Azolla filiculoides Lam.

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English Water fern Status LU: established. 1st record: LU ~2002, ITW 2009.
Lëtzebuergesch Waasserfar Status Eur.: established. 1st record: 1870s-1880s.
Français Fougère d’eau RA: ISEIA: C1. Harmonia+: 0,22
Deutsch Großer Algenfarn Wikipedia: Wikipedia - English - Water fern Wikipedia - Français - Fougère d'eau Wikipedia - Deutsch - Großer Algenfarn Wikipedia - Nederlands - Grote kroosvaren | Wikispecies: Wikispecies - Azolla filiculoides | CABI
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Report the species

Report Azolla filiculoides to the National Museum of Natural History.

Brief description

Azolla filiculoides Lam. (Syn.: A. caroliniana Willd.) originates from tropical America and is naturalised in Western, South and Central Europe and the Canary Islands. Azolla filiculoides form stands that may expand or regress rapidly, depending in particular on thermal factors. Two species of Azolla have been reported in the Flora territory: A. filiculoides and A. caroliniana, but it is now known that these two names are synonymous (Lambinon & Verloove 2012: 41).

Azolla filiculoides is an aquatic fern that 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. The plant is spread by flood waters and by the movement of birds, animals and people. It can form dense, floating and monospecific mats at the surface of water bodies that reduce light penetration and gas exchange. These mats often reduce the development of algae, other aquatic plants and animals. However, dense populations seem to be transient and well localised (Branquart et al. 2010).

Status and distribution in Luxembourg

Records of Azolla filiculoides Lam. in Luxembourg. Data source: Recorder-Lux, iNaturalist & GBIF, 2024-07-14.

In Luxembourg, Azolla filiculoides was first reported around 2002 from a private garden pond by Claude Reckinger (in litt. 2021). The water fern was first observed in the wild by Karl-Georg Gessner on 10th July 2009 in three ponds close to Bourscheid (Kesselkapp), where it grew in more or less extended carpets in 3 ponds together with Lemna minor, Alisma plantagoaquatica and Glyceria fluitans (MNHNL 2000-, Krippel & Colling 2010: 12).

Discovered in several ponds located in the open countryside, this small fern native to tropical America was known for a few years already in Luxembourg in leisure ponds in private gardens, where it had been introduced with other aquatic plants (Krippel & Colling 2010: 12).

A spectacular proliferation of the water fern appeared in autumn 2005 in the channel of Jouy nearby Metz in French Lorraine (Muller 2006).

Risk assessment

ISEIA protocol

C1 (2+2+2+2) (Ries et al. 2013: 18).

Harmonia+ protocol

Overall risk score 0,22 = (Overall Invasion score 0,66 x Overall Impact score 0,33) (Ries et al. 2020).


Worldwide distribution


  • Branquart, E., I. Stiers, L. Triest, S. Vanderhoeven, W. Van Landuyt, F. Van Rossum, F. Verloove, 2010. Harmonia database: Azolla filiculoides Lam.. Harmonia version 1.2, Belgian Forum on Invasive Species. URL: [accessed on 2019-10-02]
  • CABI, 2014. Azolla filiculoides [original text by Martin Hill]. In: Invasive Species Compendium. Wallingford, UK: CAB International. URL: [accessed 2020-02-28]
  • Krippel, Y. & G. Colling, 2010. Notes floristiques. Observations faites au Luxembourg (2008- 2009). Bull. Soc. Nat. luxemb. 111: 11-32. [PDF 209 KB]
  • Lambinon J. & F. Verloove, 2012. Nouvelle flore de la Belgique, du grand-duché de Luxembourg, du Nord de la France et des régions voisines. Sixième édition. Avec la collaboration de L. Delvosalle, B. Toussaint, D. Geerinck, I. Hoste, F. Van Rossum, B. Cornier, R. Schumacker, A. Vanderpoorten et H. Vannerom. Jardin botanique national de Belgique, Meise. CXXXIX + 1195 pp. ISBN : 9789072619884.
  • MNHNL, 2000-. Azolla filiculoides Lam. in Recorder-Lux, database on the natural heritage of the Grand Duchy of Luxembourg. Musée national d’histoire naturelle, Luxembourg. URL: [Accessed 2019-09-05]
  • MNHNL, iNaturalist & GBIF, 2019. Azolla filiculoides Lam. in MNHNL-mdata, online portal combining species observation from Recorder-Lux, iNaturalist and GBIF. National Museum of Natural History, Luxembourg. URL: [Accessed 2019-09-06]
  • Muller, S., 2006. Prolifération spectaculaire d’’Azolla filiculoides (Azollaceae, Pteridophyta) dans le canal de Jouy près de Metz (Lorraine, France) à l’automne 2005. Bull. Soc. Nat. luxemb. 107: 31-38. [PDF 828 Kb]
  • Oly, M., 2022. Kartierung invasiver gebietsfremder Wasserpflanzen im Großherzotum Luxemburg, Untersuchungen zur Verbreitung von invasiven gebietsfremden Wasserpflanzen in einer beispielhaften Auswahl an Stillgewässern, Musée national d’histoire naturelle, Luxembourg. 117 pp. 
  • Ries, C. & Y. Krippel, 2021. First records of 56 invasive alien vascular plants in Luxembourg. Bulletin de la Société des naturalistes luxembourgeois 123: 115-127. [PDF 241 KB]
  • Ries, C., Y. Krippel & M. Pfeiffenschneider, 2020. Risk assessment after the Harmonia+ protocol of invasive alien vascular plant species in Luxembourg. Bull. Soc. Nat. luxemb. 122: 197-205. [PDF 132 KB]
  • Ries, C., Y. Krippel, M. Pfeiffenschneider & S. Schneider, 2013. Environmental impact assessment and black, watch and alert list classification after the ISEIA Protocol of non-native vascular plant species in Luxembourg. Bull. Soc. Nat. luxemb. 114: 15-21. [PDF 652 KB]

 Page content last updated on 2023-08-17. Last proofread by Caroline Grounds on 2019-11-13.