Wolffia arrhiza aka Rootless Duckweed

Wolffia arrhiza is a tiny plant that floats on water. It belongs to the Araceae family. People call it rootless duckweed and least duckweed. An older name is Lemna arrhiza. Its native range spans Africa and Europe, east to northern India. It also runs from South Korea to the Philippines. The plant is native to Britain. Records call it rare in the south of Britain. Kew places it in a subtropical climate group.

This is one of the world's smallest flowering plants. It floats on quiet water, with no upright clump. It lives in ponds and other still waters. The sources differ on its life cycle. Kew calls it an aquatic annual, a water plant with a one-year life cycle. PFAF calls it an evergreen perennial, which stays green and lives for years. PFAF reports leaves, flowers and ripe seeds all year. Each flower has both male and female organs. The sources give no height or spread for garden plants.

Grow it in sun, on still water rich in nitrates and lime. PFAF says it cannot grow in shade. It grows in neutral or alkaline water, even very alkaline water. PFAF lists USDA hardiness zones 6–9. It places Britain near the edge of the plant's climate range. There, PFAF suggests growing it in a cold-water aquarium. Resting buds help it live through winter in temperate areas. They sink to the pond floor in late autumn. They rise to the surface in spring.

Once settled in a pond, it grows fast by division. PFAF says this spread means it needs no extra help to multiply. In gardens, it serves as a floating pond plant. The supplied sources name no pests or diseases. PFAF does not flag it as a weed. Its fast spread still matters when choosing where to grow it.

People grow it as a vegetable in Myanmar, Laos and Thailand. PFAF lists its leaves as edible when cooked. It notes their sweet, cabbage-like taste and high food value. Reports list protein, carbohydrate and fat among its nutrients. They also list vitamins A, B2, B6 and C. Its fast growth has led to proposals to grow it as food for sale. PFAF records no known hazards, medicinal uses or other uses. The lack of known hazards does not prove safety for every animal or use.