It's not hardy in 5; it's barely hardy here on Cape Cod and we are (officially) zone 7.

There was a thread on this topic on this forum a while back, you might try searching for it, the discussion included some good suggestions for alternatives.

Yes...crepe myrtle is not hardy in zone 5. If you buy one that would normally grow into a 15-20 foot tree in the south, the best outcome in your zone would be that it dies back to the ground each year and sprouts from the base--giving you a small shrub by the end of summer. And in zone 5, it may not even come back from the base. I had a crepe myrtle for a number of years and it did grow to about 10 feet tall before it bit the dust during January of 2004.

I have a a variety of Crape Myrtle that is tagged for zone 5. I didnt take any chances. I planted it up in a 15 gallon planter and I let it go dormant outside in the fall. Leaving it outside to about mid-november and place it in my cellar for the winter. Watering it only to prevent dissication. Around late march I water it well and about April 1st I set it outside. It leaf out in midmay and flowers every July.