Azolla pinnata aka Mosquito Fern

Azolla pinnata is a tiny, evergreen fern that floats on water. Its common names include water fern and red azolla. Kew places it in the family Salviniaceae. Kew lists Australia and New Caledonia as its native range. Other sources report a wider range across Africa, Asia and the Pacific. These records do not all distinguish native from introduced plants. It grows in ponds and quiet backwaters of rivers.

Each plant spans about 0.03–0.08 feet [0.01–0.025 metres]. Its reported height is about 0.07 feet [0.02 metres]. New plants bud from older ones and form large colonies. The foliage starts bright green and turns deep red in sun. Dense patches can cover the water with green or red mats. From a distance, these mats may resemble algae or Salvinia. Its small size contrasts with the broad area a colony can cover.

This fern grows fast in tropical and subtropical climates. It does best with daytime temperatures of 20–35°C. The cited cultivation range extends from 15°C to 40°C. Ponds and lakes provide suitable growing sites. PFAF reports that it accepts deep shade, partial shade and full sun. It tolerates water conditions across a pH range of 3.5–10.5. PFAF lists USDA hardiness zones 11–12. Gardeners can increase their stock by division.

The plant has uses in ponds, bogs and water gardens. It also supports wildlife and serves as food for aquatic animals. Waterfowl, fish, shrimp and snails are among those that eat it. Small water creatures also find shelter within its mats. A partnership with the cyanobacterium Anabaena azollae allows nitrogen fixation. Farmers use the fern as green manure in rice fields. It adds nitrogen and organic matter to the soil. Its nitrogen-rich roots can also help compost break down.

Rapid growth can turn this useful plant into an invasive weed. Water rich in nutrients can support dense growth. A full cover can reduce oxygen levels in the water. This can harm fish and other aquatic plants. The same shade can limit the growth of blue-green algae. PFAF lists no known human food or medicinal uses. The sources report no confirmed toxicity for this species. That absence does not establish that it is safe to eat.