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vert bar Hydrocotyle ranunculoides
Last updated 25th November 2007

Floating pennywort (Hydrocotyle ranunculoides)is a North American plant which grows at an alarming 30 cms a day and can choke rivers,canals, ponds, lakes and ditches.
It forms dense floating mats on the water’s surface. This causes a multitude of problems including deoxygenation of the underlying water, killing fish and invertebrates and choking of drainage systems and sluices, causing localised flooding. Most seriously, floating pennywort outcompetes native plants

In Ireland, Floating Pennywort was initially recorded from a clay pit on the Ards Peninsula in Co. Down, in 2002, and it is now known from a mill weir on the Six Mile Water, at Dunadry, in Co. Antrim.
The species was first recorded in the UK in 1990 and has spread dramatically to well over 100 sites.



On the 13th November Environment and Heritage Services (NI) and the National Trust began a major removal exercise at Glastry Clay Pits.

One of the pits following removal of ca. 30 tons of Hydrocotyle ranunculoides.

The reddish scum is Azolla filiculoides. In the ponds there was also Crassula helmsii and Myriophyllum aquaticum.

Prior to removal, the mat if floating vegetation was cut using a handsaw into manageable rafts ca. 3 m square.

These could then be pulled to the banks for lifting out.

c/o Dr. Joe Caffrey
Senior Scientific Officer,
Central Fisheries Board
joe.caffrey at

or Matthew Jebb matthew.jebb at


Klemm, V.V., Siemon, N.L. & Ruiz-Avila, R.J. 1993 Hydrocotyle ranunculoides: a control strategy for the Canning River National Park. Swan River Trust Report No. 6.

McChesney, C. 1994 Literature review of the Genus Hydrocotyle L. (Apiaceae), with particular emphasis on Hydrocotyle ranunculoides. Swan River Trust Report No. 18.

Newman, J.R. & Dawson, F.H. 1999 Ecology, distribution and chemical control of Hydrocotyle ranunculoides in the U.K. Hydrobiologia 415: 295-298.