Wild Petunia (Smooth Ruellia)

Photo of wild petunia flowers
Scientific Name
Ruellia strepens
Acanthaceae (acanthuses)

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.

Other Common Names
Limestone Ruellia
Limestone Petunia

Height: to 3 feet.

Where To Find
image of Wild Petunia Smooth Ruellia Limestone Ruellia distribution map

Scattered nearly statewide.

Occurs in bottomland forests, rich upland forests, banks of streams and rivers, edges of ponds and lakes; less commonly in bottomland prairies and fens; also in pastures, moist roadsides, and railroads. Our state has 3 ruellia species, but there are about 250 species in the world. Most of them are in tropical and warm-temperate regions.

Do not confuse these with the petunias that are so popular at garden centers; those are mostly hybrids in the genus Petunia, which is in the Solanaceae (the family containing potatoes, tomatoes, nightshades, chili and bell peppers, and so on). Those "true" petunias originated in South Africa and are quite unrelated to our "wild petunias." Ours, in the acanthus family, are more closely related to many tropical plants, including the houseplants "nerve plant" and "polka dot plant" and garden plants in the genera Justicia and Thunbergia.

With its showy flowers, wild petunia is an interesting native choice for the gardener. A clump-forming plant, it's easily grown in moist, rich, well-drained soils in part shade. Try it in woodland, shade, or native-plant gardens. Please don't dig them from the wild; buy them at a reputable native plant nursery.

Many species in the acanthus family produce two kinds of flowers: some that are open-flowering, which can be cross-pollinated by insects, and others that never open, which self-pollinate yet produce viable seeds. Either way, offspring will be produced.

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Similar Species
About Wildflowers, Grasses and Other Nonwoody Plants in Missouri
A very simple way of thinking about the green world is to divide the vascular plants into two groups: woody and nonwoody (or herbaceous). But this is an artificial division; many plant families include some species that are woody and some that are not. The diversity of nonwoody vascular plants is staggering! Think of all the ferns, grasses, sedges, lilies, peas, sunflowers, nightshades, milkweeds, mustards, mints, and mallows — weeds and wildflowers — and many more!