Research at the Morton Arboretum showed clearly that creating an 8 foot diameter mulch ring around a newly planted tree greatly accelerated tree establishment. Five years after planting, mulched sugar maple trees had root densities in the top six inches of soil that were four times those of trees with turf growing up to their trunks. Diameter increase of mulched trees was three times that of un-mulched trees. Undoubtedly, competition for water and minerals between the roots of trees and turf was responsible for some of the observed growth inhibition. There may, however, have been more to the story. Certain grass species are known to exude compounds from their roots, called allelochemicals, which inhibit the growth of other plants. The term used to describe this phenomenon in which roots of one plants give off chemicals that inhibit the growth of another plant is allelopathy.
The best know example of allelopathy is the effect of walnut trees on a wide range of other plant species. Any experienced gardener knows that it is a waste of time to plant tomatoes anywhere near a walnut tree. Walnut roots exude a chemical called juglone, which is toxic to many plants. It is, however, a little known fact that roots of tall fescue give off compounds that inhibit the growth of trees, including walnut, pecan and sweetgum. While the exact identity of the inhibitory chemicals exuded by fescue roots is not known, their effects can be dramatic. Stunted walnut trees in a planting at the University of Missouri Southwest Center at Mount Vernon increased their growth rates dramatically when a 6-foot strip if fescue was eliminated down the tree rows. This suggests that mulching of even large, established trees of certain species may be advisable if they are growing in fescue turf.
In a thought provoking review article on allelopathic effects on trees, Chick and Kielbaso suggest that this problem can be managed using various approaches. In addition to mulching, it may be possible to select fescue genotypes with low allelopathic effects. Also, improving soil drainage may reduce allelopathic effects by allowing leaching and increasing microbial degradation of allelochemicals. Keep in mind, however, that trees and turf are adapted to entirely different ecosystems and it is not surprising that they sometimes resort to underground warfare. 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.
REVISED: August 1, 2012