This is our wild petunia - I think it's either Ruellia strepens or Ruellia pedunculata [which is more southern. We'd be at the edge of it's range.] It's the smooth version. There's a hairy version, too [R.caroliniensis] but I've never seen that here.
It blooms in June along the roadsides and in fields. The flowers are shy and hide just below the tops of the grass. The plants around here have pairs of blooms. The foliage turns yellow in the fall.