Azolla filiculoides

Summary of Invasiveness

Azolla filiculoides is a small fern native to the Americas which has spread widely throughout the world by a variety of mechanisms, of which man has become the most significant (Lumpkin and Plucknett, 1982). Man has introduced A. filiculoides into Europe, North and sub-Saharan Africa, China, Japan, New Zealand, Australia, the Caribbean and Hawaii.

In eutrophic water systems, A. filiculoides grows rapidly, easily outcompeting indigenous vegetation. Decaying root and leaf matter below a mat of A. filiculoides, coupled with the lack of light penetration, creates an anaerobic environment which can reduce the quality of drinking water and make survival for other organisms in the water impossible.

Preferred Scientific Name

Azolla filiculoides Lamarck

Preferred Common Name

water fern

Other Scientific Names

These are considered to be junior synonyms of Azolla filiculoides

Azolla bonariensis Bertoloni

Azolla japonica Franch. & Sav.

Azolla magellanica Willd.

Azolla squamosa Molina

International Common Names

English: fairy moss; mosquito fern; Pacific azolla; red water fern; water fern; water velvet

Spanish: helecho acuatico; hlechito del aqua; lenteja de agua

French: fougere d’eau

Local Common Names

China: xi lu-ping; xi man jiang hong

Germany: grosser algenfarn

Japan: akaukikusa

Netherlands: grote Kroosvaren

South Africa: rooivaring


According to Lumpkin and Plucknett (1980), A. filiculoides is native to the Rocky Mountain states of the western USA and Canada, through Central America and to most of South America. It has been introduced to Europe, North and sub-Saharan Africa, China, Japan, New Zealand, Australia, the Caribbean and Hawaii.

Distribution Table and Maps

The distribution in the summary table and maps below is based on all the information available, as given in the ISC.

