The correct way to prune crepe myrtles
The wrong way to prune crepe myrtles
Do not top or knuckle your crepe myrtle, sometimes called "crepe murder". This creates poor tree structure and a significant loss of the tree's stored energy. This practice will lead to increased disease, a poor appearance, and, if used consistently year after year, the death of the crype myrtle.
This is the irregular growth that happens when a crepe myrtle has been topped. It is very unhealthy for the tree and, I've gotta say, pretty ugly too.