Answer: It sounds like your crepe myrtles are not tolerating the cold winter temperatures very well; some crepe myrtle varieties are simply more cold tolerant than others. In any case, since they bloom only on new wood, the main effect of the winter dieback is to delay their bloom a bit and to restrain their over-all size.
In general they should be pruned in late spring to both remove any dead wood and to shape the plants. Since they leaf out so late in the spring, it is important to be certain the wood is truly dead before removing it. This can be difficult! Start near the tip of a branch and work your way toward the ground, cutting experimentally. Look very closely for a fine line of green inside the bark. If there is green, or if you can see a hint of buds beginning to break, it is still alive.
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