A filicuoides world


A filiculoides asia

A filiculoides SE asia

A filiculoides africa

 A filiculoides america

 A filiculoides S americaA filiculoides C america

Country Distribution Origin Invasive References
China Present Introduced   Reed, 1954
India Present     Parimal & Kushari, 2002
-West Bengal Present     Parimal & Kushari, 2002
Iran Present Native Invasive Khoshravesh et al., 2009
Israel Present, few occurrences     EPPO, 2014
Japan Present Introduced   Seto & Nasu, 1975
Turkey Present     EPPO, 2014
Egypt Present     Magda et al., 2002; EPPO, 2014
Mozambique Present Introduced Invasive Langa, 2013
South Africa Widespread Introduced Invasive Twyman & Ashton, 1972; Twyman & Ashton, 1972
Zimbabwe Present     Chikwenhere, 2001; Chikwenhere, 2001
-British Columbia Present Native   USDA-ARS, 2003
Mexico Present Native   Svenson, 1944
USA Present     EPPO, 2014
-Alabama Present Native   Richerson & Grigarick, 1967
-Alaska Present Native   Svenson, 1944
-Arizona Present Native   Richerson & Grigarick, 1967
-California Present Native   Richerson & Grigarick, 1967
-Florida Present Native   Madeira et al., 2013
-Hawaii Present Introduced   Fosberg, 1942
-Illinois Present Native   Svenson, 1944
-Louisiana Present Native   Richerson & Grigarick, 1967
-Mississippi Present Native   Richerson & Grigarick, 1967
-Nevada Present Native   USDA-NRCS, 2002
-Oregon Present Native   USDA-NRCS, 2002
-Texas Present Native Invasive Richerson & Grigarick, 1967
-Utah Present Native   Svenson, 1944
-Washington Present Native   USDA-NRCS, 2002
-Wisconsin Present Native   Svenson, 1944
Costa Rica Present Native   USDA-ARS, 2003
Guatemala Present Native   USDA-ARS, 2003
Honduras Present Native   USDA-ARS, 2003
Nicaragua Present Native   USDA-ARS, 2003
Trinidad and Tobago Present Introduced   Reed, 1965
Argentina Present Native   Reed, 1962
Bolivia Present Native   Svenson, 1944
Brazil Present Native   USDA-ARS, 2003
-Amazonas Present Native   Reed, 1962
-Para Present Native   Reed, 1962
-Rio Grande do Sul Present Native   Reed, 1962
-Sao Paulo Present Native   Tofoli et al., 2000
Chile Present Native   Reed, 1962
Colombia Present Native   USDA-ARS, 2003
Ecuador Present Native   USDA-ARS, 2003
French Guiana Present Native   Svenson, 1944
Paraguay Present Native   USDA-ARS, 2003
Peru Present Native   Reed, 1962
Uruguay Present Native   Reed, 1962
Venezuela Present Native   USDA-ARS, 2003
Belgium Present Introduced   Lawalree, 1964; EPPO, 2014
Bulgaria Present Introduced   Sourek, 1958; EPPO, 2014
Croatia Present     EPPO, 2014
Czech Republic Present     EPPO, 2014
Czechoslovakia (former) Present Introduced   Sourek, 1958
France Present Introduced   Chevalier, 1926; EPPO, 2014
Germany Present Introduced   Schloemer, 1953; EPPO, 2014
Greece Present Introduced   Royal Botanic Garden Edinburgh, 2003;EPPO, 2014
Hungary Present     EPPO, 2014; Royal Botanic Garden Edinburgh, 2003
Ireland Present Introduced   Brunker, 1949; EPPO, 2014
Italy Present Introduced   Royal Botanic Garden Edinburgh, 2003;EPPO, 2014
-Sardinia Present     EPPO, 2014
Netherlands Present Introduced   Sculthorpe, 1967; EPPO, 2014
Poland Present, few occurrences     EPPO, 2014
Portugal Present Introduced Invasive Reed, 1962; EPPO, 2014
Romania Present Introduced   Lawalree, 1964; EPPO, 2014
Russian Federation Present     EPPO, 2014
-Central Russia Present     EPPO, 2014
Spain Present Introduced   Royal Botanic Garden Edinburgh, 2003;EPPO, 2014
Sweden Present     EPPO, 2014
UK Present Introduced Invasive Royal Botanic Garden Edinburgh, 2003;EPPO, 2014
Ukraine Present     EPPO, 2014
Australia Present Introduced   Bailey, 1902
-Victoria Present Introduced   Allinson et al., 2000
New Zealand Present Introduced   Bailey, 1902; Large & Braggins, 1993

History of Introduction and Spread

A. filiculoides, the type species of the genus, is widely distributed, having been introduced to a number of countries in which it is not indigenous (Ashton, 1982). The plant has been dispersed by a variety of mechanisms, of which man has become the most significant (Lumpkin and Plucknett, 1982).

According to Szczesniak et al. (2009), A. filiculoides was first recorded in Europe towards the end of the 19th century, and the first observations were made in 1870s-1880s. The species may have been accidentally transported in ballast tanks of ships, in water with fry, or directly as an ornamental. Janes (1998a) noted the deliberate introduction of the plant as an ornamental into Europe through mainland Britain at the end of the 19th century. Possibly as a result of its various transport routes, A. filiculoides appeared independently in different places at almost the same time. It then spread across nearly all of Europe. 

A. filiculoides was introduced into Asia from East Germany in 1977 as an alternative to the cold susceptible native strain of A. pinnata, used as a green manure in the rice industry (Lumpkin and Plucknett, 1982). It was introduced to Africa in 1948 as an aquarium plant (Oosthuizen and Walters, 1961; Jacot-Guillarmod, 1979).

A. filiculoides  has also been spread around the world as a model plant for the study of Azolla-Anabaena symbiosis (Carrapiço, 2010) Risk of Introduction

Further spread is likely as A. filiculoides continues to be sold in nurseries as a fish pond plant. In Africa and Europe, dispersal between countries will no doubt continue due to the movement of waterfowl, which can spread plant fragments between bodies of water. A. filiculoides will continue to invade countries where the presence of eutrophic waters, lack of natural enemies and an unregulated nursery trade will contribute to its status as a weed. Hosts/Species Affected

A. filiculoides is not generally considered a weed of crops. It is commonly grown in conjunction with rice, asA. filiculoides has the ability to fix nitrogen via an endosymbiotic blue-green alga (Wagner, 1997), acting as a green manure in rice cultivation. However, in its introduced range,A. filiculoides may impact upon trout and other fish farming (McConnachie et al., 2003). It has also been recorded to affect the growth of Potamogeton crispus L. (Janes et al., 1996).

