The best time to cut back or kill crepe myrtles is in the spring after they bloom. The cutting should be done during the day because otherwise, nighttime temperatures can cause them to rebloom.
What causes crepe myrtle decline?
Some things that may cause decline include: woodpeckers and other wildlife feeding on them; heavy rains washing soil chemicals into their roots; lack of winter protection; too much nitrogen fertilizer; disease such as powdery mildew fungi which are difficult to control with garden fungicides even when applied correctly twice a year.
How do I get rid of Japanese beetles on crepe myrtle trees?