Fl. Carol., 89. 1788.
Herbs often with prominent aerenchyma when base submerged, forming stolons 30–250 cm, 2–4 mm thick, creeping in mud or floating in water, sometimes bearing flowers and fruits. Stems erect, subterete, densely branched, 40–120 cm, densely hirtellous. Leaves alternate; stipules ovate to lanceolate, 0.2–0.25 × 0.1–0.15 mm, usually obscured by pubescence; stolons: petiole 0.2–0.7 cm, blade obovate or elliptic to orbiculate, 0.6–2 × 0.5–1.1 cm, margins with distinct hydathodal teeth, base attenuate, surfaces densely hirtellous to glabrate; stems: petiole 0–0.2(–1) cm, blade elliptic or lanceolate-elliptic to very narrowly elliptic, 1.5–8(–10) × 0.3–1.2(–1.4) cm, base cuneate or attenuate, margins entire with obscure hydathodal glands, apex acute or narrowly acute, surfaces ± densely hirtellous, leaves on branches much reduced; bracts much reduced. Inflorescences usually congested, leafy spikes or racemes, flowers solitary in distal leaf axils; bracteoles attached 1–2.2 mm distal to base of ovary, linear-lanceolate or narrowly elliptic, 3–6.5(–7.2)× 0.3–1.5(–1.7) mm, apex acuminate, surfaces hirtellous. Flowers: sepals ascending with reflexed tips, pale green abaxially, creamy white adaxially, often tinged with pink or red, ovate-deltate, 3.5–5.5(–6) × 2–4 mm, margins entire, apex elongate-acuminate to subcuspidate, surfaces densely hirtellous; petals 0; filaments yellowish, 1.5–2.5 mm, base dilated, anthers 0.6–0.9(–1.3) × 0.5–0.7 mm; pollen shed in tetrads; ovary obovoid to cup-shaped, 2.5–4 × 2.5–4 mm; nectary disc elevated 0.3–0.7 mm on ovary apex, bright yellow, turning black upon drying, 2–3.6 mm diam., indistinctly 4-lobed, densely hirtellous around style base and between lobes; style cream, 1–2 mm, sparsely to densely hirtellous, especially proximally, stigma capitate, 0.3–0.6 × 0.3–0.6 mm, not exserted beyond anthers. Capsules subglobose or, sometimes, oblong-obovoid, subterete or with 4 rounded corners, 3–5 × 3–4.5 mm, hard-walled, irregularly dehiscent, pedicel 0–1 mm. Seeds brown, elliptic-oblong or oblong-ovoid, slightly curved on both ends, 0.5–0.7 × 0.3–0.4 mm, surface cells ± isodiametric. 2n = 32.
Phenology: Flowering Jun–Sep.
Habitat: Roadside ditches, marshes, swales in sandy pine flats, edges of pocosins, peaty bogs, low grassy savannas, swamp forests.
Elevation: 0–300 m.
Ala., Fla., Ga., La., Miss., N.C., S.C., Tex., Va.
The distribution of Ludwigia pilosa is nearly continuous along the Atlantic and Gulf coastal plains, from extreme southeastern Virginia to northern Florida, and west to Louisiana and southeastern Texas. Disjunct populations occur in northern Alabama and central North Carolina.
Ludwigia pilosa is easily distinguished from most others in sect. Isnardia by being densely hirtellous throughout. Its showy sepals and nectary disc attract multiple insect visitors including ants, bumblebees, honeybees, moths, and wasps (C. I. Peng 1989).