Crape myrtles tend to grow numerous suckers from its base, and therefore do require some pruning every year — but only minimal pruning. Early training will help eliminate any extensive pruning later on. Extensive pruning or cutting back of crape myrtles each year only causes them to vigorously grow back what was removed. The only pruning that should be done each year is to remove suckers and to maintain its attractive shape by removing deadwood and seedpods. And heavy pruning in the winter will not help or force crape myrtles to bloom more.
Crape myrtle varieties come in all shapes, colors and sizes. Pruning large ones into small ones doesn’t make sense. If you want a small, manageable crape myrtle that looks like a shrub, buy a smaller variety. Whacking off and scarring up large crape myrtles each year serves no purpose.