Azolla filiculoides aka Pacific Azolla

Azolla filiculoides is a small, floating fern called fairy fern. It belongs to the Salviniaceae family and lives for several years. Sources disagree on its native range. Kew lists central and southern Japan as native. Useful Tropical Plants places its origin in the Americas. That source treats Japan as part of its introduced range. Its listed American range extends from British Columbia into South America.

Each plant is rarely wider than 0.08 feet [0.025 metres]. Many plants join to form thick mats across the water. Missouri Botanical Garden lists a spread of 3 feet [0.91 metres]. Its listed height reaches about 0.26 feet [0.08 metres]. Useful Tropical Plants gives a lower height of 0.03 feet [0.01 metres]. Small leaves overlap and look bright green in shade. Sun can bring out pink and purple tones. Cold or stress can turn the leaves rusty red. This fern does not produce flowers.

It grows on still water or water that moves slowly. Habitats include ponds, lakes, marshes, swamps, and streams. It needs sun or light shade and an aquatic setting. Missouri Botanical Garden lists USDA zones 7–10. Useful Tropical Plants warns that frost can damage it. In some places, resting bodies sink during autumn. They return to the surface once spring water warms. This means of winter survival does not work everywhere. The same source reports that some forms withstand much colder weather.

Gardeners have grown it for pond cover and leaf colour. It spreads fast by budding and by breaking into fragments. Under good conditions, its mass can double within four to five days. Dense cover blocks light from reaching plants below. Decay beneath the mat can also lower oxygen levels. These changes can harm other life in the water. Missouri Botanical Garden reports no major insect or disease problems. It still rates maintenance needs as high.

Its invasive spread is the main concern for gardeners. The RHS lists it as an invasive non-native species in Britain. UK law bans sale and causing it to grow in the wild. Its nitrogen-rich growth has uses in compost and as fertiliser. It also has uses in water treatment projects. Plants For A Future lists no known food or medicinal uses. It reports no known hazards, but this does not establish that eating it is safe.