The reason for trimming and the time of year dictate when, what and how much to trim from a tree, explains Better Homes and Gardens. However, trimming for safety and tree health take priority over any other considerations.
Fires and damage sometimes result from branches that get close to power lines or to buildings. V-shaped branches are weak and potentially dangerous in snow and wind. In the case of power lines, experts recommend calling a professional to trim the tree safely.
Trimming to remove a diseased branch potentially prevents the spread of the disease. A 10-percent bleach solution for dipping between cuts minimizes the risk of spread. However, if the problem is oak wilt, trimming from late spring through early summer is harmful, because beetles feeding on the tree potentially spread the fungus more than a mile away.
Summer is good for removing dead branches when they are easy to spot. If the goal is to shape the tree or change its structure, the bare branches of late fall or early winter best reveal its shape. It is best to avoid heavy pruning during late winter and spring.