Notes on Natural Enemies

Host records from around the globe show that the genus Azolla is attacked by generalist herbivores and that very few specialist insect species have evolved on these plants (Hill, 1997). However, four beetle species, the weevils Stenopelmus rufinasus and S. brunneus and the two flea beetles Pseudolampsis guttata and P. darwinii, appear to have specialised on the genus Azolla (Richerson and Grigarick, 1967; Habeck, 1979; Hill, 1999) and were identified as potential biological control agents for A. filiculoides in South Africa (Hill, 1997). Following host range testing,Stenopelmus rufinasus was released in 1997 as a biocontrol of A. filiculoides in South Africa (McConnachie et al., 2004).

Means of Movement and Dispersal

The economic impact of A. filiculoides in South Africa was examined by McConnachie et al. (2003). Thick mats on reservoirs and slow-moving water bodies caused economic losses to water-users. Among those water-uses most seriously affected were farming (71%), recreational (24%), and municipal (5%). On average, A. filiculoides was found to cause on-site damages of US$589 per hectare per year.

Environmental Impact

In eutrophic water systems, A. filiculoides grows rapidly, easily outcompeting indigenous vegetation. Decaying root and leaf matter below a mat of A. filiculoides, and the lack of light penetration, creates an anaerobic environment. Not only can very little survive under such conditions, but the quality of drinking water is reduced, caused by bad odours, colour and turbidity (Hill, 1997). Cases have been reported where both livestock and game farmers have lost animals due to them refusing to drink from infested water bodies or drowning as a result of mistaking the mat for solid ground. The weed also reportedly increases water loss through evapotranspiration and promotes the development of waterborne, water-based and water-related diseases (Hill, 1997).

Impact: Biodiversity

A. filiculoides infestations may form thick mats (5-20 cm thick), on water bodies up to 10 hectares in size (McConnachie et al., 2003). Such infestations have been shown to severely impact the biodiversity of aquatic ecosystems and have serious implications for all aspects of water utilization (Gratwicke and Marshall, 2001).
One of the last remaining habitats of the endangered fish species, the eastern Cape rocky (Sandelia bainsii Castelnau, 1861; Anabantidae) in South Africa, had become so overgrown with the weed that had the biological control programme not been so successful, S. bainsii faced extinction.

Biological Control

South Africa is the only country that has initiated a classical biological control programme against A. filiculoides. Four insect species were identified as potential biological candidates – all frond-feeding beetles: Pseudolampsis guttata (Leconte) (Chrysomelidae), P. darwinii (Sherer),Stenopelmus rufinasus Gyllenhal and S. brunneus Hustache (Curculionidae). All species do extensive damage to the plants in the country of origin (Hill, 1997).

The biology and host range of P. guttata was investigated by Hill (2002), and although it was found to be fairly damaging, the weevil S. rufinasus was identified as the most suitable of the four for release in South Africa, and was imported into quarantine for host-specificity screening. The weevil was released in 1997 and results have been dramatic, causing local extinction of A. filiculoides at the sites where it was released (McConnachie et al., 2004).

The surface area of weed controlled totalled 203.5 ha and infested sites were controlled in approximately seven months on average (in a range of 3-11 months). Five years after the release of the weevil, A. filiculoides no longer poses a threat to aquatic ecosystems in South Africa and its effects on the utilization of water resources have been significantly reduced (McConnachie et al., 2003). S. rufinasus has been released in Mozambique and Zimbabwe with material provided by South Africa, has established, and is proving a successful biocontrol agent in these countries also (Cilliers et al., 2003).

S. rufinasus has been present in the UK since it was first reported there by Janson (1921) and was probably brought into Europe with the plant. Since then it has been recorded in Ireland, France, Belgium, the Netherlands and Spain (Pratt et al 2013). This agent has been used as in an augmentive manner in England and Ireland and is under consideration for mass-rearing and releasing in the Netherlands, France and Belgium (Pratt et al., 2013).

