mulch volcano

Mounds of mulch that is piled up around the trunk at the base of trees have earned the name “mulch volcano.” Harmful to trees, this ill-advised yet well-intentioned practice is becoming more commonplace.

William M. Johnson/Courtesy

How do I decide on a topic or topics to discuss in my weekly garden columns? Typically, I do not have a firm concept until the weekend when I reflect on the gardening issues presented over the preceding week. Not so with this column.

One homeowner indicated that after reading last week’s column about termites and improper mulching along a home foundation, he immediately went outside to his yard and removed mulch that had been applied over the first layer of brick at his home. He indicated that a yard maintenance crew had applied the mulch. He also indicated that his first thought was to call the yard maintenance crew and demand that they remove the excess mulch but had second thoughts about the matter since he had paid for the excess mulch (and its application).

Then another reader emailed me the link to an online article published on March 14 by the Galveston Daily News entitled “Volcano mulching kills trees.” It was authored by Robert Komarowski who resides in League City. Mr. Komarowski was intrigued about the concept of volcano mulching and concluded his article with the following query: Perhaps Dr. Johnson could provide us with his opinion regarding mulch volcanos.

Voilà! The topic for this week’s column had been presented to me (and I finally get to use my high school French course). If that circumstance was not a sufficient matter of serendipity, I then came across another action that yelled write-about-me. While on an evening walk around my neighborhood last Friday, I walked past a home that had been recently mulched. Even the landscape beds around my HOA (Homeowners Association) are improperly mulched. Now I’m thinking it’s payback time — I should send my HOA one of those dreaded letters.

I do not routinely carry a camera on my walks so I had to lug a camera along on my next evening walk to capture the photo accompanying this column. Not only had the tree been mulched volcano-style but the front landscape beds along the concrete foundation had been mulched above the first layer of brick.

The benefits of mulch

Although mulch is not particularly glamorous, it can be your best friend. It can be used in a variety of locations to help with weed control and to help save water.

Research conducted through Texas A&M University shows that two-thirds of the water applied by irrigation is lost through evaporation. That means the water evaporates from the soil surface without ever being absorbed by the plant. When mulch is applied, evaporation is limited, leaving more water for plants to use and thereby reduces water application. Not only does mulch conserve water, it also moderates the soil temperature during our hot Gulf Coast summers. In winter, mulch helps to keep the soil warmer and promotes early root and shoot growth in the spring.

Disease control is another benefit of mulch. Many soil-borne diseases in the vegetable garden can be reduced or prevented by eliminating “splash up” from the soil from water droplets. Mulch creates a barrier between the soil and the plant, which keeps the disease-causing pathogens (such as fungal spores) from splashing on plant leaves.

Mulch should be applied to a 3-to-4 inch depth to provide maximum benefits. This depth is thick enough to reduce soil moisture loss from evaporation but will allow water that is applied by rain or irrigation to filter down and promote the development of a strong root system.

Properly applied mulch around the trunk of a tree has the added benefit of providing a physical barrier around the tree’s trunk which helps to reduce damage from line trimmers.

What is volcano mulching?

Mounds of mulch that are 10, 12, 18 or more inches thick, piled up around the trunk at the base of trees have earned the name “mulch volcano.” Harmful to trees, this ill-advised yet well-intentioned practice is becoming more commonplace.

Mulch applied too thick on top of tree roots results in suffocation of the existing uppermost roots. In a struggle to survive, the tree then grows new roots into a mulch volcano. Trees growing under these stressful conditions can actually appear healthy for a while. Eventually though, these aboveground roots can encircle the tree and strangle it to death as the trunk increases in diameter.

The root zone of an established tree extends far beyond its drip line, so mounding the mulch against the trunk does little for the roots. The mulch volcano can hold the water that was intended for the roots. Also, moisture trapped by the mulch can soften the bark of trees, opening the door to insect pests and disease problems.

Now is a great time to get out there and mulch new beds or replenish mulch to existing beds. If you’ve never used mulch in your landscape, this is a great time to give it a try. You will be amazed at the fresh appearance it gives your yard right now and the water savings you’ll see in the future.

Just be sure to say NO to volcano mulching.

Dr. William M. Johnson is a horticulturist with the Galveston County Office of Texas A&M AgriLife Extension Service. Visit his Web site at

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