The late Fleming brothers had a serious case of zone denial - in the frozen tundra of Nebraska they bred hibiscus and crepe myrtles. Their primary goal of course was increased hardiness. This selection of crepe myrtle, Lagerstroemia indica 'Red Filli', is about as hardy as you can get in the genus. Reports from up north say this plant is hardy as a die-back shrub to zone 4. I'm betting that is with a whole lot of mulch. We were also told that it would grow like a ground cover 18" tall by 36" or more wide. It may grow like that in Nebraska but here in crepe myrtle country at the JC Raulston Arboretum, it is forming a small, multi-stemmed shrub. In the 5 years we've been growing it, 'Red Filli' has become a 4' shrub that begins flowering early with big clusters of bright pink flowers. The foliage is tinged purple and deepens to plum with red highlights in fall. So far, it has been fairly mildew resistant. It is exceptionally long blooming, starting as early as the beginning of June and lasting through September for a flower show that covers a full third of the year. We're not sure quite how large it will ultimately grow for us, but if treated as a cut-back shrub it should stay in the 4' range although it will not flower until mid-July. Full sun and average soil.
Follow me at @jcramark because life is too short for boring plants.