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© Gérald Duhayon, Parc Naturel des Plaines de l'Escaut
© Gérald Duhayon, Parc Naturel des Plaines de l'Escaut
© Emmanuel Delbart, FUSAGx
© Etienne Branquart, Belgian Biodiversity Platform
© Vilda
© Gérald Duhayon, Parc Naturel des Plaines de l'Escaut
Azolla filiculoides - Water fern

Synonym: Azolla caroliniana
French name:  Azolla
Dutch name: Grote kroosvaren
Family: Azollaceae
Group: Vascular plants
Origin: North America, South America
Habitat: freshwater
Introduction:  aquariums and ponds
ISEIA Score : 9
Naturalization in Belgium
First observation in the wild: 1912
Invasion stage: spread
Spatial distribution: restricted
Reproduction in the wild: yes
Dispersion potential: high
Natural habitats: medium
More on invasiveness: A. filiculoides settles in ponds, ditches, water reservoirs, wetlands, channels and slow flowing rivers, often together with Lemna minuta. It does not tolerate turbulence or fast flowing water. It is able to grow in nitrogen-deficient waters through its symbiotic association with Anabaena azollae but is limited by phosophorus availability; it generally prefers (very) eutrophic and alkaline waters with organic bottoms. Although the fern has been reported to die at temperatures below –4°C, there is some evidence that it might have adapted to West European climate since its introduction. The plant is spread in flood waters and by the movement of birds, animals and man.
Distribution in Belgium
Established populations
absent from district
isolated populations (1-5 localities per district)
widespread (>5 localities per district)
Endangered areas
low risk
medium risk
high risk

Endangered Natura 2000 habitats ():
freshwater habitats: 313031503260
Impacts on Species
Predation / Herbivory: low
Competition: medium
Disease transmission: low
Genetic effects: low
Impacts on Ecosystems
Nutrient cycling: medium
Physical alteration: medium
Natural successions: likely
Food web alteration: low
More on impacts: A. filiculoides can form dense floating monospecific mats at the surface of water bodies that reduce light penetration and gas exchanges, causing the predominance of respiratory activities and the reduction in dissolved oxygen in water beneath the mats. These mats often reduce the development of algae, other aquatic plants and animals. Dense populations seems however to be transient and well localised in Belgium due e.g. to consumption by Cyprinid fish and the North American azolla weevil, Stenopelmus rufinasus. Free-floating weeds can be drawn into water intakes, blocking pumps and filters, and can mat together forming floating rafts, which cause flow problems and obstructions to weirs, locks and other structures.
Data Source & References
Authors: Branquart Etienne, Stiers Iris, Triest Ludwig, Vanderhoeven Sonia, Van Landuyt Wouter, Van Rossum Fabienne, Verloove Filip
Published on:  04 December 2007
Last update:  31 July 2022
AEF (2006)
Cartes provisoires de la distribution de néophytes en Wallonie.
Unpublished document.
Agbede, S.A., Adeyemo, O.K., Ajani, F. & Adedeji, O.B. (2004)
Selectivity of three aquatic weeds as diet for Nile tilapia (Oreochromis niloticus).
African Journal of Livestock Extension 3: 8-12
Denys, L., Packet, J. & Van Landuyt, W. (2004)
Neofyten in het Vlaamse water : signalement van vaste waarden en rijzende sterren.
Natuur.focus 3(4): 120128.
Felzines, J.C. & Loiseau, J.E. (1990)
Lemna minuscula et Azolla filiculoides dans les vallées de la Loire moyenne et du Bas-Allier.
Le monde des Plantes 441: 6-9.
Gratwicke B. & Marshall B.E. (2001)
The impact of Azolla filiculoides Lam. on animal biodiversity in streams in Zimbabwe.
African Journal of Ecology 39 (2): 216-218.
Hussner A. (2006)
Invasive alien species fact sheet Azolla filiculoides.
From online database of the North European and Baltic network on invasive alien species (NOBANIS).
Janes R. (1998)
Growth and survival of Azolla filiculoides in Britain. 2. Sexual reproduction.
New Phytologist 138: 377-384.
Janes R., Eaton J.W. & Hardwick K. (1996)
The effects of floating mats of Azolla filiculoides Lam. and Lemna minuta Kunth on the growth of submerged macrophytes.
Hydrobiologia 340: 23-26.
Lambinon, J., Delvosalle, L. & Duvigneaud, J. (2004)
Nouvelle fore de la Belgique, du Grand-Duché de Luxembourg, du Nord de la France et des régions voisines.
Editions du Patrimoine du Jardin botanique national de Belgique, Meise.
Muller, S. (2004)
Plantes invasives en France : état des connaissances et propositions d'actions.
Publication scientifique du Museum d'Histoire naturelle, Patrimoines naturels n°62.
Thiébaut, G. (2007)
Non-indigenous aquatic and semiaquatic plant species in France.
In: F. Gherardi (Ed.), Biological invaders in inland waters: profiles, distribution and threats, Springer: 209-229.
Toussaint, B. & Bedouet, F. (2005)
Les espèces végétales invasives des milieux aquatiques et humides du bassin Artois-Picardie.
Agence de l'Eau Artois-Picardie, 38 pp.
Van Landuyt, W., Hoste, I., Vanhecke, L., Van den Bremt, P. Vercruysse, W. & De Beer, D. (2006)
Atlas van de Flora van Vlaanderen en het Brussels gewest.
Nationale Plantentuin en het Instituut voor Natuur- en Bosonderzoek i.s.m. Flo.Wer vzw.
Verloove, F. (2006)
Catalogue of the Neophytes in Belgium (1800-2005).
Scripta Botanica Belgica 39, 89 pp.
Other Internet Links

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