Published on 10/31/06

Mulch volcano too much of good thing in landscape

By Dan Rahn
University of Georgia

In many manicured landscapes, plants often seem to grow out of little mulch volcanoes. But piling mulch 1 to 3 feet deep around trees is way too much of a good thing, experts say.

"Piling mulch around the base of the plant does more harm than good," said Gary Wade, a Cooperative Extension horticulturist with the University of Georgia College of Agricultural and Environmental Sciences.

"It generally sets the plant up for long-term stress," he said, "which makes it more prone to injury from insects, diseases and environmental extremes."

Proper mulching

Generally, mulching plants is good, Wade said. Mulches:

  • Keep the roots evenly moist.

  • Insulate roots from extreme heat and cold.

  • Prevent weeds that compete with plants for moisture and nutrients.

  • Serve as a barrier to certain soil-borne diseases.
"Done properly," he said, "mulching is one of the best things you can do to help plants get established and survive our environmental extremes."

But there's a right way to do it, he said. And making mulch volcanoes isn't it.

Mulch volcanoes

Mulch volcanoes can be particularly deadly to newly planted trees and shrubs, Wade said. It causes problems in two ways.

First, it encourages roots to grow the wrong way. At first, the conditions in the top of the volcano are moist and attractive to roots while the waterlogged soil under the volcano suffocates deep roots. The roots grow up, rather than down.

In the long run, though, mulch can't hold nearly as much water as the soil. So when the mulch volcano dries out, the plants can be severely stressed.

Second, the volcano can become an umbrella, shedding water to the surrounding area. Fungal activity can make the umbrella-shaped surface become resistant to wetting. Water runs off the mulch, rather than moving into it. This is more common, Wade said, in high-carbon mulches like ground wood, wood chips or sawdust.

You need to remember, he said, that newly planted trees and shrubs are still like container plants, making their living from the original root ball. If the volcano-umbrella keeps the root ball dry, the plant will struggle and may not survive.

More problems

And that's not all, Wade said. Mulch volcanoes can cause problems for even established trees. They can:

  • Keep plants from getting enough oxygen, resulting in root death and decay.

  • Make the lower trunk more constantly moist, promoting fungal canker diseases.

  • Cause stress from poor gas exchange by the cells in the bark, resulting in bark death and decay.

  • Invite damage from termites and rodents that may live in the volcano.
The key to proper mulching, Wade said, is to take a good look at how nature mulches plants. The natural mulch of fallen leaves is flat and rarely more than 2 inches thick. It never looks like a volcano.

A 6-inch layer of bark will last a long time, he said. But it may restrict gas exchange in the soil and may keep the soil too wet during rainy periods.

A mulch layer 3 inches deep after settling is enough for most plants. If you can, extend the mulched areas out to the outermost leaves (called the drip line) and beyond. And pull the mulch back a few inches from the main trunk.

Mulching properly will help keep your plants healthy, Wade said. Mulch volcanoes can have the opposite effect.

(Dan Rahn is a news editor with the University of Georgia College of Agricultural and Environmental Sciences.)

Dan Rahn is a news editor with the University of Georgia College of Agricultural and Environmental Sciences.