Hist. Pl. Remarq. Brésil 3/4: 245. 1825.
Herbs, 25–100 cm; indumentum of glandular and nonglandular hairs, glandular hairs coarse, erect, thick-based, and gland-tipped (especially abundant on stems and petioles). Stems erect, moderately to densely hairy, with gland-tipped hairs and finer, usually appressed, nonglandular hairs; older stems 4–7 mm diam. Leaves: petiole 0.3–2.5(–3.5) cm; blade narrowly ovate, lanceolate, or linear-lanceolate, 6–15 × (0.6–)1–6 cm (L/W = 2–7(–11)), base usually rounded or obtuse, rarely acute, apex acute or narrowly acute, surfaces glabrescent or sparsely, mostly appressed-hairy. Inflorescences 2–14 cm, peduncle 1–7 cm, fertile portion 1–9 cm, with (1–)2–4 pistillate flowers. Staminate flowers: petals narrowly obovate, 1.4 mm, ± equal, not or somewhat exerted beyond calyx. Pistillate flowers: sepals ovate or elliptic, longest 2–3.2 mm, becoming 3.5–5.5 mm in fruit; petals 1–2(–2.4) mm; ovary densely covered in bulbous-based, gland-tipped trichomes. Capsules 5–7 mm wide, trichomes conic proximally, hairlike distally, gland-tipped. Seeds brown, 2.5–3 mm diam. 2n = 22.
Phenology: Flowering and fruiting Jul–Nov.
Habitat: Disturbed wet areas, ditches, swampy areas, rice fields.
Elevation: 0–100 m.
Introduced; Ala., Ark., Fla., La., Miss., Tex., West Indies, Central America, South America.
Caperonia palustris is a major weed in rice fields in parts of the southern United States (R. K. Godara et al. 2011). Known in the flora area at least since the 1920s from Texas, this species was first collected in Arkansas in 1971 and Mississippi in 1982.