Smooth-stemmed perennial, branching above, with stems nearly square, generally hairless except near the flower stalks. Flowers in many-flowered terminal panicles, small, arising from a green, 5-lobed leafy involucre; there are no true petals; sepals are petal-like, fused into a trumpet shape, light pink to purple; stamens 3-5, protruding, with yellow tips. Blooms May–October. Called “four-o’clock” because the flowers open in late afternoon. Leaves opposite, heart-shaped or triangular, to 3½ inches long, on petioles (leaf stalks) (except a few of the uppermost leaves; this is the only Mirabilis in Missouri with petioles).
Similar species: White four-o'clock (M. albida) has narrowly lanceolate or oblong leaves without stalks, white, lilac or pink flowers, and grows on bluff edges, glades, and other dry sites. Narrow-leaved four-o'clock (M. linearis) has very narrow (linear) leaves and is quite rare in Missouri.