It almost doesn't matter how careful you are or how much you try and avoid nicking trees with a string trimmer or mower, over time it is bound to happen. The key is to not leave the damaged tree unattended, and there is a very simple way to help a damaged tree after a chunk has been taken out by a mower. This situation happen to one of my employees recently, which provided the perfect opportunity to show what should be done if a tree is damaged by a mower. The video below is a real situation where the rear tire of the the mower clipped the base of a tree and took out a 3" chunk of bark. Since the trees in the neighborhood are only a few years old, this damage could end up killing the tree if it is not attended to. It is important to recognize the damage right away and keep the bark that has been torn away from the base. By reapplying this piece of bark, like a missing puzzle piece, and duct taping the damaged area to prevent insects and other diseases from harming the tree during its recovery, the tree should heal over a 3 to 6 month period of being wrapped up and cared for.