When  
Unlike many other tree species, pruning must be done at a very specific time of year. Because open wounds attract the elm bark beetle (the major vector for Dutch elm disease), pruning should never be performed from about mid-April to late-July.  In fact, some communities have bylaws to this effect.  Also, due to presence of a variety of fungal spores in the fall, if possible, pruning should be avoided.  This leaves early spring as an ideal pruning season.  With the growing season to follow, the tree has ample time to close the wound and regain its vitality. 

How Often    
Mature deciduous ornamental trees such as elms should be thoroughly pruned once every three years with annual minor pruning.  In areas where Dutch elm disease is present, more vigilance is required to allow timely removal of dead or dying branches.  If a tree is noticeably slow to bounce back from a pruning session, less frequent pruning may be in order.

Pruning Pitfalls