Introduced in the 1980s from North America as an ornamental plant for the aquatic nursery trade, Floating pennywort (Hydrocotyle ranunculoides) is a fleshy-stemmed plant that has become a problem in parts of Britain, including the Broads. Sales bans in the UK in 2014 and across the European Union in 2016 came too late. By then, it had already escaped into parts of Essex and spread into a number of rivers and canals. Today, the annual cost of controlling it, together with the impact on tourism and recreation, may exceed £25 million.
Floating pennywort is highly invasive, growing up to 20cm per day in late summer, and quickly creating mats of lush foliage on the water surface, preventing navigation, blocking water flow and flood defences, deoxygenating the water and crowding out native plants. Until recently, the only way to control it was physical removal by hand or with machinery, followed by chemical treatment with weedkiller, but it can grow vegetatively from tiny fragments left behind, making complete control very difficult.
Since 2011, the Centre for Agriculture and Bioscience International (CABI), with Defra funding, has investigated the potential use of a biocontrol agent for Floating pennywort. They found that Listronotus elongatus, which is a weevil native to South America, controlled it by eating the plant and laying eggs on it, following which the hatched larvae eat into the stems, reducing its ability to spread. Unlike other methods, which are costly, labour-intensive, and all too often ineffective, biocontrol with the weevil presents a long-term, self-sustaining solution. The weevil has evolved to feed and develop only on Floating pennywort, reducing biosecurity issues that can otherwise arise.
After a decade of safety testing, the weevil was approved for release in the UK in 2021 and has since been successfully monitored on several sites, most recently in the Cam Washes SSSI, which became reinfested in 2024. Prior to the release, drone footage captured the extent of the Floating pennywort, and the authorities hope that drone monitoring will help show how effective the weevils are and whether this is a cost-effective solution.
Floating pennywort has been found in the Broads since at least 2020, most notably in the catchment of the River Ant between Dilham and Hunsett Mill and also in the dyke near Smallburgh. The Norfolk Non-native Species Initiative provides online expert guidance on dealing with it, and the Broads Authority employs a standard procedure. So far, control using mechanical and chemical means has generally been successful, but the time may soon come when the South American weevil will be introduced here.
The Broads Authority has invited the public to look out for Floating pennywort and report its location when found. It’s usually easy to spot with its shiny, crenellated leaves, but be careful not to confuse it with our native Marsh pennywort, which grows on damp ground in bogs and fens and is always rooted in the ground, never free-floating. The leaves of the native pennywort are small and round, with the stalk attached to the centre of the leaf, like an umbrella, whereas Floating pennywort’s leaves are kidney-shaped and have the stalk attached between the lobes.
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