Fl. Bor.-Amer. 1: 88. 1803.
Herbs slender, sometimes suffrutescent from woody base, often with aerenchyma, rarely creeping and rooting at nodes, often forming slender stolons 4–15(–25) cm, 0.4–0.8 mm thick. Stems usually erect or ascending, rarely prostrate, slightly to distinctly winged, wings to 1.8 mm wide, unbranched to densely branched, 5–60 cm, glabrous. Leaves alternate; stipules lanceolate-deltate, 0.13–0.15 × 0.1–0.13 mm; stolons: petiole attenuate, 0.1–0.5 cm, blade broadly elliptic to suborbiculate, 0.2–0.7 × 0.2–0.5 cm; stems: petiole winged, 0.1–0.5 cm, blade obovate-spatulate or oblanceolate, sometimes narrowly oblanceolate-elliptic, 0.4–1.7 × 0.15–1 cm, base attenuate, margins subentire or often with hydathodal glands forming minute teeth, or minutely papillose-strigillose, apex acute or mucronate, surfaces glabrous; leaves on side branches much reduced, glabrous; bracts near apex and on branches reduced. Inflorescences leafy spikes or racemes, flowers solitary in axils, usually not crowded; bracteoles attached at base of ovary, sublinear or narrowly oblong, 0.4–1.2(–1.5) × 0.1–0.4 mm, usually with swollen base. Flowers: sepals ascending or spreading, pale green to cream adaxially, ovate-deltate, 0.9–2 × 1–1.9 mm, margins minutely papillose-strigillose or entire, apex acuminate, surfaces glabrous; petals 0; filaments translucent, 0.4–0.55 mm, anthers 0.1–0.2 × 0.2–0.3 mm; pollen shed singly; ovary pale green, obovoid-subglobose, 0.8–1 × 0.8–1.2 mm, glabrate; nectary disc nearly flat on ovary apex, light green, 0.5–1.2 mm diam., 4-lobed, glabrous; style light green, 0.3–0.6 mm, glabrous, stigma subcapitate, 0.15–0.3 × 0.05–0.15 mm, not exserted beyond anthers. Capsules obconic, subterete, 1–1.5 × 1.4–1.9 mm, thin-walled, seeds often visible as bumps, dehiscent by apical ring, pedicel 0–0.2 mm. Seeds dark reddish brown, oblong-ovoid, 0.5–0.6 × 0.3–0.4 mm, surface cells transversely elongate, glabrous or, sometimes, densely covered by waxy hairs. 2n = 16.
Phenology: Flowering Mar–Nov (year-round).
Habitat: Roadside ditches, marshes, borders of ponds and streams, low meadows, low areas in open woods, edges of swamp forests, brackish marshes, hammocks, solution pits of limestone on marl prairies.
Elevation: 0–400 m.
Ala., Ark., Fla., Ga., La., Miss., Mo., N.C., S.C., Tenn., Tex., Va., West Indies (Bahamas, Cuba, Jamaica).
The diploid Ludwigia microcarpa has the smallest stature, leaves, flowers, fruits, and fewest seeds (ca. 10–20) per capsule of any species in sect. Isnardia (C. I. Peng 1989). Most plants start to flower when young.