Two spp are documented in Oregon (and Metro area), A. microphylla and A. filiculoides. FNA says these are difficult to ID. "Azolla is the most frequently studied genus of ferns in the world because of its economic importance. The three North American species are naturalized in Europe and South Africa, and they have been introduced into Hawaii for horticulture and into Asia for agriculture. All species have been studied for agricultural uses in rice-producing areas." http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=103297
Azolla rubra and Lemna fighting for dominance of the last water in the quickly drying upper Heathcote River.
Azolla (mosquito fern, duckweed fern, fairy moss, water fern) is a genus of seven species of aquatic ferns in the family Salviniaceae. They are extremely reduced in form and specialized, looking nothing like conventional ferns but more resembling duckweed or some mosses.