Home Gardening

Volcanoes are coming!
The Coast Gardener Newspaper and Web Column - April 28, 2008

When you hear the word volcano what comes to mind? Some people may think about Hawaii and the Kilauea Volcano. Astronomers might envision Olympus Mons on Mars. It's the biggest volcano in the solar system and is as big as Arizona.

Typical mulch volcano with the mulch mounded high around the base and trunk of this tree.

Still others remember bad Hollywood movies. How about “Volcano” (a real and original title) about a volcano forming under Los Angeles?

I do not want to alarm anyone, but there are multiple volcanoes forming in our Mississippi communities as you read this very column.

These volcanoes do not spew lava and noxious gases and only reach a height of about 12 inches, but they are just as damaging. Every year in the spring they begin to form. Some are cone shaped and some have wide, high-rimmed caldera. Generally we find them forming around the bases of trees. The volcanoes I am talking about are mulch volcanoes.

Mulch volcanoes form because we love our shade trees, sometimes a little too much. We read articles in the newspaper and gardening magazines, watch all of those gardening shows on HGTV, and listen to landscape professionals. They tell us to spread mulch around the base of our trees to help reduce weeds, cool the soil, and most importantly, to conserve precious water in the tree’s root zone. And so we mulch, and mulch, and mulch. No matter that most recommendations tell us to use only a 2” to 3” layer, 6” to 10” has to be better. WRONG!!!!!

The roots circling the trunk of this tree grew in this manner because of excess mulch being applied around the trunk of this tree.

Circling roots are another problem that is commonly found in mulch volcanoes. In this moist environment a tree will begin to grow roots into the mulch instead of outward into the surrounding soil. Most mulch volcanoes are circular, since most homeowners mulch in a circle around trees, and roots will start to circle the tree staying in the mulch. An example can be seen with bedding plants or any plant grown in a container. Many consumers will look at the roots of a container-grown plant before purchasing. What are they looking for? Why roots that are circling the inside of the container, of course. These roots never grow out into the soil and the plant will not perform well in the landscape.

The tree in the background is surrounded by a mulch volcano while the tree in the foreground in mulched correctly.

The same is true for roots growing in a mulch volcano.  As the tree grows the circumference of the trunk grows larger.  Eventually the circling roots begin to strangle the tree.  However, the tree does not die immediately but grows through a prolonged period of decline. 

The proper way to mulch a tree is to first spread an even 2” to 3” layer around the base of the tree. The diameter of this mulch is up to the homeowner. I personally like big mulch beds around trees to ease lawn mowing and reduce edging, but this is a topic for another column.

After the mulch is applied, use a rake or by hand pull the mulch back away from the tree trunk. It is OK to leave a thin layer so there is not any bare soil exposed as long as the mulch does not touch the tree trunk. As the mulch is pulled back, contour the mulch to resemble a bowl. This will help to collect water and direct it towards the root system of the tree during rain or irrigation.

Written by Dr. Gary R. Bachman, Assistant Extension Professor of Horticulture, Coastal Research & Extension Center.