We have been preaching to anyone who will listen for years. Do not top, pollard, knuckle, shorten, etc. your crepe myrtles. It is bad for the tree's health, it looks bad, everything about it is wrong. It doesn't matter if your parents, grandparents, neighbors, or friends have done this for years. If they or you are still doing this... stop, pretty please.
How to Fix Your Crepe Myrtle
If you have been topping your crepe myrtles for years, there is a way to get them back on track over time. If you have a knuckled mess from years of trimming branches to the trunk, you are going to have to top the tree for the last time. Cut just below the knuckled mess that has been created. If your tree was clear cut at the tops, then you can skip the previous step.
Next you have to wait a few weeks until new growth starts at the top. There should be a lot of small, new growth. Pick a few to keep from the tops of each trunk that are growing up and away from the center of the tree. The rest can be removed with pruners. You will let these continue to grow. Every year to six months, you can remove the smaller new growth and branches that form that grow towards other branches. It is going to take a while to look better. We are talking three to five years. If you are impatient, look into transplanting a new tree to replace it.
Train Your Crepe Myrtle to Grow the Right Way
Traditional pruning, or training, for crepe myrtles should keep splits in branch growth in threes. While not definitive, a good guide to go by is three limbs every three to five feet of height. Start of with three, then nine, then twenty seven. Always train your tree branches to grow up and away from the center. Remove small limbs that cross other limbs. Your goal for crepe myrtles is to have them resemble flowers in a clear vase meaning blooms up top and a center you can generally see through.
Get Rid of Crepe Myrtle Suckers
Small pruning for suckers can be done anytime they pop up. Those are the micro branches that pop out of fresh cuts or just about anywhere. Take some pruners and remove them, especially from the base of a mature tree.
What If My Crepe Myrtle Is Too Tall
If your crepe myrtle is too tall, you need a different variety of crepe myrtle. I know it is not what you want to hear, but cutting the top off looks bad and the point of a crepe myrtle is to look good. Transplant your existing crepe myrtle where it can grow and plant something else in it's place.