What is Floating Pennywort?
Floating pennywort (Hydrocotyle ranunculoides) is an aquatic plant that was introduced to the UK in the 1980s by the aquatic nursery trade. It is a fleshy stemmed plant which grows into mats of foliage across waterways.
Floating pennywort is native to the Americas, and since being introduced has caused several issues as spread rapidly from garden ponds into waterways. Across Great Britain and Europe, these costs are estimated to exceed £25 million annually!
Why is it a problem?
Floating pennywort grows very rapidly in late summer, swamping waterways, blocking water flow and crowding out native plants. As the plant can grow up to 20cm per day, it easily takes oxygen from fish and insects and becoming more and more of a problem throughout the UK. The plant blocks canals, rivers and other waterways such as ponds.
Floating pennywort on a pond in Wynyard, Stockton.
Management
Similar to Japanese knotweed, floating pennywort has the ability to grow from the smallest plant fragments. This makes removing the plant even more difficult and a huge expense. There are a few different management techniques used for removal.
Pulling/Cutting: Regular cutting from May until October will prevent complete dominance which helps to manage this plant. Once cut, needs to be removed by hand pulling.
Spraying: Chemical treatments can work well on this plant however can take up to six weeks in slow flowing water bodies. Similar to giant hogweed, established colonies spray applications may not reach all the leaves at the first attempt. It is therefore essential to carry out a follow-up treatment about 2 to 4 weeks after the first application. Agreement must be obtained from the local Environment Agency office before application of herbicides in, on or near water.
Biocontrol: The Centre for Agriculture and Biosciences International (CABI) are currently working on controlling floating pennywort using biological control, specifically a weevil from South America. Although this method is still in its youth, it is looking promising for the management of floating pennywort.
Weevil biocontrol (CABI)
Floating pennywort is present at a few locations within the Tees catchment, which is being controlled using various methods. We hope to have biocontrol sites in the future, working with local authorities to reduce the problems this species causes to our waterways.
We are currently working with other organisations in the North East region to not only control floating pennywort but to also look at how we can prevent spread into other catchment areas!
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