Floating pennywort, pictured in the Norfolk Broads, can kill river wildlife
Boaters and walkers using rivers and canals in Northamptonshire are being asked to look out for a virulent weed that threatens to choke the waterways.
The Environment Agency has spent £15,000 in the past two years eradicating floating pennywort.
The weed has colonised waterways after being introduced for garden ponds.
Large accumulations affect boating and increase the risk of floods by slowing water flow so the Environment Agency wants any sightings reported to them.
The weed, which is native to North America, also poses a threat to fish and other aquatic life by starving the water of light, oxygen and nutrients.
The weed was first spotted on a pond near the River Nene in 2004 but was later found in the river itself, prompting the Environment Agency to begin an eradication programme in 2007.
Chris Randall, from the Environment Agency, said: "This plant creates havoc for boaters and water managers.
"It clogs river channels and sluices and gets wrapped around boat propellers.
"It is also an arch enemy of native plants and can smother native flora through its ability to grow up to 20cm (8ins) in a single day.
"Once out of control, pennywort along the whole of the River Nene would cost us at least £50,000 per year to keep the river clear.
"We need people who are out on the river to keep their eyes out and to tell us where they find it.
"If they give us the location, we can add it to our control programme and they will have helped out both wildlife and boating on the river."
A recent survey of the river revealed that the number of sites where floating pennywort could still be found had dropped from eight when the annual control programme started, to three.
When the work to remove and spray the plant began, it was found at locations in Islip, Higham Ferrers, Titchmarsh, Wadenhoe and Lilford.
Now, only small quantities remain on a 5km stretch of river between Islip and Higham Ferrers.