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How volunteers are winning the war against pennywort on the upper River Cam




Grantchester Mill Race before and after its clearance. Pictures: Mike Foley
Grantchester Mill Race before and after its clearance. Pictures: Mike Foley

The scourge of the floating pennywort menace is on the verge of being eradicated on the upper River Cam following the diligent work of an army of volunteers.

Cam Valley Forum members Mike Foley and Anne Miller have coordinated the resistance against one of the UK’s worst invasive weeds, Hydrocotyle ranunculoides, also known as floating pennywort.

Having been imported from South America as a garden pond plant, it spread and was first recorded in the wild in this country in 1990. It is now designated by the European Union as an ‘alien species’ and has been illegal to sell in shops and garden centres since 2014.

The huge mats can cause flood risk by creating dams, disrupt navigation, damage life in rivers by restricting light and oxygen and cause fish stocks to disappear. Mats can weigh as much as 75kg per square metre.

It was first spotted on the Upper Cam in 2012, and by 2016, the pennywort was in the upper reaches of the Cam – in the Bourne Brook, Byron’s Pool area and Grantchester Meadows, where it totally blocked several places.

The Cam Valley Forum, an organisation that tries to influence people about the whole Cam catchment and its tributaries and streams, focused on the upper Cam to Cambridge centre, with Cam Conservators, to Bottisham Lock, and the Environment Agency looking after further down river.

Financing allowed contractors and weed attack boats to be brought in to combat the threat by early spring in 2017, and a punt working party was formed with six punts and members of Cambridge Canoe Club.

But the pennywort was caught in the branches of the willows along a 2km stretch in the Grantchester Meadows area.

Floating Pennywort growth for identification. Picture: Mike Foley (35231712)
Floating Pennywort growth for identification. Picture: Mike Foley (35231712)

“We couldn’t reach it with our bare hands, we couldn’t use nets because they got torn up by the branches and you couldn’t get rakes in there because of the tangle of the branchwork,” explains Mike.

“With pennywort you only have to look away in the summer and it is growing so rapidly, that it just takes over the space that you have already cleared.

“If you do half a job, it comes back again with a vengeance.

“The idea was a good plan, the weed boats coming in, clearing the main bulk, we would then follow it up and just take out the remnants of it and thereafter if any small part dared to show itself, we would pounce on it, maybe with canoes and/or kayaks and just simply remove it.”

The weed boats referred to come in two varieties – one with a vertical cutting bar that slices through the pennywort and another that can lift up a cut-out partial mat to take it to the bank.

The manual approach, using rakes, was working on the Grantchester Meadows side of the river, but the other bank was more inaccessible because of the lines of willows and the private farming land, so the best method was in boats.

Grantchester Mill Race before clearance 2017 (Acknowledgment, Environment Agency). Picture: Mike Foley (35231679)
Grantchester Mill Race before clearance 2017 (Acknowledgment, Environment Agency). Picture: Mike Foley (35231679)

However, with not quite enough working parties going out in 2017, it meant the pennywort got out of hand again, and it was meeting in the middle of the river.

Users were unable to enjoy pastimes and the Cambridge Canoe Club could not race as they did not have the width of water to paddle fast.

With the menace hiding in the branchwork of the willows, something had to be done and with funding from a variety of different sources, contractors were brought in to cut back the branchwork to remove the tangle in the water – although not a hard prune so the willows kept the same structure.

“Willows that were actually growing in the water, and the branches that were falling into the water, were cut away leaving it clear,” says Mike.

“As part of that, the pennywort was dislodged from all the branchwork and drifted down river.

“Being responsible, you’ve got to stop that drift down the river so we put chicane bombs down the river to catch pennywort that was being dislodged during the branchwork.

“It was very successful. By spring 2018, we had cleared as much of the branchwork as we wanted and the pennywort had coincidentally gone as well. We started almost with a clean sheet.”

Cambridge Canoe Club autumn 2017 Grantchester Meadows. Picture: Mike Foley (35231694)
Cambridge Canoe Club autumn 2017 Grantchester Meadows. Picture: Mike Foley (35231694)

It put them in a far stronger position to use boat volunteers more effectively.

By June of that year, Mike took a kayak and net up the river and scooped out 40 different strands of pennywort into a supermarket bag between Cambridge and Byron’s Pool and back – staying on top of the menace is crucial as one small part can become up to a meter squared in just a few days.

“In 2019, we just kept on doing the same,” says Mike. “We had dedicated people who would go very slowly up and down the river in a canoe looking for it, occasionally. But we would also have a lot of casual observers, Cambridge Canoe Club especially.

“By the end of last year, there were approximately 10 small sites that we found strands in up river and only a single site so far this year.”

The pennywort is now almost beaten and gone from the entire upper Cam to Baits Bite Lock.

Mike may have been doing the main organising with Anne, but he describes the success in combating the water menace as “an overall Cambridge community effort”.

Cam Valley Forum volunteer Anne Miller searching for small remnants along Grantchester Meadows. Picture: Mike Foley (35231708)
Cam Valley Forum volunteer Anne Miller searching for small remnants along Grantchester Meadows. Picture: Mike Foley (35231708)

“I feel that we’ve done a good job for the community,” he says. “The river is used by a whole range of people – punts, canoes, paddleboards.

“All these things go up and down that part of the river so much, and people enjoy it so much that I feel we have done a good job there so other people can enjoy the river again.

“I do feel uneasy as I have seen it carried on the flank of a mute swan and if some pennywort attached to a mute swan and it flew off a mile and landed, that could be the start of a new infestation.”

Grantchester Mill Race after clearance by weedboat 2017 (Acknowledgment, Environment Agency). Picture: Mike Foley (35231690)
Grantchester Mill Race after clearance by weedboat 2017 (Acknowledgment, Environment Agency). Picture: Mike Foley (35231690)

So Mike remains on his guard.

Prior to the lockdown, there was a 1km infestation found at Great Shelford, Great Shelford Award watercourse which feeds into Hobson’s Brook City Wildlife
Site and the brook provides water
to the Botanic Garden lake and fenland area.

A third of the area was dealt with before restrictions were enforced.

“It is pretty good down to Baits Bite Lock, and I think we will be able to say by the end of this year, the river should be clear of pennywort [to Baits Bite Lock],” adds Mike.

Further down river, revetments and reed beds have proved difficult to overcome as a hiding place for the pennywort, but members of the Cam Valley Forum have been supporting and volunteering with the Cam Conservators, which manages the navigable part of the river between Cambridge and Bottisham Lock.



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