In general, you should prune crape myrtles in spring when they are dormant. Wait until the temperature is around 10 to 15 degrees F and then take a look at your shrub. You may need to remove some branches that have broken off or become weak from disease or winter damage. If your plant has small twigs, these can be removed as well. Once again, wait until the weather cools down before removing any larger branches from near the ground since this will make it easier for diseases such as powdery mildew to enter the foliage and spread throughout your plant quickly.