Tips on Pruning a Crape Myrtle by Master Gardener John Phelps

When it comes to pruning crape myrtles, it would be better to leave them alone than to prune them improperly. We've all seen over-pruning - people chop back crape myrtles below the knuckles each and every year.
When a crape myrtle is pruned back too far it has two effects:Reduces the number of blooms that will be produced during summer.New branches will grow far too long and therefore not be able to support the weight of heavy blooms - particularly when wet. These long branches weep over and often break off during heavy rains.When a crape myrtle is pruned properly:It will produce twice the number of branches and therefore twice the number blooms as it did during the previous year.The new branches will be strong enough to support blooms.
Wrong-season pruning would mean November and December. Don't let "peer pressure" by neighbors and commercial gardening crews get to you. If you trim the crapes in the last two months of the year, and we get a warming trend in January or February, the trees might actually start putting on new growth. That new growth will be highly susceptible to freezing weather should it come on the heals of a warm spell. New growth will also tend to draw the cold right into the plant, causing needless damage to a tree that should be resting in dormancy.
So, the best time to trim crapes in is late winter or early spring - just prior to new growth emerging. For years, we've suggested this as a great time to trim them, because at that time we're also trimming back our roses and many other plants and trees.