Case Study:A. filiculoides in South Africa

A. filiculoides was first recorded in South Africa in 1948 from the Oorlogspoort River in the Northern Cape Province of South Africa (30°37’58.22”S 25°21’28.89”E), and by 1999 it was recorded from 152 sites in South Africa, largely in the Free State Province.

Owing to the adverse environmental and economic effects of the weed, a biological control programme was initiated with the importation of the weevil Stenopelmus rufinasus Gyllenhal (Coleoptera: Curculionidae) from Florida, USA, in 1995. Following host-specificity testing, the weevils were released in South Africa in 1997. By 2004, nearly 25,000 weevils had been released throughout South Africa, and their feeding damage has resulted in local extinctions of A. filiculoides from the majority of sites that were surveyed at the time. It took, on average, ten months for a site to be cleared after release of the weevils. The dispersal abilities of the weevils were originally underestimated, but they are capable of dispersing up to 350 km unaided.

Just five years after the release of the weevils, A. filiculoides was no longer considered a threat to South African water bodies (McConnachie et al., 2004). These successes were carefully monitored between 1999 and 2006, documenting the rapid control of the plant, and proving that just four years after the studies by McConnachie et al. (2004) the weevils had succeeded in controlling A. filiculoides at every site where they had been released (Hill et al., 2008).

Country-wide surveys from 2008 produced further evidence of the success of this program. In 2010, of the 102 A. filiculoides sites investigated (40% of water weed sites surveyed), the weed was present at 19 (19%) of these sites, and S. rufinasus was recorded from 14 (73%) of the infested sites. A. filiculoides is no longer a significant problem in South Africa, and where it does occur, S. rufinasus is usually present. Biological control of A. filiculoides is now widely regarded as the most successful biological control programme against an invasive alien weed in South Africa (Coetzee et al., 2011).


Allinson G, Stagnitti F, Colville S, Hill J, Coates M, 2000. Growth of floating aquatic macrophytes in alkaline industrial wastewaters. Journal of Environmental Engineering, 126(12):1103-1107.

Ashton PJ, 1982. The autecology of Azolla filiculoides Lamarck with special reference to its occurrence in the Hendrik Verword Dam catchment area. PhD thesis, Rhodes University, South Africa.

Ashton PJ, 1992. Azolla infestations in South Africa: history of the introduction, scope of the problem and prospects for management. Water Quality Information Sheet.

Ashton PJ, Walmsley RD, 1984. The taxonomy and distribution of Azolla species in southern Africa. Botanical Journal of the Linnean Society, 89:239-247.

Axelsen S, Julian C, 1988. Weed control in small dams. Part II Control of Salvinia, Azolla and of water hyacinth. Queensland Agricultural Journal, 114(5):291-298.

Bailey FM, 1902. The Queensland flora, Pt VI. Brisbane, Australia: HJ Didams and Co.

Brunker JP, 1949. Azolla filiculoides Lam. In Co. Wicklow. Irish Naturalists’ Journal, 9:340.

Carrapico F, 2010. Azolla as a Superorganism. Its Implication in Symbiotic Studies. Symbioses and Stress Cellular Origin, Life in Extreme Habitats and Astrobiology, 17:225-241.

Chevalier A, 1926. La culture des Azolla pour la nourriture des animaux de basse-cour et comme engrais vert pour les riziers. Rev. Bot. Appl. Agr. Trop., 6:356-360.

Chikwenhere GP, 2001. Current strategies for the management of water hyacinth on the Manyame River System in Zimbabwe. Biological and integrated control of water hyacinth: Eichhornia crassipes. Proceedings of the Second Meeting of the Global Working Group for the Biological and Integrated Control of Water Hyacinth, Beijing, China, 9-12 October 2000, 105-108.

Cilliers CJ, Hill MP, Ogwang JA, Ajuonu O, 2003. Aquatic weeds in Africa and their control. In: Neuenschwander P, Borgemeister C, Langewald J, eds. Biological Control in IPM Systems in Africa. Wallingford, UK: CAB International, 161-178.

