Floating Pennywort (Hydrocotyle ranunculoides) is an aggressive aquatic ‘invasive non-native species’ (INNS). As a native of North America, it first naturalised in 1990, as a result of discarded plants from garden ponds. It grows up to 20cm a day, quickly establishing dense mats of vegetation that can extend from bank to bank.
It is most common in the south-east of England but is now spreading to other parts of the UK including Lincolnshire. It tends to favour still or slower moving watercourses such as those heavily regulated by the presence of structures, such as the Witham in Lincolnshire. A population can become established by a single fragment drifting downstream and once present in a waterbody can rapidly dominate until it is present in extreme densities.
Large populations will rapidly out-compete native species, blocking out light, causing deoxygenation, obstructing air breathing insects from reaching the water surface and reducing water temperatures. It can become a significant flood risk, primarily by clogging weirs and other water management structures, including their controls.
Any populations that are not controlled become a source for the rapid expansion of new populations. Early identification and removal is therefore essential, as is a clear and consistent approach to its control. Treatment is dependent on the density of the plant but is limited to: mechanical removal, especially where populations are significant; the use of approved herbicides; and removal by hand if the population is limited.
It is essential that whichever control method is employed, they must all be followed up with regular monitoring. Of equal importance is having a suitable surveillance plan to record and monitor new populations.
What we are doing
Lincolnshire Rivers Trust are working in partnership with the Environment Agency and a trusted local contractor; a husband and wife team able to work in their ‘social bubble’ during these unprecedented times. We have been able throughout most of the year to undertake regular monthly treatment cycles, involving mapping, treating with herbicide and pulling by hand when the fragments are small or near to boat inhabitants. So far, we’ve treated and surveyed more than 50km of the main river and its tributaries. The good news is that treatment is working and there is a general decrease in the volumes we’ve recorded.
With our partners at RiverCare we are also asking river users to help us monitor and control the spread. People can now help by spotting and recording pennywort using i-record (download the poster below to find out more) to help us map and focus our efforts, alongside the usual advice of check, clean and dry of course!
Promotional posters will be visible along the publicly accessible sections and postcards provided to local river user groups for their members. We also hope to produce some floating keyrings, as a handy reminder for boaters, canoeists and fisherman to get online and record those sightings.