Floating Pennywort (Hydrocotyle ranunculoides) is a species native to North America. It was first discovered naturalised in Essex in 1990 and had been introduced to the UK as an ornamental plant for ponds. Since April 2014 it has not been available to buy in the UK. It is now common in the south-east of England and occasionally found in the north-west of England and Wales. Floating Pennywort is found emergent or floating on still or slow moving waterways and prefers areas of full sunlight.

Spread

Spreads easily via plant segments that break off and move through the waterways via currents or attached to boats. Roots form at the plant nodes, allowing it to quickly regenerate. During the summer it can double its biomass between 4-7 days. Floating pennywort is listed under Schedule 9 of the Wildlife and Countryside Act 1981 with respect to England, Wales and Scotland. As such, it is an offence to plant or otherwise cause this species to grow in the wild.

Environment

This plant can outcompete native species by blocking sunlight, reducing water temperature and preventing air-breathing insects from reaching the water’s surface. It can deoxygenate the water by reducing available light to waterweeds and algae, then causing nutrient overload when it dies back.

Boating

Floating pennywort can grow up to 20cm a day forming dense mats of vegetation that quickly dominate waterways making navigation difficult and impeding water flow and increasing flood risk.

The plant is difficult to control as the smallest fragment can lead to the plant re-establishing. Mechanical removal is best but the area should be netted off to prevent plant fragments from travelling downstream. Once the main bulk has been removed it is important to go over the area and remove all smaller fragments to prevent spread.

Chemicals can be used but only by those with suitable training.

Floating Pennywort should be reported via the Recording Invasive Species Counts scheme.