Pistia stratiotes aka Water Lettuce

Pistia stratiotes, or water lettuce, belongs to the arum family. It is an evergreen perennial that floats on fresh water. Kew lists its native range across the tropics and subtropics. It grows in lakes, ponds, and other wet habitats. Its leaf rosettes look like small heads of lettuce. Long, fine roots hang in the water below them.

Size reports vary quite widely between sources. NC State gives heights of 0.25–0.5 feet [0.08–0.15 metres]. It lists widths of 0.5–1.5 feet [0.15–0.46 metres]. RHS gives an upper height and spread near 1.6 feet [0.5 metres]. The soft leaves have a thick, spongy feel. Fine hairs cover their surface, and deep veins form ridges. They are pale green above and whitish green beneath. Smooth green stolons link new plantlets as the colony spreads. NC State reports stolons reaching 25 feet [7.6 metres].

Small flowers sit among the leaves in spring and summer. They lack petals and draw little attention in a pond. Male flowers surround a female flower on a spadix. Insects pollinate the blooms, according to Useful Tropical Plants. The small green fruits are berries. Water lettuce grows fast, with still water aiding its spread. It needs an aquatic setting rather than a dry bed. RHS lists full sun; NC State also allows partial shade. The plant cannot withstand frost. NC State lists USDA zones 8–10, while RHS assigns H1B.

Gardeners use it in ponds, water gardens, and some aquariums. Its floating leaves provide the main visual interest. Yet its spread can outweigh its value as an ornamental. Dense mats block boating and fishing and reduce water oxygen. Fish can die, and the mats provide mosquito breeding sites. In cold regions, seeds can renew the population. NC State reports few major pest or disease problems. Aphids may occur, and the plant needs substantial upkeep. RHS warns against releasing it into the UK wild. Sales are banned in Northern Ireland, and EU restrictions also apply.

All parts contain calcium oxalate crystals and other unidentified toxins. Eating large amounts can burn and swell the mouth and throat. Vomiting and diarrhoea may follow. RHS also warns of skin and eye irritation. Reports describe cooked leaves as a poor-quality famine food. Such use does not make the raw plant safe. Useful Tropical Plants records folk treatments for urinary and skin complaints. It also notes uses in water cleanup and organic matter production. Other reported uses include stain removal and insecticidal leaves.