The Lawn Mower vs. the Tree Trunk

When the lawn creeps too close to tree trunks, a lawn mower or weed trimmer can damage the tree by cutting into the base which can hamper the movement of water and nutrients. Wounds also allows fungi and wood-rotting organisms an opening to the heartwood of the tree which can weaken or even kill the tree. There are a number of ways to prevent tree trunk damage caused by mowers and weed trimmers; remove grass or weeds that get too close to the tree, spray an herbicide to kill off vegetation, add a sufficiently large circle of mulch, or put a tree guard around the tree (but don’t let that guard squeeze the tree trunk as it grows).
See the Purdue Extension guide, Mechanical Damage to Trees: