Azolla filiculoides | Water Fern | Raithneach uisce



Conservation status

Not Assessed

Legal status

Third Schedule listed species under Regulations 49 & 50 in the European Communities (Birds and Natural Habitats) Regulations 2011. (Note: Regulation 50 not yet enacted). Listed as a schedule 9 species under Articles 15 & 15A of the Wildlife Order (Northern Ireland) 1985 (Article 15A not yet enacted).

First reported in the wild



Invasive species - risk of Medium Impact

Irish status


Introduction pathways - 1

Escape from Confinement

Introduction pathways subclass - 1

Ornamental purpose

Introduction pathways - 2

Escape from Confinement

Invasive score


NAPRA Ireland risk assessed


Species Biology


Small water fern, green free floating on the surface in dense mats, with short slender floating stems(Booy et al., 2015). May become bright red when exposed to stresses, leaves composed of densely overlapping scales, 1-1.5mm arranged on opposite sides of the stem, dark brown and hair like roots (Booy et al., 2015).


Lowers the dissolved oxygen concentrations in water bodies where it is present, as well as lowering macroinvertebrate abundance and diversity in streams in Zimbabwe (Gratwicke & Marshall, 2001). Decreases the growth of submerged macrophytes in lab experiments, affecting a native species Potamogeton crispus more than a fellow invader Elodea nuttalli, potentially facilitating the invasion of Elodea nuttalli (a process known as 'invasional meltdown') (Jones et al., 1996). Thought to be limited in Ireland due to winter frost (Lucey, 1998) but increasing winter temperatures are likely to be linked to an increase in Azolla abundance, as observed in Spain (Espinar et al., 2015).


Inland surface waters


Heterosporous (produces spores of two different sizes and sexes), producing acorn-shaped megasporocarps (c. 1 mm in length) and larger microsporocarps which rupture when ripe, releasing microspores embedded within massulae with an estimation that a thick mat of 8 kg m2 fresh biomass can produce 380000 microsporocarps and 85000 megasporocarps per m2 (Janes, 1998).

Pathway and vector description

Introduced as an ornamental and oxygenating plant in ponds and aquaria (Lucey, 1998).

Mechanism of impact

Competition, Bio-fouling

Broad environment


Habitat description

Slow moving, freshwater including canals, lakes and ponds.

Species group


Native region

North America, South America

Similar species

Azolla pinnata, an invasive species in Asia, Australia & the Pacific Islands, not currently present in Ireland


World distribution(GBIF)

Irish distribution

Establish - Widespread & Locally abundant. Appears to be expanding its range. Quantities at sites in Ireland may vary year on year depending on winter temperatures (Lucey, 1998).

Native distribution

Native to the Americas, it has been spread to all continents, except Antarctica.

Temporal change

Date of first record category


Fifty year date category


Records submitted to Data Centre in 2021

The following map is interactive. If you would prefer to view it full screen then click here.

How can you help

Report any sightings to the National Biodiversity Data Centre.

Further information

The weevil Stenopelmus rufinasus, used as a biological control agent in South Africa (McConnachie et al., 2003), is present in Cos Cork & Fermanagh, and may control populations of Azolla in these areas ( Baars & Caffrey, 2008). Weevils resulted in extirpation at 81% of sites in South Africa (McConnachie et al., 2004).



Baars, J-R. and Caffrey, J.M. (2008). Invasive Species Ireland, Case Study 4, Water fern, Azolla filiculoides – Under biological control in Ireland. Report prepared for Invasive Species Ireland.

Gratwicke, B., & Marshall, B. E. (2001). The impact of Azolla filiculoides Lam. on animal biodiversity in streams in Zimbabwe. African Journal of Ecology. Vol. 39, Issue 2. pages 216-218

Espinar, J. L., Díaz-Delgado, R., Bravo-Utrera, M. A., & Vilà, M. (2015). Linking Azolla filiculoides invasion to increased winter temperatures in the Doñana marshland (SW Spain). Aquatic Invasions, 10, 17-24.

Janes, R. A., 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(1-3), 23-26.

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

Lucey, J. (1998). Azolla filiculoides Lam.(Water Fern) in fresh and brackish water in E. Cork. Irish Botanical News 8:5.

McConnachie, A. J., De Wit, M. P., Hill, M. P., & Byrne, M. J. (2003). Economic evaluation of the successful biological control of Azolla filiculoides in South Africa. Biological Control, 28(1), 25-32.

McConnachie, A. J., Hill, M. P., & Byrne, M. J. (2004). Field assessment of a frond-feeding weevil, a successful biological control agent of red waterfern, Azolla filiculoides, in southern Africa. Biological Control, 29(3), 326-331.

CABI Datasheet