Crape myrtles are generally cold-hardy to USDA Zone 7 or possibly 6, depending on the cultivar. They bloom on the current season's growth, so some gardeners in cooler zones try their hand at growing them as perennials.
You can't beat a tree that blooms long and beautifully in the heat of summer. Pair that with a multi-trunked form, compact habit and lovely bark in shades of cinnamon, cream, gray, brown or white (and sometimes even good fall foliage color) and you have a winner. In the South, the crape myrtle (Lagerstroemia indica) shows up as often in parking lot islands as it does in front yards, and nobody seems to tire of them. Gardeners in USDA Zones 7 to 10, where crape myrtles thrive, need only worry about what color to choose and how to mix things up with early-, mid- and late-blooming varieties so the bloom extends from mid-June to September.
For gardeners who live in colder climates, sometimes it's a trip to a southern beach that does it. They see the crape myrtle and wonder if they can somehow manage to have a little crape myrtle they can call their own.
For sure, gardeners in USDA Zone 6 have an excellent chance with cultivars that are known to have more cold hardiness or any that have parentage from Lagerstroemia fauriei, a slightly hardier species. Good choices along either of these lines are 'Sarah's Favorite' (white) and 'Velma's Royal Delight' (magenta). In this zone, gardeners know not to yearn to see the plant grow to the magnificent form and heft of southern regions and are happy enough with blooms and some wood.
Gardeners in zones where the crape myrtle is marginal or who grow crape myrtles as perennials don't get a chance to see one of the tree's attributes — smooth and sinewy bark in shades of cinnamon, gray, cream or tan.
In USDA Zone 5, gardeners who love crape myrtles experiment with growing the hardier ones as perennials or woody sub shrubs. In winter the tops are likely killed to the ground, but in spring, a flush of new growth appears. Growing crape myrtles as perennials doesn't give you beautiful peeling bark or the lovely tree form, but if the summers are hot enough, gardeners still get the blooms on shrublets about four feet tall, a welcome site on any July or August day.
Gardeners in Knoxville, Tennessee, learned about that phenomenon the hard way. One night during the winter of 1984 the temperature dropped — dramatically and uncharacteristically — to -24 degrees F. Quite a few marginal plants didn't survive in that USDA Zone 6b region, where 0 degrees is considered cold. Among them were the crape myrtles — or so it seemed. As winter waned, people pruned away the dead limbs and trunks. When spring came, a flush of new growth sprung up around the stumps, and since crape myrtles bloom on new wood, that first summer those stems flowered.
Other tips for improving hardiness: