In the penultimate article of our ‘Invasive Non-Native Species’ series, Elizabeth Kimber looks at floating pennywort…
Floating pennywort (Hydrocotyle ranunculoides) is a an aquatic plant which forms dense mats of rounded leaves that float across the water’s surface. It is most common in the wild in south east England, but is appearing more often across the rest of the country. The first record of floating pennywort in the United Kingdom was in 1990.
Identification & Ecology
Identification features of floating pennywort include leaves that can either be floating or emergent., fleshy stalks, fine roots, horizontal growth and shiny, kidney-shaped leaves with a crinkled edge. The leaves are usually broader than they are long. Identification of the species does not vary throughout the year, although in winter it is most likely to be found at the water’s edge. Tiny white flowers are rare, but if present, appear between July and August.
Floating pennywort occurs in shallow pools, drawdown zones of lakes and in shaded, seasonally inundated wetlands. In England it has been recorded from still or slow flowing water in lakes, ponds, streams, ditches and canals. Floating pennywort occurs in freshwater.
Natural erosion-deposition processes can be disrupted by large masses of floating pennywort. It can also disrupt movement of animals, outcompete native aquatic plants, block light needed for photosynthesis, disrupt predator-prey relationships, create mosquito breeding areas, increase water temperature by absorbing sunlight and increase nutrient loads to the water through die back.
Floating pennywort may impact tourism, fishing and water sports as dense mats can prevent access to recreational areas. This could have an economic impact either through clearance costs or the cost of the lost tourism to the area. Across Europe and Great Britain these costs are estimated to exceed £25 million per year. Furthermore, the impacts on biodiversity and habitats are equally important though harder to quantify.
Legislation which attempts to control the distribution of floating pennywort includes Schedule 9 of the Wildlife and Countryside Act, 1981 (as amended), which makes it illegal to distribute or allow the release of floating pennywort into the wild.
Floating pennywort is difficult to control due to rapid growth rates (up to 20cm per day), and it’s ability to re-grow from a small fragment. Cutting at regular intervals from May-October will prevent complete dominance and help to manage the rapid growth. The cut material needs to be removed from the water immediately. Following cutting, hand pulling should be utilised to reduce re-growth. Chemical treatment should only be used at the end of the season when all other plants have died back.
CABI is currently undertaking a project which aims to identify the safest and most effective biological control agent for floating pennywort through comprehensive host range testing in the quarantine facilities. A damaging weevil (Listronotus elongatus) was prioritised for study, as well as a number of fungal species. Tests are currently being undertaken to source, propagate and test an agreed plant list to ensure the weevil and other agents don’t attack related species and important native species. A comprehensive scientific dossier has been submitted to UK regulators, who will carefully consider the suitability for release based on scientific evidence and peer consultations. If the application is approved, the weevil could be released into the environment in the near future.
Header Image: GBNNSS ©Crown Copyright 2009
About the Author: Elizabeth Kimber works for an ecological consultancy based in Dorset (Lindsay Carrington Ecological Services Ltd). As part of the role she manages the ecological works for a multi-phase development. She conducts protected species surveys and holds a class 1 bat licence, smooth snake and sand lizard licence. She can be contacted via email: liz (at) ecological-services.co.uk.