Distribution: Ludwigia sedoides forms beautiful colonies of plants with symmetrically arranged leaves floating on the surface of lakes and ponds and occasionally in seasonally dry marshes, from the Yucatán Peninsula of Mexico to Paraguay, and from Piaui, Brazil to western Ecuador. It is scattered in southern Mexico, Central America where it grows in all countries except Belize, and the West Indies (Cuba, Jamaica). In South America, it is fairly common but scattered in Guyana, Venezuela, eastern Colombia, eastern Ecuador, Bolivia, Paraguay and the northern and central states of Brazil.
Habit: Aquatic perennial herbs. Stem very slender, 0.2-0.5 cm thick, freely branching below, rooting along nodes, terete, glabrous, bearing floating rosettes of densely packed leaves at the ends.
Leaves: Stipules 3-4 mm long, 1-2 mm wide, deltoid, deciduous with age. Petioles 5-100 mm long, usually flattened, often reddish, glabrous or strigillose. Leaf blade 0.5-2 cm long and wide, rhombicovate, thick, acute at base, crenate-serrate in upper half, entire in lower half, strigillose or sometimes glabrous, usually bright pinkish beneath and along margins and bright green above, minutely glandular-punctate above; venation faint, with 4-6 veins on each side of the midrib; submarginal veins absent.
Flowers: Flowers solitary, emerging out of the water, pedicels 10—30 mm long, often reddish, bracteoles 4—6 mm long, ca. 1 mm wide, setaceous, not subtended by stipels, sepals 0.6—1 cm long, 0.2—0.6 cm wide, oblong-ovate, obtuse or acute at tip, 3—5-nerved, thick, petals 1—1.5 cm long, 0.8—1.3 cm wide, broadly obovate, emarginate, subsessile, yellow, stamens subequal, 2.8—3.2 mm long, or sometimes unequal in larger-flowered individuals, when unequal, the filaments of the episepalous anthers ca. 4 mm long, those of the epipetalous ones 3-3.5 mm long, filaments dilated toward the base, anthers ca. 2 mm long, rounded, ovary 10—15 mm long, 2—4.5 mm thick, obconic, 4-angled, glabrous, disk plane, hairy, style 2.5—4 mm long, 0.2—0.3 mm thick, white, stigma 0.7—1 mm long, 1—2 mm thick, depressed-globose, sometimes 4-lobed.
Fruit: Capsule 8—14 mm long, 2—4 mm thick, obconic, gradually narrowed towards the base, 4-angled, sometimes rounded, glabrous.
Seeds: Seeds 0.55—0.6 mm long, 0.2—0.25 mm thick, obovoid-oblong, slightly curved, brown, striate; raphe reduced and inconspicuous. Probably autogamous.
Chromosomes: Gametic chromosome number, n = 8.