Crape Myrtles are slow to leaf out in the spring but once they are breaking dormancy, you can easily see if there is winter damage that needs to be pruned out. This is a great time to prune, thin, and shape up these beautiful trees.
One way is to train them to tree form, such as the white blooming cultivar 'Natchez' seen in the forground of the photo above and the other is to maintain them as a multi-stemmed shrub like Andre has the pink blooming cultivar 'Hopi' pruned in the background of the photo above. See photos below to see how these shrubs looked after pruning in the spring of the year!
In our area of the Shenandoah Valley, the hardiest hybrids of crape myrtle can usually be trained into tree form by pruning out all but 1-6 main stems. As it grows each year, prune out any lower branches that may develop along the main trunks. Because crape myrtle blooms on new wood (current season's growth), even tree forms bloom more prolifically if they are pruned or thinned in the late spring. Prune out crossing branches and those that are growing into the center of the canopy. If necessary, remove dried flower clusters from the previous season.
In the colder areas of Zone 6, crape myrtles may experience some severe die back over the winter. In these colder areas, you may need to treat your crape myrtle as a shrub rather than as a tree. Once they break dormancy in late spring, prune these shrub forms back to about 12" - 18" or sometimes even lower depending on where there is live wood. Your crape myrtle will then flush out in beautiful new, fresh growth that will bloom well and remain compact.