Most trees can be pruned at any time of year. Some exceptions are American elms and crabapples, which should be pruned in the winter to abate disease spread. Aspen, silver maples, birch, and walnut trees exude sap if pruned in the later winter or early spring, but this will not harm the tree.
When pruning, remove all dead, damaged, and diseased wood. Make clean pruning cuts adjacent to, but not flush with, tree trunk or leader. Leaving a small stub, or collar, will allow the tree to compartmentalize the pruning wound and heal it naturally. No additional wound dressing is needed. For additional information, see: Follow Proper Pruning Techniques