Answer: Growing even the hardiest crepe myrtles in zone 5 is somewhat risky in terms of winter hardiness. The best you can do is mulch generously over the root area in late fall after a few freezes have occurred. You might also consider a windbreak if it is in a windy or exposed location.
Do not prune it now. The branches provide a certain amount of winter protection to the crown. Wait until late spring when the buds begin to swell on it and at that time remove any winter damaged branches, cutting back to live wood. This plant blooms on new growth of the season, so it should still grow and bloom for you even if it is killed back to the ground in the winter.
Crepe myrtles do not actually need much pruning apart from removal of dead wood, although in southern areas where they are not killed back at all they may need a bit of shaping or can be trained into a tree form. When winter damage is usually severe, as will probably be the case for you, they generally are maintained as multistemmed shrubs.
Enjoy your crepe myrtle!
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