Azolla cristata

Azolla cristata, the Carolina mosquito fern, Carolina azolla, or water velvet, is a species of the Azolla genus native to the Americas, in eastern North America from southern Ontario southward, from the east coast west to Wisconsin and Texas, the Caribbean, and in Central and South America from southeastern Mexico (Chiapas) south to northern Argentina and Uruguay.

It is a freshwater aquatic fern, with scale-like fronds 5–10 mm long, green to reddish, most often reddish in winter and in strong light. They are covered in tiny trichomes that give it the appearance of velvet. It is able to fix nitrogen from the air by means of symbiotic cyanobacteria. It can survive winter water temperatures of 5 °C (41 °F), with optimum summer growth between 25–30 °C (77–86 °F).