The species Buddleja lindleyana (http://davesgarden.com/guides/pf/go/57955/#b) survives and prospers in Gainesville FL/z9a and is very attractive to butterflies, bees and hummingbirds. It is very drought tolerant and seems to require no fertilizer. It is colonial and will spread out from the original, with suckers showing up as much as a foot or two away, but they can be dug out if you want to control the spread. However, it doesn't appear to seed about.
I have had no luck with any of the B. davidii cultivars as they all eventually die of spider mite infestation due to drought intolerance.
B. x Weyeriana and B. indica also work and slowly develop into large clumps.