Ludwigia alternifolia is a perennial herb, 0.4-1.2 m tall, from clumps of fleshy-fibrous roots; stems slightly wing-angled (Radford et al. 1968), erect, branched, often reddish in sunny sites.
Leaves alternate, stalks 0-0.2 cm long, blade 5-10 cm long, to2 cm wide, lance-shaped, pointed at both ends, surface slightly rough from minute stiff bristles, secondary veins loop-connected along margin.
Flowers yellow, 4-parted, ca 1.5 cm in axils, petals free, 5-7 cm long, soon deciduous; sepals 4, 0.6-0.9 cm long, arising from top of floral base (hypanthium); stamens 4, ca 0.2 cm long; ovary inferior, surrounded by winged, 4-sided hypanthium 0.4 cm long and wide; style 0.2-0.3 cm long, stigma broad, rounded (Yatskievych 2006); blooms July-Aug.
Fruit dry, a square pod 0.4-0.6 cm wide, with rounded base, at first surrounded by persistent, pointed sepals, mature fruit with pore at top, eventually splitting open for seed dispersal by wind, persistent into winter.
Wetland status: FACW+.
Frequency in NYC: Very infrequent.
Habitat: Open marshes, moist to wet edges.