Purdue Extension Service

Vanderburgh County, Indiana

Using Mulch in the Garden

By Larry Caplan, Extension Horticulture Educator, Vanderburgh County, IN

For the Evansville Courier and Press, April 5, 2009

I've had lots of questions about using wood chips from the ice storm cleanup.

First of all, there is nothing wrong with using fresh wood chips in the landscape. There is a concern that the chips will heat up as they break down, and this heat can injure tender perennials and shrubs. But if you don't spread your mulch too deeply, this shouldn't happen.

Wood chip mulch should be spread about 2 to 4 inches thick over the landscape beds. Any less than that, and we get no benefits. The weeds will push their way through, and the soil will still dry too quickly.

Too much mulch, though, can cause problems. A deep mulch layer (6 inches or more) across a landscape will encourage the roots of trees and shrubs to move upwards so that they can get oxygen. This can continue to the point where the plant's roots are growing in the mulch, and not in the soil. There aren't many nutrients in mulch, so the plants may start getting deficiencies. Also, the mulch may dry out quicker than the soil, causing more drought injury. Trees may blow over easier, since the mulch can't hold the plant down as well as soil can. And a thick mulch layer will heat up as the composting process begins.

Piling the mulch high around the trunk of the tree can lead to stem and root rot. Unlike the roots, the bark on the trunk is not designed to spend long periods of time in wet conditions. A "mulch volcano" can hold enough moisture against the trunk that eventually the bark and underlying wood start rotting away.

Wood chips generally do not attract termites or carpenter ants to your home. If there were insects in the damaged tree, the action of cutting it apart and shredding it will disrupt the colony, leading to a quick death of the queen and most of the larvae. Termites need a constant supply of moisture in order to stay alive, and a 2 to 4 inch mulch layer dries out too quickly for a colony to survive. If you do find termites or carpenter ants in the mulch, they most likely came from an infestation that was already on your property, either from a nearby tree or an already-infested home.

It's not very likely for disease or insect pests to spread from the mulch to your plants. Any borers in the wood will quickly die as the chips dry out. Any insects that hatch out from overwintering eggs will likely starve to death before they can reach the edible part of your tree. Likewise, most disease agents won't survive long in dead wood; they need live tissue to infect.

For more information on mulch or storm recovery, stop by the Purdue Extension Booth at today's Tri State Home Show, or call the Extension office at (812) 435-5287.