Neptunia oleracea aka Water Mimosa

Neptunia oleracea, or water mimosa, belongs to the pea family. This perennial legume fixes nitrogen and grows near fresh water. Its native range spans the tropics and subtropics. Sources place it in Africa, southeast Asia, Australia, and South America. It grows in ponds, swamps, canals, and slow streams. Plants may float freely or root along muddy banks.

In gardens, plants reach about 0.5 feet [0.15 metres] tall. Their stems spread 3–5 feet [0.9–1.52 metres] across the water. One source lists a height of 4.9 feet [1.5 metres]. Floating stems can join into thick mats of leaves. White, spongy tissue helps these stems stay afloat. Stems on land lack this tissue. The land form also has smaller leaves and flowers. Fine, bipinnate leaves fold shut when touched. The plant has a thick taproot and few stem branches.

Greenish-yellow flowers add colour from June to September. Seedpods follow, then split open to release their seeds. The plant grows fast in warm, humid conditions. Full sun suits it best, though it accepts part shade. It needs wet ground or still to gently moving water. It favours acidic conditions, with a preferred pH of 5–6.5. Plants tolerate a wider pH range of 4.5–7.

Water mimosa is winter hardy in USDA zones 9–12. Gardeners can grow it as an annual in colder areas. Set plants out after frost, once water reaches 70°F. Roots can sit at pond edges with water above their crowns. Plants can also float on the surface or grow in containers. Aquariums and greenhouse ponds provide other growing sites. For winter shelter, move plants indoors when outdoor temperatures reach 50°F. Seeds and stem cuttings provide ways to raise new plants.

Water mimosa adds leaf texture and flowers to small water gardens. It needs close management and can invade warm regions. It also grows as a weed in irrigation channels. The Missouri Botanical Garden reports no major pest or disease issues. People eat the leaves and stems raw or cooked. They also cook the young pods. The plant forms part of Thai cuisine. Useful Tropical Plants reports no known hazards, but this does not establish pet safety. It records traditional medicinal uses for roots and stem juice. Its cited 2013 IUCN assessment lists the species as Least Concern.