Both this plant database and the Missouri Botanical Garden PlantFinder database list this plant as hardy to zone 8. The MBG database does, however, say it might be hardy in zone 7 if planted in a protected location.

It should be pointed out that at least 5 specimens of this plant have been growing in Oklahoma City gardens for many years. They do, of course, die to the ground in winter... but they all return very vigorously each spring. The two I am most familiar with grow into 5x5 foot specimens each year and bloom prolifically.

All of the photos I have loaded to this location are of a plant growing in Oklahoma City [zone 7a.]