Floating Pennywort

Grantchester Mill September 2017, before clearance. Image (c) Environment Agency

Click here for Pennywort Resources

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 clearance of the large rafts by contractors.

No single body has responsibility for the eradication of invasive species, so in 2017 Cam Valley Forum set up the ‘The Pennywort Project‘ with 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.

During 2018 we engaged contractors to clear the bulk of the pennywort and the tree banches that were trapping Floating Pennywort, allowing volunteers to deal with the strands as they emerged.  The first year’s work was very successful, leaving just a few strands of pennywort emerging during 2019. These were all removed by volunteers. We’ll need to remain vigliant in 2019  and hopefully we can “finish” the job onthe upper Cam

In 2019 we started to turn our attention to progressively eradicating Floating Pennywort from the Cam below Cambridge, working in partnership with Cam Conservators and others.

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 or email us. If your community group would like help in getting started with pennywort pulling on other rivers, please contact us.


Status Maps of floating pennywort on the Upper Cam from 2016 -2019, together with a log of recent status reports.
Downloadable advice sheet on pulling pennywort
Environment Agency advisory note on Floating Pennywort
Pennywort pulling Seasons:  A CVF guidance note for volunteers
PlantTracker: A very useful website and app for tracking a wide range of invasive species throughout the UK.