Contrary to popular belief, this is not the way to prune your crape myrtle trees. Do not be led to believe this is necessary by landscape crews or neighbors!
The only "major" pruning Crape Myrtles require is to reduce the number of trunks on young trees leaving somewhere around 3 to 7 permanents trunks. Each year around late winter or early spring, all that is needed is to remove any new suckers that appear from the ground or from you main trunks. It is best to remove them before getting thicker than a pencil.
There’s no telling how many thousands of Crape Myrtles are incorrectly pruned/beheaded each year. An unpruned Crape Myrtle is almost always superior in appearance to a "professionally" pruned/topped one. If you are beheading a Crape Myrtle because it is too tall, you are actually making things worse for yourself. Trees respond to this kind of pruning by increasing growth over the next season to recover from the beheading. You will quickly be right back where you started, plus battle scars.
If you have the space, you can get a variety that will grow up to 30 feet tall. But if you are looking to plant a Crape Myrtle and are worried about it eventually growing too large, ask your local nursurey about one of the many smaller or dwarf varieties that won't grow above 10-12 feet tall. Zero pruning, ever!
Texas SmartScape principles include avoiding unnecessary pruning, so next time you get the thought of pruning your Crape Myrtle, don't. Sit on the porch, drink iced tea, and revel in the beauty of your SmartScape.