Azolla filiculoides
American water fern
(c) Kevin Thiele, some rights reserved (CC BY) · licence
Family: Azollaceae · Type: Herb · Native
Azolla filiculoides (American water fern) is a native California herb in the Azollaceae family.
American water fern is a native aquatic fern found in California's Foothills Periphery, Great Basin, and Mojave Desert regions in ponds and slow-moving streams at elevations below 1,300 meters. Often displaying a distinctive green to reddish coloration, this small fern grows in prostrate and ascending forms with stems generally 1 to 3 centimeters long. Growing with delicate, tiny leaves that are smooth or bearing inconspicuous papillae on the upper leaf lobes, the plant creates dense floating mats across water surfaces. Its growth pattern is characterized by very short internodes, with mature plants developing ascending stems less than 1 millimeter between leaf attachments. The plant produces both male and female sporangium cases, often with a distinctive equatorial girdle and tubercled, pitted wall structure.
Identification and appearance
Key characters: 1 sympatric species. Key differentiator is leaf papillae visibility and growth form.
Habitat and range
Habitat: Common. Ponds, slow streams
Elevation: < 1300 m
California counties: Tulare, Ventura, Los Angeles, Riverside, San Bernardino, Alameda, Butte, Colusa, Fresno, Glenn, Humboldt, Inyo, Kern, Lake, Madera, Mendocino, Modoc, Mono, Monterey, Orange, San Diego, San Francisco, San Joaquin, San Luis Obispo, San Mateo, Santa Barbara, Santa Clara, Santa Cruz, Sonoma, Stanislaus, Mariposa, Merced, Marin, San Benito, Yuba, Tehama, Yolo, Placer, Napa, Contra Costa, Sacramento
Outside California: to Washington, Arizona, South America, also eastern United States, Eurasia, Africa
How to tell it apart
vs Lemna: Lemna has smooth round pads; Azolla has branching stems with minute scale-like leaves.; vs Spirodela: Spirodela has flat green fronds; Azolla has tiny overlapping scales creating fuzzy texture.; vs Eichhornia: Eichhornia has large inflated leaf stalks; Azolla is minuscule at under 3 cm.; vs Pistia: Pistia forms 10+ cm rosettes; Azolla is tiny at under 3 cm with branching stems.; vs Salvinia: Salvinia leaves are fingernail-sized (5+ mm); Azolla leaves are pinhead-sized (under 2 mm).
Frequently asked questions
Is Azolla filiculoides native to California?
Yes. Azolla filiculoides is native to California (recorded as native).
Where in California does Azolla filiculoides grow?
Azolla filiculoides occurs in Tulare, Ventura, Los Angeles, Riverside, San Bernardino, Alameda, Butte, Colusa and other counties, at elevations of < 1300 m.
What plant family does Azolla filiculoides belong to?
Azolla filiculoides is in the Azollaceae family, genus Azolla.
What habitat does Azolla filiculoides grow in?
Azolla filiculoides grows in Common. Ponds, slow streams.
Related species
Other California species in the same genus, family, or bioregion.
Sources: Jepson eFlora · Index of California Plant Names · CalPhotos
Last updated 2026-10-07
Data from The California Species Project — 14,000+ California species with verified data from CNPS, CDFW, USFWS, Cal-IPC, and more.