Ludwigia palustris is a perennial herb, small, floating or creeping, stems prostrate, colonial from stem fragments and rooting at nodes, stems 10-60 cm long (Radford et al. 1968), weak, often reddish in full sun.
Leaves opposite, stalk to 2.5 cm long, blade 0.5-3 cm long, 2 cm wide, elliptic to egg-shaped, tip pointed, base narrowed to stalk, often reddish where exposed to sun.
Flowers green, tiny, no petals, 4-parted, sepals 0.1 cm long, egg-shaped, tips pointed; stamens 4, white, ovary inferior surrounded by 4-sided hypanthium; in leaf axils; blooms and fruits June-Oct.
Fruit green banded, 0.2-0.4 cm long, 0.2 cm wide (Radford et al. 1968), bluntly 4-sided; splitting open to release seeds.
Wetland status: OBL.
Frequency in NYC: Occasional (more common than indicated by collections).
Habitat: Shallow water and wet soil, open areas, part shade, pond edges; tolerant of anaerobic, low nutrient soils, pH 5-8.5. Intolerant of salt and full shade (USDA, NRCS 2006).