Coetzee JA, Hill MP, Byrne MJ, Bownes A, 2011. A review of the biological control programmes on Eichhornia crassipes (C.Mart.) Solms (Pontederiaceae), Salvinia molesta D.S.Mitch. (Salviniaceae), Pistia stratiotes L. (Araceae), Myriophyllum aquaticum (Vell.) Verdc. (Haloragaceae) and Azolla filiculoides Lam. (Azollaceae) in South Africa. African Entomology, 19(2):451-468.

Diatloff G, Lee AN, A new approach for control of Azolla filiculoides. Proceedings of the 7th Asian-Pacific Weed Science Society Conference, Sydney, Australia, 1979., 253-255

Edwards DJ, 1974. Weed preference and growth of young grass carp in New Zealand. New Zealand Journal of Marine and Freshwater Research, 8(2):341-350.

EPPO, 2014. PQR database. Paris, France: European and Mediterranean Plant Protection Organization.

Evrard C, Hove Cvan, 2004. Taxonomy of the American Azolla species (Azollaceae): a critical review. Systematics and Geography of Plants, 74(2):301-318.

Forni C, Chen J, Tancioni L, Grilli Caiola M, 2001. Evaluation of the fern Azolla for growth, nitrogen and phosphorous removal from wastewater. Water Research, 35(6):1592-1598.

Fosberg FG, 1942. The uses of Hawaiian ferns. American Fern Journal, 32:15-23.

Garcia-Murillo P, Fernández-Zamudio R, Cirujano S, Sousa A, Espinar JM, 2007. The invasion of Donana National Park (SW Spain) by the mosquito fern (Azolla filiculoides Lam). Limnetica, 26(2):243-250.

Gratwicke B, Marshall BE, 2001. The impact of Azolla filiculoides Lam. On animal biodiversity in streams in Zimbabwe. African Journal of Ecology, 38:1-4.

Habeck DH, 1979. Host plants of Pseudolampsis guttata (LeConte) (Coleoptera: Chrysomelidae). The Coleopterists Bulletin, 33:150.

Henderson L, 2001. Alien Weeds and Invasive Plants. Plant Protection Research Institute Handbook No. 12. Cape Town, South Africa: Paarl Printers.

Hill MP, 1997. The Potential for the Biological Control of the Floating Aquatic Fern, Azolla filiculoides Lamarck (red water fern / rooivaring) in South Africa. Report No. KV 100/97. Pretoria, South Africa: Water Research Commission.

Hill MP, 1998. Life history and laboratory host range of Stenopelmus rufinasus, a natural enemy for Azolla filiculoides in South Africa. BioControl, 43(2):215-224; 25 ref.

Hill MP, 1999. Biological control of red water fern, Azolla filiculoides Lamarck (Pteridophyta: Azollaceae), in South Africa. Biological control of weeds in South Africa (1990-1998)., 119-124; [African Entomology Memoir, No. 1].

Hill MP, McConnachie AJ, Byrne MJ, 2008. Azolla filiculoides Lamarck (Pteridophyta: Azollaceae) control in South Africa: a 10-year review. In: Proceedings of the XII International Symposium on Biological Control of Weeds, La Grande Motte, France, 22-27 April, 2007 [ed. by Julien, M. H.\Sforza, R.\Bon, M. C.\Evans, H. C.\Hatcher, P. E.\Hinz, H. L.\Rector, B. G.]. Wallingford, UK: CAB International, 558-560.

Hill MP, Oberholzer IG, 2002. Laboratory host range testing of the flea beetle, Pseudolampsis guttata (LeConte) (Coleoptera: Chrysomelidae), a potential natural enemy for red water fern, Azolla filiculoides Lamarck (Pteridophyta: Azollaceae) in South Africa. Coleopterists Bulletin, 56(1):79-83.

Hussner A, Weyer Kvan de, Gross EM, Hilt S, 2010. Comments on increasing number and abundance of non-indigenous aquatic macrophyte species in Germany. Weed Research (Oxford), 50(6):519-526.

Jacot-Guillarmod A, 1979. Water weeds in southern Africa. Aquatic Botany, 6:377-391.

Janes R, 1998. Growth and survival of Azolla filiculoides in Britain. I. Vegetative reproduction. New Phytologist, 138(2):367-375.

