Ruellia humilis is the night flowering wild petunia. It bears the small funnel-shaped purple flowers typical of a Mexican petunia but its stems and leaves are more like those of a petunia hybrid.
Its drought tolerance and small stature (6-12 inches) are characteristics it also shares with Ruellia brittoniana, the dwarf Mexican petunia. It differs from R. brittoniana in that it is more cold hardy. Wild petunias can be grown from USDA zone 3a-9b.
This U.S. native grows wild all along the east coast and west to Texas. It is endangered in some states, weedy in others. Where it is weedy, it spreads by dropping seeds. Wild petunia seeds can be sown outdoors in the fall to propagate the plant.
Its one drawback is that it flowers less abundantly than most petunia annuals. This is forgivable as R. humilis is a rare perennial petunia. The only other perennial petunia I know of is Petunia Surfinia.
Ruellia humilis Culture
If you want a purple, creeping Ruellia for a dry sun or shade location, this is the plant you need.
There is no easier petunia to care for. Plant it in any well drained soil. Water it regularly until it takes hold. Feed it whenever you feed your other plants.
Wild petunia does not need pinching back and is suitable for xeric landscapes.
In caring for petunias you may encounter a few petunia pests. Horn worms like to bore holes into newly formed flower buds and, to add insult to injury,they leave their black droppings all over the plant.