Prune a crape myrtle by removing any crossing limbs or limbs that grow inward, and then removing any spent blooms to encourage more. To develop a tree shape, remove all but three to five of the strongest limbs, and remove lower branches of the trunk as the plant develops.
Remove dead or diseased limbs anytime you find them. Remove excess lateral growth to open the plant during its dormant season. Remove any branches that interfere with people walking under the canopy or that rub against houses or other structures.
If you prefer to keep the crape myrtle as a low, compact shrub, there are two options. One is to replace the larger plant with a smaller variety of crape myrtle and the second is pruning the stems to within 6 inches of the ground annually. If the plant is healthy, this type of severe pruning does not injure it.
Contrary to popular belief, the Clemson Cooperative Extension indicates crape myrtles do not require heavy trimming to encourage blooms. Instead, it recommends choosing the correct type of crape myrtle to minimize the need to prune the plant. They are available in dwarf varieties that reach maturity at 3 to 6 feet and semi-dwarf varieties that reach 7 to 15 feet, eliminating the need to top the plant.