Specialist “weed boats” are being used to try and clear a carpet of greenery which is choking a Lincolnshire river.

The vehicles are being used four days a week on the River Witham in Boston after plants covered the surface of the waterway.

Up to 4in (10cm) thick in places, the verdant mat is made of non-native fast-growing Azolla weed and duckweed.

Richard Bennett, from the Canal and River Trust, said wildlife and smaller boats had been affected.

“When you get to the late summer as we’ve had this year and the plant just proliferates,” he said.

“To be honest we’re overwhelmed. We are spending sort of £80,000 this year on weed boats four days a week in order to try and clear a passage through.

“But the size of the Witham and the scale of problem means this year we’ve just been overwhelmed.”

Azolla can reduce oxygen in the water and harm fish and other aquatic life. Each plant can double in size every three days.

Last year, thousands of weed-munching weevils were released into the Witham to help clear the growth.

Rodney Bowles, who runs day trips on the river in the Boston Belle boat, said the weeds “stink”.

“It’s contaminated the river,” he said.”You can smell it. People walk past here it’s horrible.

“It’s affecting the café and the people who are living on the boats.”

The Environment Agency (EA) said that river flows in Lincolnshire are low as the county is in a period of drought, which can contribute to algal blooms.

“We are providing advice in terms of flood risk and water level management with our partners the Canal and River Trust regarding the River Witham at Boston”, an EA spokesperson said.

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