Janes R, 1998. Growth and survival of Azolla filiculoides in Britain. II. Sexual reproduction. New Phytologist, 138(2):377-384.

Janes RA, Eaton JW, Hardwick K, 1996. The effects of floating mats of Azolla filiculoides Lam. and Lemna minuta Kunth on the growth of submerged macrophytes. Hydrobiologia, 340(1/3):23-26.

Janson OE, 1921. Stenopelmus rufinasus Gyll. an addition to the list of British Coleoptera. Entomologist’s Monthly Magazine, 57:225-226.

Khoshravesh R, Akhani H, Eskandari M, Greuter W, 2009. Ferns and fern allies of Iran. Rostaniha, 10(Supplement 1). Tehran, Iran: Iranian Research Institute of Plant Protection, iv + 134 pp.

Kröck T, Alkämper J, 1991. Azolla’s contribution to weed control in rice cultivation. Plant Research and Development, 34:117-125.

Langa SDF, 2013. PhD Thesis. Grahamstown, South Africa: Rhodes University.

Large MF, Braggins JE, 1993. Spore morphology of the New Zealand Azolla filiculoides Lam. New Zealand Journal of Botany, 31:419-423.

Lawalree A, 1964. Azollaceae. In: Tutin TG, Haywood VH, Vallentyne DH, Walters SM, Webb DA, eds. Flora Europaea. UK: Cambridge University Press.

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

Lumpkin TA, Plucknett DL, 1982. Azolla as a green manure: use and management in crop production. Azolla as a green manure: use and management in crop production. Westview Press Boulder, Colorado, 230 pp.

Madeira PT, Center TD, Coetzee JA, Pemberton RW, Purcell MF, Hill MP, 2013. Identity and origins of introduced and native Azolla species in Florida. Aquatic Botany, 111:9-15.

Magda SA, Ghazi IM, Atalla KM, Nahed MA, 2002. Influence of salinity and source of water on growth and protein content of Azolla pinnata and Azolla filiculoides under Egyptian conditions. Annals of Agricultural Science, Moshtohor, 40(2):871-884.

McConnachie AJ, 2003. Post release evaluation of Stenopelmus rufinasus Gyllenhal (Coleoptera: Curculionidae) – a natural enemy released against the red waterfern, Azolla filiculoides Lamarck (Pteridophyta: Azollaceae) in South Africa. PhD thesis, University of the Witwatersrand, Johannesburg, South Africa.

McConnachie AJ, de Wit MP, Hill MP, Byrne MJ, 2003. Economic evaluation of the successful biological control of Azolla filiculoides in South Africa. Biological Control, 28:25-32.

McConnachie AJ, Hill MP, Byrne MJ, 2004. Field assessment of a frond-feeding weevil, a successful biological control agent of red waterfern, Azolla filiculoides, in southern Africa. Biological Control. In press.

Moore AW, 1969. Azolla: biology and agronomic significance. Botanical Review, 35:17-35.

Nayak SK, Singh PK, 1989. Cytological studies in the genus Azolla. Cytologia, 54:275-286.

O’Keeffe JH, 1986. Ecological research on South African rivers – a preliminary synthesis. South African National Scientific Programmes Report, 121:1-121.

Oosthuizen GJ, Walters MM, 1961. Control of water fern with diesoline. Farming in South Africa, 37:35-37.

Parimal Chattopadhyay, Kushari DP, 2002. Adaptation of two Azolla species in a laterite zone of West Bengal. Environment and Ecology, 20(1):126-132.

Pratt CF, Shaw RH, Tanner RA, Djeddour DH, Vos JGM, 2013. Biological control of invasive non-native weeds: an opportunity not to be ignored. Entomologische Berichten, 73(4):144-154.

Preston CD, Croft JM, 1997. Aquatic plants in Britain and Ireland. Colchester, UK: Harley.

Rajendran R, Reuben R, 1991. Evaluation of the water fern Azolla microphylla for mosquito population management in the rice-land agro-ecosystem of south India. Medical and Veterinary Entomology, 5(3):299-310

Rao HS, 1936. The structure and Life-history of Azolla pinnata R. Brown. With remarks on the fossil history of the Hydropteridae. Proceedings of the Indian Academy of Science 2, 175-200.

