David Robson: The results of too much mulch

David Robson

Most gardeners know mulch is the key to surviving summer’s heat and winter’s cold.

Applied correctly, mulch can cut down the amount of time you spend weeding and watering. On the other hand, applying mulch so it looks like volcanoes about to erupt with the plants in the middle can lead to insect and disease problems.

The ideal mulch layer of wood chips or bark is never greater than four inches, except in the winter. Watch out if you use cocoa bean –– keep mulch layers to less than one inch or you’ll discover how quickly cocoa beans can grow mold.

You can use grass clippings, too, but they’re probably best kept in the vegetable garden. Avoid using clippings for four mowings if you’ve used weed killers on the lawn.

In this age of cutting costs, you probably can get by with only two inches of mulch around most of your vegetables and annual and perennial flowers.

What happens if you apply too much mulch?

1. You mess up the air and water movement between, above and below ground. Roots need both to survive. Putting on an inch or two of extra mulch makes it harder to wet the soil, and plants may start to wilt.

Part of the problem is mulch can absorb water, especially as the materials start to decompose. It’s nothing like sphagnum moss, which absorbs 40 times its weight in water. But when a sprinkling would help your tomatoes or roses and the mulch prevents the water from soaking through, you’ve created a stress problem.

You can also look at the reverse. Too many wood chips can keep wet soil from drying out. Soggy soil may be worse than dry soil. With many new subdivision yards composed of clay, this can be a big problem.

2. Excess mulch around trees and shrubs can keep trunks wet. This leads to crown and root rot. The next thing you know, the bark is turning mushy and the woody plant falls over in a storm.

Soggy trunks also lead to insect attacks. Coupled with the rot, the plants can topple sooner rather than later.

3. Too many wood chips can lead to termite infestations. Termites like moist, humid environments.

Two to four inches of wood chips dries out readily under the summer sun, and termites won’t build their tunnels through such minuscule levels. But give termites six to eight inches of mulch, and they’ll be in termite heaven, feeding on the decaying wood chips.

Now, as long as the chips are around a living tree or shrub, they won’t bother the plant. But if the deep layers of chips are next to your house’s foundation, the termite scouting parties trumpet “Charge!” and you’ll need an exterminator.

4. Thick layers of fresh mulch may rob the soil of nitrogen, resulting in stunted plants. Wood chips and bark should be composted before being used as mulch, but that would mean creating a pile somewhere and allowing the wood particles to decompose for a year. That’s not going to happen, which is why a thickness of two to four inches is crucial.

5. You have the extra expense and/or work moving that much mulch around the yard. Save time and money for something more constructive, like sitting in the shade and reading.

When applying mulch, follow a couple basic steps.

1. Water the soil first, or wait until after a good soaking rain. While you don’t want a soggy soil, having some moisture in the ground means you won’t have to water as soon. The soil should be wet eight inches below the ground. An inch of water should do the trick.

2. Make sure all the weeds are killed. Mulch controls weeds, but if there are weeds already there, two to four inches of material probably won’t suffocate them. Pull the weeds, or use your hoe to scratch weeds away. If you irrigate beforehand or wait until after a rain, the weeds are easier to remove.

If weeds do pop up after mulch has been applied, they should be easy to pull. Ideally, you would mulch out to the drip line of the plant. Reality sets in soon if you have big mature trees and shrubs. Mulching out to the drip line may mean you have no grass to frolic on.

3. Mulch trees and shrubs in at least a three-foot radius from the trunk, which creates a six-foot ring.

David Robson is a specialist with the University of Illinois Extension. For more gardening information or for your local extension unit office, go to www.extension.uiuc.edu/mg. The Sangamon-Menard Unit Sangamon County office can be reached at 782-4617.