Why are trees pruned and why is winter the best time to prune?
Trees are pruned to preserve their health and appearance and to prevent damage to human life and property. Health reasons for pruning include the removal of rubbing and/or crossing branches, to decrease the chance of self-wounding, and removing insect and diseased affected branches to prevent decay from spreading. Safety pruning includes pruning branches that may impede vehicular (when you are sitting on your riding mower) or pedestrian traffic and removing branches that interfere with lines of sight for automobiles, bicycles and pedestrian traffic.
Pruning should be done when it is cold for several reasons. It is easier to make pruning decisions without leaves obscuring the branches. Diseases are less prevalent at this time and fresh wounds are only exposed for a short time before the sealing process begins to occur. Oaks and elms are particularly susceptible to diseases and dormant pruning greatly reduces the possibility of infection.
Unless absolutely necessary for safety reasons, do not prune while the leaves are expanding until they are fully mature. Energy reserves are limited at this time leaving little for defensive activities like wound sealing and compartmentalization. Also avoid pruning during the leaf color season in the fall when fungi are sporulating and absorbing roots are forming.