The pale purple flowers of Ruellia caroliniensis do resemble our commercial petunias, however, those annual flowers sitting in greenhouses across the country each year have more in common with a tomato than they do with this plant. The carolina wild petunia is one of several closely related species in the Acanthaceae family that are noted for pale purple flowers that bloom off and on all summer.
This cheerful little wildflower is a North American native that seems to thrive in a number of conditions and soils. From sunny, sandy conditions to shady woodland edges, it carves out a place wherever its seeds land. Even though this is a perennial that can spread underground, the little seed capsules explode with enough force that new plants tend to pop up far from the original clump. It never reseeds with enough vigor to become a nuisance, but you will find babies popping up here and there. The underground spread is more like a mild mannered clump of daylilies as opposed to something like bamboo. Transplant them to another part of your garden or pass them along to friends.
Here in west Kentucky these plants favor the dappled shade of overgrown fence rows or open woods. They grow about a foot tall , but also happily bloom where they've been pruned. There's a small cemetery nearby where I've seen them blooming right on the ground where the mowers have clipped them short. The wild petunia does tend to get a bit leggy in shady areas, but they still produce the pretty little purple flowers just the same. The flowers only last for one day, but that does not stop butterflies from seeking the nectar. Apparently, this is a rich source of nectar, because there always seems to be several fluttering about where the plant is blooming. It is also a host plant for the common Buckeye, Junonia coenia.
Ruellia caroliniensis has some invasive relatives that you do not want in your garden. The Mexican wild petunia is similar in appearance, but has some very bad garden manners. Ruellia tweediana is almost impossible to eradicate once it gains a foothold in frost-free areas and is considered a noxious weed in Florida. It spreads by a vast network of underground runners and often chemical control is the only answer as it is tough and resilient.There are a number of named cultivars available commercially, however, it is best to only use them where winters are cold enough to freeze the plants each winter.
I didn't find any medicinal or culinary uses for this plant, so enjoy it in your butterfly and pollinator garden since it isn't considered a useful herb. It doesn't require rich soil and seems to thrive with a minimum of care. From sun to shade, as long as the soil is well-drained, the carolina wild petunia is a cheerful addition to the garden. Be aware that the top growth completely disappears in the winter, so it is best to give it plenty of neighbors to prevent an unsightly bare spot.
Our native wildflowers are a treasure that we should cherish. They may not be as bright or showy as their cultivated cousins, but with global urbanization, our world's wildflower population is shrinking daily. If you can add just a few of these plants to your garden, the chances they will be around for future generations are much better.