Depending on the climate, there are two times in a year to trim crepe myrtles. Major pruning is best done during the late winter or early spring. Late fall is a good time to cut branches and prune off dead wood.
Plants are dormant during the winter season, and flowers only appear after a new growth, so trimming the crepe myrtles in the early spring or late winter encourages the growth of new branches. Pruning during the late fall is best in mild climates. The United States National Arboretum advises against pruning in the late fall because it may result in cold injury.