A “Safe Zone” is an area around a tree trunk in which grass and weeds are excluded in order to prevent damage to trees from the use of lawn equipment close to the tree trunk. The point is to keep mowers and weed whips well away from tree trunks. Weed whips can cut through the bark and into the vascular layers that are essential to tree health and growth. The damage often extends all the way around the tree.
Cuts in the trunk that go beyond the bark will first disrupt the downward flow of sugars from the leaves, leaving the roots without needed resources for health and growth. Then if wounded just a little further, the flow of water and minerals from the roots to the leaves is disrupted. Damage to these systems will also affect tree support, storage ability, and defense against the spread of disease and decay.
The total effect of damage to a tree by turf maintenance equipment also depends on the width and depth of the damage, as well as the size of the tree. Taking a half inch deep chunk out of a mature pine tree will probably not go beyond the bark, but the same chunk out of a newly-planted tree can extend well past water and sugar conducting systems. Weed whipping all the way around a young tree, cutting in only a small fraction of an inch, can fully disrupt the flow of sugars and water, cutting off sustenance needed for growth. The result is a tree that is barely hanging on, one that dies, or one that may fall over.
“Better to be safe than sorry” is definitely true in this case. Provide a safe zone for all of your trees and prevent one second of “oops” from wiping out a sizeable investment of money and time.
Community Canopy recommends a grass and weed-free safe zone be established for at least 18” from the trunks of trees. That would be a 36” diameter area for a newly planted tree. An outer barrier, such as plastic edging or cement curbing, could be used.
If there is already turf in this area, establish the safe zone by removing the grass and roots by hand. Alternatively, use a contact herbicide, such as Round-Up, to kill the grass, taking care not to get spray on the tree or tree roots.
A composted mulch layer will help reduce the germination and growth of weeds. It has many added benefits, such as maintaining soil moisture and improving soil fertility. Mulch should be at least 2, but no more than 4 inches deep, tapering to zero as it approaches the tree trunk. Periodic maintenance will be needed. As in establishment, this can be done by hand or by use of an appropriate herbicide.