Crepe Mertle Care
I apologize for the delay in this response. As for pruning Crapemyrtles, it is similar to pruning any other flowering tree/shrub. In most cases, Crapemyrtle needs very little pruning. The most common mistake, and it's a big mistake, is topping the plant. For some reason some folks think these plants should be cut back hard and that's just not the case. In fact, topping on this plant or any other woody plant is detrimental to it's health.
We have a great publication on this topic that also refers to other resources. Please checkout: http://pubs.ext.vt.edu/430/430-451/430-451.html
Thank you for contacting eXtension.