Mulching defends trees from chemical attack by turf grasses

The grass in your lawn might be stunting the growth of your trees, says a University of Missouri Extension horticulturist.

Certain grass species, including tall fescue, give off chemicals that can inhibit tree growth, said Chris Starbuck. “The effects can be dramatic.”

The chemical inhibition of one plant by another is called allelopathy.

Creating a 6- to 8-foot diameter mulch ring around a newly planted tree can help overcome this problem and speed establishment before the tree has to do battle with turf roots, Starbuck said.

At the MU Southwest Center in Mount Vernon, stunted walnut trees doubled their growth rates after mulching eliminated a 6-foot strip of fescue along tree rows. This suggests that mulching can aid even some large, established trees if they are growing in fescue turf, he said.

Improving soil drainage also may reduce allelopathy by speeding the leaching and microbial degradation of the inhibitory chemicals.

Many tree species, including black walnut, wield chemical weapons of their own. “Any experienced gardener knows that it is a waste of time to plant tomatoes anywhere near a walnut tree,” he said.

“Trees and turf are adapted to entirely different ecosystems. It’s not surprising that they sometimes resort to underground warfare,” he added. “In many landscape situations, it may be best to create some turf-free areas where trees and shrubs are mulched together to simulate the forest floor.”