Grantchester Mill September 2017, before clearance. Image (c) Environment Agency
Floating pennywort (Hydrocotyle ranunculoides) arrived in the upper Cam in about 2012, completely covering it for months in 2016. This makes it difficult to punt or canoe up the river, out-competes native species and deoxygenates the water, killing fish. It also increases the flood risk.
Floating pennywort grows rapidly (up to 20cm/day in summer) from tiny fragments of stem which drift down the river. These then grow into large rafts that float on the water, which, when left undisturbed, can take root in muddy banks. It dies back in a cold winter before regrowing in spring, but climate change is increasing its growing season.
The Environment Agency advises that the best way of eradicating it is by a combination of volunteers pulling out the strands while they are still small (carefully disposing of them somewhere dry) supplemented by an annual mechanical clearance of the large rafts by specialist contractors.
No single body has responsibility for the eradication of invasive species, so Cam Valley Forum is coordinating ‘The Pennywort Project’. We have the aim of eradicating floating pennywort from the upper Cam within 5 years. This volunteer led effort is supported by community groups, landowners, agencies, charities, councils and local businesses.
How to help eradicate Floating Pennywort
If you would like to join pennywort pulling working parties on the upper Cam, please register as a volunteer here. If your community group would like help in getting started with pennywort pulling, or if you can help in other ways, please contact us
Status Map of floating pennywort on the Upper Cam in 2016 and 2017
Environment Agency advisory note on floating Pennywort
Downloadable poster on The Pennywort Project
PlantTracker: A very useful website and app for tracking a wide range of invasive species throughout the UK.