Invasive Alien Species in Northern Ireland

Hydrocotyle ranunculoides, Floating Pennywort

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Hydrocotyle ranunculoides
© Graham Day
Hydrocotyle ranunculoides
© Graham Day
Hydrocotyle ranunculoides
© Paul Hackney
Hydrocotyle ranunculoides
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Hydrocotyle ranunculoides L.f.
One of a number of aquatic invasive species which pose a threat to Northern Ireland's ponds and lakes, and which have been distributed through the aquatic nursery trade.
Floating pennywort resembles a very large, robust version of the native Irish marsh pennywort (Hydrocotyle vulgaris L.). It is a creeping, stoloniferous, perennial aquatic, with floating or emergent leaves. The leaves have long stalks and roundish leaf-blades about 2 6cm across. Flowers are pale-coloured, tiny, and are followed by small, round, dry fruits.
Country of origin
North America.
Location in Ireland
Currently known from two sites a flooded clay pit in the Ards Peninsula, County Down and a mill weir at Dunadry, County Antrim.
Life cycle
The plant overwinters on the margins of waterbodies as a smaller, flatter plant than the summer growth. During summer, it can grow from the bank out into open water at the rate of 20cm a day, or up to 15m from the bank in a single season! Propagation is probably mainly vegetative, including fragmentation caused by physical damage. Spread of fragments to new sites in flowing water is possible.
Wildlife and habitat impacts
The plant outcompetes native species, and chokes waterbodies.
Human impacts
The dense floating mats of this species can pose problems for recreational users of lakes and ponds.
Key vectors
There are no special key vectors for transport of this plant, but long-distance dissemination can be by the trade in garden pond plants.
What you can do as an individual
You should avoid buying or planting this species in your garden pond. If you do have it, take care of disposing of any excess material burn or compost or bury it. Do not throw it into household refuse bins, or tip it on to waste ground, or dispose of it into streams, lakes or ponds. If you own or manage a garden centre selling plants for garden ponds, do not stock this plant. In the Netherlands it is now illegal to sell or even possess this plant.
Advice to specific groups relevant to this species
Gardening clubs should inform their members of the undesirability of growing this species. The Royal Horticultural Society actively campaigns against the use of this and other invasive alien aquatics.
Management measures
The Centre for Aquatic Plant Management (CAPM) gives details on their web site of methods of control. Bulk material should be removed physically before treating with a suitable herbicide, but since a single node of this plant can regrow, make certain that fragments do not escape down watercourses during treatment. CAPM point out that, once established in a pond or lake, it may prove impossible to eradicate this plant. Since the occurrences in Northern Ireland are few, the best approach at present is to prevent further spread.
Further information
Royal Horticultural Society - general information and advice on invasive alien plants, including Crassula helmsii
Royal Botanic Gardens Kew; information sheet T4 on invasive species
European & Mediterranean Plant Protection Organisation (EPPO) reporting Service
See the Invasive Species Ireland web site for further information -
Text written by:
Paul Hackney, Keeper of Botany, Ulster Museum