References: Lumpkin in FNA (1993b).
Azollaceae consists of the single genus Azolla, with about 6 species.
Azolla Lamarck in Lamarck et al. (Mosquito Fern)
A small genus of about 6 species, in tropical and warm temperate regions.
Azolla caroliniana Willdenow, Eastern Mosquito Fern, Water fern. Cp, Mt, Pd (NC, SC, VA): stagnant waters of interdune ponds, limesink ponds, old millponds, beaver ponds, floodplain sloughs; uncommon (though often locally abundant, rare in Mountains and Piedmont of Virginia only). June-September. Widespread in the se. United States, extending irregularly north (partly from introductions) into s. New England and MN, and south into the tropics. Very un-fernlike, this floating aquatic looks superficially more like an aquatic liverwort. In some years and some places it occurs in great abundance, covering the surface of the water with a green or red mass of vegetation. Azolla has a symbiotic, nitrogen-fixing cyanobacterium, Anabaena azollae Strasburger. The nitrogen-fixing capabilities of Azolla have resulted in its use as a fertilizer, green manure, and livestock feed, much promoted in recent years, but used historically in Asian rice paddies for centuries (Lumpkin in FNA 1993b). [RAB, C, F, FNA, G, K, S]
Other species in eastern North America: Azolla mexicana C. Presl of midwestern North America, as far east as AR, MO, OK, and TX.
Flora of the Carolinas and Virginia, Working Draft of 27 October 1997 -- KEY TO PTERIDOPHYTE GENERA. Alan S. Weakley. The Nature Conservancy, Southeast Regional Office, Southern Conservation Science Department.