Tree wounds allow disease to enter the tree through the trunk and spread throughout the tree. If a trimmer has injured a tree, trim away the broken bark with a sharp knife. Carefully remove the bark around the wound, making sure not to cut to deep into the tree. Shape the wound into an elliptical shape, pointing downward. Do not paint over wounds, as this will seal-in diseases and infections. The tree will naturally fight the spread of these agents and heal the area.