This is limestone wild petunia, Ruellia strepens. In Michigan, it has been known only from two spots here in Lenawee County. At one spot it has almost died out. The last search only turned up eleven plants. At the other, the new owner yells and curses at you if you ask permission to go there. Now there is a third, about three miles from the yells and curses. All the owners there are nice quiet souls who are very hospitable. I've already found these plants, and sure look forward to exploring more.