IN A matter of weeks, a North American weed has carpeted a stretch of the River Ray. Low water flows have aided the Azolla’s growth and in the past four weeks a 150-metre stretch of the river, in Islip, has been covered in the floating water fern.

Environment Agency conservation officer Pedro Collins said the weed was not harmful to fish and would die off naturally in the next few weeks as the weather cools. And plans are afoot to stop it in its tracks next year by using weevils, an insect that feeds and reproduces on Azolla, which was brought to the UK in the 1840s as an ornamental pond species.

The plant has created a carpet effect on the river, and even ducks have been spotted walking across it.

Mr Collins said: “It does look like terrestrial landscape, like grass, and the problem is sheep or cattle could wander into it thinking it was hard ground.”

He said the problem of imported plants such as Japanese Knotweed and Himalayan Balsam cost the British economy about £1.7bn a year to deal with.