she says. “It provides plants with a constant supply of oxygen, keeps the soil at a more even temperature, prevents weed growth, conserves moisture, adds organic matter to the soil, reduces soil erosion, improves aesthetic values and most likely adds value to the landscape property.”

The proper way to mulch a tree or shrub after planting them at the proper depth is to apply 2 to 4 inches over the entire root system of the plant. The depth of organic mulch should not exceed 3 inches after settling.

Keep mulch at least 6 inches from the base of a trunk, she notes. Some people prefer to

start tapering off when they reach within a foot of the trunk. You should see the flare of the bottom of the trunk. If you already have mulch and roots have not begun to grow within it, just pull it away from the trunk and use that extra supply elsewhere.

“Simply because you have seen this practice done in commercial landscapes certainly doesn’t mean that you should do the same in your own landscape,” says Sanders. “Research has shown a dramatic decrease in the size of trees and shrubs that have fallen victim to mulch volcanoes.”

For more information, contact your county extension office or visit The Cooperative Extension Service is part of the U of A Division of Agriculture.