Injury and infection started by a lawn mower or weed trimmer can seriously threaten a tree's health.The most severe injury occurs when the tree bark is slipping ( loose due to cambium growth) in early spring during leaf emergence, and in early fall during leaf drop.The site of injury is usually the root butress,although large wounds are more serious,repeated small wounds can also add up to trouble .Injury can be prevented by the removal (by hand trimming) or prevention (use of a herbicide or mulch) of grass and weeds from growing at the base of the tree.
Wounds are serious enough by themselves, but the wounded tree must also protect itself from pathogens that invade the wound. These microorganisms often attack the injured bark and invade adjacent healthy tissue, greatly enlarging the affected area. Trees can be completely gridled from microbial attack following injury. Decay fungi also become active on the wound surface, and structural deterioration of the woody tissues beneath the wound will often occur.