Wild petunia is a perennial, either single-stemmed or branched. The stems can be smooth or more commonly slightly hairy in 2 narrow, lengthwise bands on opposite sides of the stalk, the hairs appearing crinkled. The stems are bluntly 4-angled. Flowers few, arising from nodes near middle of stems, tubular with 5 lobes, resembling petunia flowers; lavender to lilac-blue, very rarely white. Blooms May–October. Leaves opposite, on short stalks, ovate.
Similar species: There are 2 other wild petunias in our state, both of which have stems that are evenly hairy on all sides. One, hairy wild petunia or fringeleaf ruellia (R. humilis), is scattered nearly statewide, has leaves that are stalkless or with only very short stalks, and has flower clusters in the axils of the main stem leaves. Our other wild petunia, R. pedunculata, is scattered in the Ozarks and Ozark Border, has leaves with definite stalks, and has flower clusters in axils of small leaves at the tip of inflorescence stalks.