The Nonindigenous Occurrences section of the NAS species profiles has a new structure. The section is now dynamically updated from the NAS database to ensure that it contains the most current and accurate information. Occurrences are summarized in Table 1, alphabetically by state, with years of earliest and most recent observations, and the tally and names of drainages where the species was observed. The table contains hyperlinks to collections tables of specimens based on the states, years, and drainages selected. References to specimens that were not obtained through sighting reports and personal communications are found through the hyperlink in the Table 1 caption or through the individual specimens linked in the collections tables.

Azolla pinnata
Azolla pinnata
(mosquito fern)

Copyright Info
Azolla pinnata R. Br.

Common name: mosquito fern

Synonyms and Other Names: water velvet, Azolla africana Desv., Azolla guineensis Schum., Azolla pinnata var. africana (Desv.) Baker

Taxonomy: available through www.itis.govITIS logo

Noxious: This species is listed by the U.S. Department of Agriculture as a noxious weed.


Stem/Rhizoids: Azolla pinnata is a free floating aquatic plant typically found in clusters or in large mats (Bodle 2008). Each plant is 1-2.5 cm in diameter with a feathered triangular shape; midsection is typically straight with pinnately arranged side branches that are longer towards their base (Sweet and Hills 1971; Saunders and Fowler 1992).

Leaves: Each leaf is 1-2 mm long and overlap in a two-ranked pattern (Pereira et al. 2011).

Look-a-likes: Azolla microphylla (A. caroliniana) Mexcian mosquitofern, A. pinnata ssp. africana, and A. pinnata ssp. asiatica. Azolla pinnata differs from native Azolla species by the triangular leaf arrangement, and A. pinnata ssp. pinnata differs morphologically from other non-native subspecies of Azolla pinnata by having unicellular rhizome papillae and an elliptical dorsal lobe shape (Pereira et al. 2011, Madeira et al. 2013). Azolla pinnata populations in Florida were also determined to be genetically similar to A. pinnata ssp. pinnata populations in Australia (Madeira et al. 2013).

Size: 1.5-2.5 cm long, 1-1.5 cm wide (Sweet and Hills 1971)

Native Range: Australia (Saunders and Fowler 1992; Madeira et al. 2013).

Hydrologic Unit Codes (HUCs) Explained
Interactive maps: Point Distribution Maps

Nonindigenous Occurrences:

Table 1. States with nonindigenous occurrences, the earliest and latest observations in each state, and the tally and names of HUCs with observations†. Names and dates are hyperlinked to their relevant specimen records. The list of references for all nonindigenous occurrences of Azolla pinnata are found here.

StateFirst ObservedLast ObservedTotal HUCs with observations†HUCs with observations†
AZ200220022Agua Fria; Rillito
FL200720248Apalachee Bay-St. Marks; Everglades; Florida Southeast Coast; Kissimmee; Lake Okeechobee; Lower Ochlockonee; New; Upper St. Johns
HI201720222Hawaii; Maui
LA200920091Lower Mississippi-New Orleans

Table last updated 6/12/2024

† Populations may not be currently present.

Ecology: Plants reproduce vegetatively and sexually. Vegetative fragments form when the main axis deteriorates and lateral branches break free (Rao 1935; Shen 1960; Lumpkin 1983). When reproducing sexually, round sporocarps (1-1.5 mm in diameter) form on the underside of the leaves (Rao 1935; Shen 1960; Lumpkin 1983).

Azolla pinnata grows optimally between 29-33ºC, although only subspecies asiatica was studied (Watanbe and Berja 1983). It tolerates salt concentrations up to 30 mM, but can be preincubated in lower concentrations to increase salinity tolerance up to as high as 60 mM (Rai and Rai 1999).

Upper lobes of A. pinnata's leaves are host to a cyanobacteria symbiont that fixes atmospheric nitrogen (Strasburger 1873; Moore 1969; Wagner 1997). It is sometimes introduced by rice farmers as a natural fertilizer for this reason (Lumpkin and Plucknett 1980). Typical habitats are wind-protected, slow-moving waters, such as ponds, small lakes, swamps, wetlands or drainage canals (Svenson 1944).  

Means of Introduction: The main pathway in the U.S. is hitchhiking with ornamental pond or aquarium plants (Kay and Hoyle 2001). In other countries it has also been introduced by farmers to help fertilize rice fields and control mosquito populations (Moore 1969; Lumpkin and Pucknett 1980; Holm et al. 1997).

Status: Established in three drainages in Florida; status is unknown in Arizona and Louisiana.

Impact of Introduction:
Summary of species impacts derived from literature review. Click on an icon to find out more...


Azolla spreads rapidly via vegetative reproduction and quickly covers water surfaces (Moore 1969). It forms dense surface mats, which interfere with boating, fishing and recreational activities as well as degrade water quality by reducing oxygen levels and limiting light to native plants (Lumpkin and Plucknett 1980; Kay and Hoyle 2001; Pemberton and Bodle 2009).

Azolla is commonly used as a fertilizer in rice paddies and forage for livestock in southeast Asia (Moore 1969; Lumpkin and Plucknet 1980).

Remarks: Color ranges from green to maroon-red. Red hues form when anthocyanin is produced as a reaction to unfavorable pH, temperature, moisture or nutrient availability (Holm et al. 1997).

The common name, mosquito fern, may originate from the use of Azolla as a measure to prevent mosquito reproduction in Europe and the United States by covering the water surface (Moore 1969).

