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vert bar Azolla filiculoides
Last updated 12th May 2007

An aquatic fern, Azolla was introduced as an aquatic plant for ponds and aquaria. Persistent in garden ponds, it is an occasional escape from cultivation, and is now apparently becoming more widespread. As well as establishing in still fresh and brackish water, but it can also infest flowing water, such as the river Barrow and Limerick canal. The quantity at a site fluctuates from year to year (e.g. Wolfe-Murphy 1997, Lucey 1998).


Close up of Azolla shoots.
A healthy mat may be up to 12 cm thick.
Azolla in the Limerick Canal. the red colouration is characteristic of Azolla in late summer and autumn.


Information Sheet 22: Azolla filiculoides, Water fern from Centre for Ecology and Hydrology (NERC) in UK.

Biological Control

BBC Manchester article on Stenopelmus rufinasus, Waterfern Weevil.

Azollacontrol, from CABI Bioscience, makes use of the North American weevil Stenopelmus rufinasus. This weevil is a highly effective natural enemy of Azolla filiculoides. The weevil, which can only feed and reproduce on Azolla spp., has proved itself to be successful biological control agent in laboratory and field trials in South Africa as well as the UK.


Dr Jan-Robert Baars Ph.D. of the School of Biology & Environmental Science at UCD, is experimenting on the possible use of the water fern weevil in Ireland.

Known Sites for Azolla filiculoides (based on Reynolds (2002)):
Vice-County Location
4 Lough of Cork, 8 miles (c.13 km) W of Queenstown (Cobh) Junction, first seen there 1934 (Praeger 1946). Marsh by Tramore River, N of Frankfield c.1950 (DBN). Lee Fields 1970s, 1995; Clogheenmilcon marsh, Blarney 1995 (O'Mahony 1996).
5 Near railway between Little Island and Glounthaune 1907, plentiful in large pond (DBN).
Near Queenstown (Cobh) Junction 1911, plentiful in brackish water having spread from adjoining garden (Druce 1911, 1912).
Little Island 1974, 1995 (O'Mahony 1996). Little Island 1994-95, and Great Island since 1994 (Lucey 1998).
Castlemartyr demesne 1994, artificial lake (M. Scannell).
Dower Lough near Castlemartyr 1999
Ballybutler Lough (Scannell 2000).
12 Inish and Ballyteige Slob, Duncormick, 700 m S of Yaletown Bridge 1997 (DBN).
20 {Mount Usher garden, Ashford (Pim 1893).}
Buckroney marsh 1949, considered a deliberate introduction (Brunker 1949)
pond in Buckroney sand dunes 1965 (DBN).
21 [Near Dublin c.1904 (record in Simpson (1960, p. 413) from an unavailable publication).]
Grand Canal and several ponds since early 1980s (FDub 1998, DBN).
22 S of Drogheda 1957 (Wallace 1959a, but 22 not 31).
36, 37, 39 Lough Neagh 1988, washed up on the shore (Weyl et al. 1989).
37 S of Derrytrasna 2001, ditch (I. McNeill).
38 Clandeboye Lake 1985, introduced there (FNE 1992).
38, 39 River Lagan in Belfast 1995-96 (Wolfe-Murphy 1997, FBel 1997).
39 Dunore Point, Lough Neagh 1990, small ponds (FNE 1992).

If you have information about any other populations of Azolla filiculoides please notify the following:-
Matthew Jebb
National Botanic Gardens
Dublin 9
matthew.jebb at

Dr Jan-Robert Baars Ph.D.
School of Biology & Environmental Science
121 Research 2-Limnology Unit
Limnology Unit
School of Biology and Environmental Science
UCD, Belfield, Dublin


Lucey, J. (1998) Azolla filiculoides Lam. (Water Fern) in fresh and brackish water in E. Cork (v.c. H5). Ir. Bot. News No. 8: 5-7.

O’Mahony, T. (1996) A report on the flora of Cork ( H3-H5), 1995. Ir. Bot. News No. 6: 35-40.

Scannell, M.J.P. (2000) Cloyne, East Cork (H5), 26-27 June 1999. Ir. Bot. News No. 10: 50-53.

Wolfe-Murphy, S.A. (1997) Some non-native aquatic plants in the lower River Lagan. Ir. Bot. News No. 7: 27-28.