There are a number of reasons for pruning trees and overgrown shrubs from November through March:
1. Some trees should only be pruned during the dormant season due to the threat of insect or disease. Oaks (oak wilt disease), American elms (dutch elm disease), ash (emerald ash borer) and crabapples (bacterial fireblight) are some that this is especially true for.
2. Leafy plants store their nutrients in their leaves, than send those nutrients back to the roots in the fall. Heavy pruning in the summer can rob the trees of this valuable energy source. Pruning in the dormant season is a special advantage when renovating overgrown shrubs. Did you know that some overgrown shrubs (such as privet or forsythia) can be cut right to the ground and will re-grow into manageable, smaller shrubs the next growing season?
3. We prune many trees to train them for stronger, storm resistant structure. This is especially helpful to younger trees. It is much easier for a trained arborist to evaluate the structure of a tree when the leaves are gone. It is also easier to identify hazards in larger trees.
4. If large equipment is required for trimming or tree removal, frozen ground reduces the chance of damage to your lawn. Also, many folks have wonderful plantings of annuals and perennials under their trees- pruning in the winter eliminates the potential for damage to these seasonal plants.
Fall through March is an excellent time to evaluate your trees and shrubs for dormant pruning schedules.