Hi Lane, Kimberly here from landscape solutions for you. Thank you for asking about your white Crepe Myrtles. Crepe Myrtles should be pruned in February. This is the general rule of thumb and I recommend this to my clients as well as do it myself.
Always prune at an angle and I suggest to try and keep the Crepe Myrtles in their natural form. Trim off the dead flowers and branches associated with the flowers. If you prune in February, you will have a full tree come summertime. Thanks again for asking the question here at landscape solutions for you. Feel free to ask a question anytime. Hope your day is going great!
Kimberly