Ruellia humilis (prairie petunia)

This low-growing plant of the Great Plains is not really a petunia, but resembles one. It grows about a foot high and can spread to 2' in diameter, making a great sun-loving ground cover, though the leaves come up late in the spring. Ruellia thrives in sunny, dry conditions and produces lavender flowers throughout the hottest months, starting in July. Once it is established it reseeds freely.