A FOREIGN pest is invading local towns, the countryside and river edges. Japanese knotweed is an "alien" species which grows incredibly fast, very tall – and is almost impossible to kill.

It is so strong it can push through masonry, and will re-grow from bits of stem. It is spreading particularly fiercely this summer because so much of the countryside has been closed to maintenance work.

"It is noxious, and is a notifiable species on the Dangerous Weeds List," says Richard Halford of Gloucestershire County Council's environmental department. "If you see it growing on roadside verges, you should contact the divisional surveyor."

Divisional surveyor for the Forest of Dean district is Robin Peacey at the Cannop depot (Dean 860777).

"The gist of advice about its eradication is: don't cut it. It just grows back from the rhysomes under the soil.

"You need to spray it with a weedkiller containing glyphosate. But there are two schools of thought about it when it's best to spray.

"Some think it's best in the spring when the new shoots are coming up. Others suggest late summer when the leaves are beginning to die back and the poison gets drawn into the roots.

"Either way it's going to take up to five years. It's a very tough old bird."

His colleague Richard Halford has a further thought.

"The best remedy is to cut it back and spray when the new shoots come through. You have to be careful to burn the cuttings, or it will start growing again."

Both stress that their own workforce wear protective clothing and face masks when spraying. Anyone doing it themselves should take expert advice beforehand.

At Forest Enterprise's Coleford HQ, Val Long comments: "We have been fighting it for years. We have trained staff to be able to deal with it.

"It looks a bit like bamboo, and was introduced by Victorians as a decorative addition to their gardens."

The plant often grows in clumps alongside streams, bits of stem being carried down by the current. In these circumstances, the Environment Agency should be informed – the area being covered by the Tewkesbury office (01684 850951).