Tackling the growing menace of invasive weed on River Cam

PUBLISHED: 16:31 01 December 2017

Pennywort at Grantchester Mill

Pennywort at Grantchester Mill

ILIFFE

ANNE MILLER, of Cam Valley Forum, explores the huge problem of floating pennywort on the river – and explains what you can do about it.

Clearing pennywort from the River CamClearing pennywort from the River Cam

I’m no botanist, but over the last few years I’ve watched with growing horror as huge mats of a strange weed spread along and across the Cam at Grantchester Meadows.

Earlier this year I discovered that this is one of the UK’s worst invasive weeds: floating pennywort (Hydrocotyle ranunculoides) and joined Cam Valley Forum and the fight to eradicate it from the upper Cam.

Floating pennywort is originally from South America and is spreading in the UK, clogging rivers and destroying habitats. Climate change is increasing its growing season, and it loves the nitrates in our water.

It was first spotted on the upper Cam five years ago, and spread quietly, so by this time last year, the Cam was totally blocked in two places at Grantchester Meadows.

Pulling floating pennywort

Stay safe. Pennywort is safe to handle, but working near a river is hazardous, so be careful.

Leave it somewhere dry to die. Lift the strands out of the water with a rake, and put them somewhere dry, several metres from the water. Be careful not to contaminate neighbouring ditches.

Don’t shred it, don’t spread it. Even a 1cm length of stem floating down the river can start a new colony, so remove patches cleanly and try hard to pick up all the little fragments.

A net or springbok rake can be useful. Clean your equipment before going to other river systems.

As punts and canoes pushed through they broke the pennywort into little pieces which then flowed downstream to start new colonies. We are suffering the consequences of that this year.

As you will see if you go for a walk on Grantchester Meadows, in places the river is nearly blocked by floating pennywort. However, we are making progress thanks to the hard work by volunteers and those on the “pennywort pulling” working parties. These are fun and effective, and have been run with Cam Conservators, The Wildlife Trust, Cambridge Canoe Club and Cambridge City Council.

One success has been in tackling the upstream “source”, which was above Byron’s Pool and Grantchester Mill. This was a good example of collaboration: the Environment Agency cleared the bulk mechanically, while volunteers collected the little pieces that the mechanical clearance inevitably left behind, thus stopping them spreading downstream.

On Grantchester Meadows, where the river is accessible from the bank, volunteers have kept it relatively clear. The problem, however, has been the pennywort trapped in the many fallen branches of the trees. The little pieces were impossible to remove, and over the year they have grown into huge mats that now nearly block the river. We are therefore extremely pleased to have been awarded a grant of £4,385 from Cambridge Water’s PEBBLE fund for professional contactors to prune back the most problematic branches this winter.

Volunteers clearing pennywortVolunteers clearing pennywort

This work will be done in a “habitat sensitive” way, aiming to preserve the beautiful natural appearance of the river bank. Trumpington Farm Company, Scudamores, Cam Conservators, Cam Valley Forum and Grantchester Parish Council are contributing too, giving us enough to do about 600m of our target 1km of riverbank.

If anyone would like to help, either by joining a pennywort pulling working party, or to make a donation to help us do more, contact anne@camvalleyforum.uk.

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