Most wouldn't think it, but DC is pretty damn tropical. Its summers were not terribly different from the Saudi summer I experienced--although I was on the more temperate area along the coast of the Red Sea, that just meant it was schorching and humid. In any case, I wouldn't want to be outside more than a few minutes in either heat.
Many plants, however, love that wet, oppressive heat. And there's a Yahoo group dedicated to growing those plants in DC: the DC Tropics group. I went to their swap last Sunday with some chocolate babka to share and walked home with more plants than I'd have space for even if I didn't have any other plants vieing for the space on my steps and in my community garden plots.
I also picked up a red stripe Canna, a Streptocarpus, and some Fatsia japonica seeds. The heirloom Hungarian peppers and tomatoes from that swap took a walk home with me, too, but they were generously donated for today's DC State Fair Seedling Swap!