Tough Strokes' Aster can withstand hot, humid weather. More plants for humid conditions: http://www.bhg.com/gardening/gardening-by-region/south/top-native-plants-for-southern-gardens/?socsrc=bhgpin062112#page=7
Catmint: Nepeta, or catmint, the plant has rich blue flowers that stand up to heat and drought. Plus, after they finish blooming, you can shear the plant back by a third of its height and it’ll bloom again in the late summer and early fall. Taller varieties that grow 3 feet tall. Shorter catmints that grow just 12 inches tall excel at the border’s edge.