I'm not positive but based on the hairy leaves, I believe this is Ruellia humilis, commonly known as Fringe-leaf Wild Petunia, Hairy Ruellia, Low Ruellia, or Low Wild Petunia. If you take a look at the Lady Bird Johnson Wildflower Center's listings for Ruellia, you can see why I found it difficult to be certain. It could be Carolina Wild Petunia, Ruellia caroliniensis ssp. ciliosa var. cinerascens. I'm hoping it's the R. humilis, since that one is a larval food plant for several species of butterflies, including Buckeyes. The LBJWC says the plants prefer moist soil, which might explain why they're doing so well this summer. In my garden, the plants range from 6 inches to 14 inches tall, some of them upright, some sprawling. I much prefer these pale lavender flowers to those of Ruellia cultivar 'Katie' and I also prefer the mannerly behavior of this wild variety. Although it reseeds for me, it does not do so aggressively and seedlings are easy to pull. The behavior of 'Katie', on the other hand, is best described by the words of my friend Genny, who has spent years attempting to eradicate it: she calls it "the cockroach of the garden".
Thank you to my delightful friend Gail at Clay and Limestone, who sponsors Wildflower Wednesday. Mosey on over to C&L and check out her Mr. Linky list of other WW posts!