Azolla pinnata was discovered in nursery tanks in Idaho (Thomas Woolf, ID Dept. of Ag., pers. comm.) and North Carolina (Stratford Kay, NCSU, pers. comm.) and quickly removed. There are no established populations in waterbodies in those states.

References: (click for full references)

Anderson, L.C. (curator). 2001. Herbarium specimen voucher data, Florida State University Herbarium (FSU). Florida State University, Tallahassee, FL. http://herbarium.bio.fsu.edu/.

Bodle, M. 2008. Feathered mosquito fern (Azolla pinnata R. Br.) comes to Florida. Aquatics 30(2):4,6,8-9. http://www.fapms.org/aquatics/issues/2008summer.pdf.

Center for Invasive Species and Ecosystem Health. 2015. EDDMapS: Early detection and distribution mapping system. The University of Georgia, Tifton, GA. http://www.eddmaps.org.

Holm, L., J. Doll, E. Holm, J. Pancho and J. Herberger. 1997. World weeds : natural histories and distribution.  Wiley & Sons, New York, New York, USA.

Kay, S.H., and S.T. Hoyle. 2001. Mail order, the internet, and invasive aquatic weeds. Journal of Aquatic Plant Management 39(1):88-91.

Kunzer, J.M., R.P. Wunderlin, L.C. Anderson, and J.R. Burkhalter. 2009. New and noteworthy plants from Florida. Journal of the Botanical Research Institute of Texas 3(1):331-337.

Lumpkin, T.A., and D.L. Plucknett. 1980. Azolla: botany, physiology, and use as a green manure. Economic Botany 34(2):111-153.

Lumpkin, T.A. 1983. Taxonomy, physiology, and agronomic potential of Azolla spp. Unpublished Ph.D. dissertation. University of Hawaii, Honolulu, HI.

Madeira, P.T., T.D. Center, J.A. Coetzee, R.W. Pemberton, M.F. Purcell, and M.P. Hill. 2013. Identity and origins of introduced and native Azolla species in Florida. Aquatic Botany 111:9-15.

Madsen, J.D. 2010. Invasive Plant Atlas of the MidSouth. Geosystems Research Institute, Mississippi State University, Starkville, MS. http://www.gri.msstate.edu/ipams/.

Moore, A.W. 1969. Azolla: biology and agronomic significance. Botanical Review 35(1):17-34.

Pemberton, R.W., and J.M. Bodle. 2009. Native North American azolla weevil, Stenopelmus rufinasus (Coleoptera: Curculionidae), uses the invasive old world Azolla pinnata as a host plant. Florida Entomologist 92(1):153-155.

Pereira, A.L., M. Martins, M.M. Oliveira, and F. Carrapiço. 2011. Morphological and genetic diversity of the family Azollaceae inferred from vegetative characters and RAPD markers. Plant Systematics and Evolution 297:213-226.

Rai, V., and A.K. Rai. 1999. Growth behaviour of Azolla pinnata at various salinity levels and induction of high salt tolerance. Plant and Soil 206:79-84.

Rao, H.S. 1935. The structure and life-history of Azolla pinnata R. Brown with remarks on the fossil history of the hydropterideæ. Proceedings of the Indian Academy of Sciences - Section B 2(2):175-200. https://www.ias.ac.in/article/fulltext/secb/002/02/0175-0201.

Saunders, R.M.K, and K. Fowler. 1992. A morphological taxonomic revision of Azolla Lam. section Rhizosperma (Mey.) Mett. (Azollaceae). Botanical Journal of the Linnean Society 109(3):329-357.

Shen, E.Y. 1960. Anabaena azollae and its host Azolla pinnata. Taiwania 7:1-7.

Strasburger, E. 1873. Ueber Azolla. Hermann Davis, Jena, Germany.

Svenson, H.K. 1944. The new world species of Azolla. American Fern Journal 34(3):69-84.

Sweet, A., and L.V. Hills. 1971. A study of Azolla pinnata R. Brown. American Fern Journal 61(1):1-13.

Thomas, K.A., and P. Guertin. 2007. Southwest Exotic Mapping Program 2007: occurrence summary and maps of select invasive non-native plants in Arizona. US Geological Survey Open-File Report 2007-1277. http://pubs.usgs.gov/of/2007/1277/.

Wagner, G.M. 1997. Azolla: a review of its biology and utilization. Botanical Review 63(1):1-26.

Watanabe, I., and N.S. Berja. 1983. The growth of four species of Azolla as affected by temperature. Aquatic Botany 15:175-185.

Author: Pfingsten, I.A., D.D. Thayer, and V. Howard

Revision Date: 8/30/2023

Citation Information:
Pfingsten, I.A., D.D. Thayer, and V. Howard, 2024, Azolla pinnata R. Br.: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, https://nas.er.usgs.gov/queries/FactSheet.aspx?speciesID=2745, Revision Date: 8/30/2023, Access Date: 6/12/2024

This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.


The data represented on this site vary in accuracy, scale, completeness, extent of coverage and origin. It is the user's responsibility to use these data consistent with their intended purpose and within stated limitations. We highly recommend reviewing metadata files prior to interpreting these data.

Citation information: U.S. Geological Survey. [2024]. Nonindigenous Aquatic Species Database. Gainesville, Florida. Accessed [6/12/2024].

Contact us if you are using data from this site for a publication to make sure the data are being used appropriately and for potential co-authorship if warranted.

For general information and questions about the database, contact Wesley Daniel. For problems and technical issues, contact Matthew Neilson.