Reed CF, 1954. Index Marsileata et Salviniata. Biol. Soc. Bot., 2(a):5-61.

Reed CF, 1962. Distribution of Salvinia and Azolla in South America and Africa, in connection with studies for control by insects. Phytologia, 12(3):121-130.

Richerson PJ, Grigarick AA, 1967. The life history of Stenopelmus rufinasus (Coleoptera: Curculionidae). Annals of the Entomological Society of America, 60:351-354.

Royal Botanic Garden Edinburgh, 2004. Flora Europaea Database. Royal Botanic Garden Edinburgh, UK.

Sadeghi R, Zarkami R, Sabetraftar K, Damme Pvan, 2012. Use of support vector machines (SVMs) to predict distribution of an invasive water fern Azolla filiculoides (Lam.) in Anzali wetland, southern Caspian Sea, Iran. Ecological Modelling, 244:117-126.

Sanyahumbi D, Duncan JR, Zhao M, van Hille R, 1998. Removal of lead from solution by the non-viable biomass of the water fern Azolla filiculoides. Biotechnology Letters, 20(8):745-747.

Saunders RKM, Fowler K, 1992. A morphological taxonomic revision of Azolla Lam. Section Rhizosperma Mey.) Mett. (Azollaceae). Botanical Journal of the Linnaean Society, 109:329-357.

Saunders RKM, Fowler K, 1993. The supraspecific taxonomy and evolution of the fern genus Azolla (Azollaceae). Plant Systematics and Evolution, 184:175-193.

Schloemer VA, 1953. Ein verwilderter wasserfarn, Azolla filiculoides. Natur. and Volk, 83:131-134.

Sculthorpe CD, 1967. The Biology of Aquatic Vascular Plants. London, UK: Edward Arnold Publications Limited.

Searg MS, El-Hakeem A, Badway M, Mousa MM, 2000. On the ecology of Azolla filiculoides Lam. in Damietta District, Egypt. Limnologica, 30:72-81.

Seto K, Nasu T, 1975. Discovery of fossil Azolla massulae from Japan and some notes on recent Japanese species [In Japanese]. Bull. Osaka Mus. Nat. Hist., 29:51-60.

Sourek J, 1958. Azolla filiculoides Lam. – neue eingeschleppte Art in der CSR. Preslia, 30:84-85.

Stergianou KK, Fowler K, 1990. Chromosome numbers and taxonomic implications in the fern genus Azolla (Azollaceae). Plant Systematics and Evolution, 173:223-239.

Steyn DJ, Scott WE, Ashton PJ, Vivier FS, 1979. Guide to the use of herbicides on aquatic plants. Technical Report TR95, Department of Water Affairs, South Africa, 1-29.

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

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

Szczesniak E, Blachuta J, Krukowski M, Picinska-Faltynowicz J, 2009. Distribution of Azolla filiculoides Lam. (Azollaceae) in Poland. Acta Societatis Botanicorum Poloniae, 78(3):241-246.

Tofoli GR, Negrisoli E, Velini ED, Martins D, 2000. Chemical Azolla filicoides L. control. (Controle químico de Azolla filicoides L.) Ecossistema, 25(2):181-183.

Twyman ES, Ashton PJ, 1972. An autecological investigation of the aquatic fern (Azolla species) in relation to the Hendrik Verwoed Dam. The Civil Engineer in South Africa, 14:88-89.

USDA-ARS, 2003. Germplasm Resources Information Network (GRIN). Online Database. National Germplasm Resources Laboratory, Beltsville, USA.

USDA-NRCS, 2002. The PLANTS Database, Version 3.5. National Plant Data Center, Baton Rouge, USA.

van Cat D, Watanabe I, Zimmerman WJ, Lumpkin LA, de Waha Baillonville T, 1989. Sexual hybridization among Azolla species. Canadian Journal of Botany, 67:3482-3485.

Wagner GM, 1997. Azolla: A review of its biology and utilization. The Botanical Review, 63(1):1-25.