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Dr. Gary Bachman Click to enlarge. Avoid dangerous ‘mulch volcanoes’ around trees

By Gary R. Bachman

MSU horticulturist and assistant Extension professor
Coastal Research & Extension Center

I don’t want to alarm anyone too badly, but there are multiple volcanoes forming in our Mississippi communities as you read this column.

When you hear the word “volcano,” you might think of active volcanoes in Hawaii or other places around the globe. Some might envision the big volcanoes in the solar system, such as the one on Mars that is as big as Arizona. Still others remember bad Hollywood movies like Volcano, about a volcano forming under Los Angeles.

Typical mulch volcanoes have been formed high around the base and trunk of these trees. This thick layer of mulch is bad for the trees and can cause bark decay, root circling, and other problems. (Photo by Gary Bachman) Click to enlarge
Typical mulch volcanoes have been formed high around the base and trunk of these trees. This thick layer of mulch is bad for the trees and can cause bark decay, root circling, and other problems. (Photo by Gary Bachman)

These volcanoes do not spew lava and noxious gases and only reach a height of about 12 inches. They begin to form early each spring around the bases of trees. The volcanoes I am talking about are mulch volcanoes.

Landscape professionals tell us to spread mulch around the base of our trees to help reduce weeds, cool the soil and conserve precious water in the tree’s root zone. We listen to their advice, and many go out and mulch and mulch and mulch. Even though recommendations usually say to use only a 2- to 3-inch layer, these gardeners think 6-10 inches has to be better. Wrong!

When a thick layer of mulch is spread around the trunk of a tree, several things can happen, and they are all bad. The mulch will indeed hold moisture, but it will hold it around the tree trunk where it will create conditions for the bark to decay. This allows fungi, bacteria and insects to get under the bark and cause problems internally.

Mulch volcanoes also can lead to circling roots. In this moist environment, a tree will begin to grow roots into the mulch instead of outward into the surrounding soil. Since most mulch volcanoes circle around trees, the roots will start to circle the tree much like the roots do in a container-grown plant.

Roots that have circled a container never grow out into the soil, and the plant will not perform well in the landscape. The same is true for roots growing in a mulch volcano. As the tree grows, the circumference of the trunk gets larger and eventually the circling roots strangle the tree. The tree does not die immediately but grows through a prolonged period of decline.

The proper way to mulch a tree is to spread an even 2- to 3-inch layer around the base of the tree. The diameter of this mulch circle is up to you. I like big mulch beds around trees to ease lawn mowing and reduce edging, but that is another subject. After the mulch is applied, use a rake or your hands to pull the mulch away from the tree trunk.

As you pull the mulch back, make sure there is only a fine layer of mulch close to the tree so there is no bare soil exposed. Also make sure the mulch does not touch the tree trunk. Contour the mulch layer to resemble a bowl. This will help collect rainwater or irrigation and direct it toward the root system of the tree.

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Released February 25, 2010
Contact: Dr. Gary Bachman, (228) 546-1009
1815 Popps Ferry Road, Biloxi, MS 39532

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