String trimmers that use a monofilament line for cutting down weeds and grass can be very damaging to young trees. Young trees have relatively thin bark. If the line is allowed to hit the trunk, part of the bark will be removed with each contact of the line. If you are not careful, you might even remove an entire ring of bark all the way around the trunk, girdling the tree. Mowers pushed hard up against or dragged around the base of young trees can be almost as damaging.
The part of a tree’s circulatory system that carries food manufactured by the leaves to the roots (which can make no food for themselves) lies just under the bark. The damage that occurs when mowers or string trimmers remove patches of bark interferes with the tree’s ability to send food to its roots. As the roots are deprived of food, they become stunted and function poorly, and this leads to a stunted, unhealthy tree. Remove a complete ring of bark, and you may cut off food to the roots altogether. Then the roots die of starvation, leading to the death of the plant.
In addition to interfering with food movement, open wounds created by mowers and trimmers can provide entry points for disease organisms that can cause decay.
Many sickly, stunted trees that have been planted for years but don’t grow well have been damaged in this way. Look at the base of their trunks, and you will often see scars and callus growth from repeated injury done to the base of the tree.
To prevent these problems, do not allow grass to grow close to the base of young trees for the first three to five years after planting. Keep an area at least a foot out from the trunk grass-free (2 feet or more is better). A mulch 2 or 3 inches thick spread evenly over the area, but pulled back slightly from the trunk, will help a lot. Any stray weeds can be killed with a quick spray of glyphosate herbicide, if necessary.
Shrubs are generally planted in beds so are less at risk. But I have seen this problem occasionally when ground covers, such as Asiatic jasmine, are trimmed away from the base of shrubs with string trimmers. Whether you maintain your landscape yourself or pay someone to do it for you, don’t let this kind of needless damage happen to your trees and shrubs.
The LSU AgCenter and the LSU College of Agriculture