WORK to clear a beauty spot of the longest stretch of the most invasive weed in Britain gets underway today when contractors start injecting every single stem with poison.

Japanese knotweed has taken over a seven-mile stretch of the Tregeseal river and its tributaries in the Kenidjack Valley in Cornwall.

The valley is designated an area of outstanding natural beauty, but has been overwhelmed by the densely growing knotweed, which can reach 12 ft tall. Native plants and animals have been driven away by the knotweed, which has spread so widely across the county that it is estimated to cover a total of 250 hectares.

The warmth of Cornwall makes perfect for a host of invasive plants originally imported from abroad, including the Hottentot fig, New Zealand pigmy weed and Rhododendron ponticum. But it is the knotweed that is regarded as the worst of them all and the National Trust will today begin work on a £20,000 initiative to return the valley to its natural state.

Japanese knotweed is notoriously difficult to kill. Its roots can go 10 ft beneath the surface and it can regenerate from a missed or dropped segment as small as a drawing pin.

Contractors will tackle the Kenidjack Valley invasion near St Just by hacking down the stems and then injecting almost every stump with a herbicide. The herbicide, a specially licensed glysophate, is drawn down to the roots of the plant and kills it, though the process needs to be repeated for up to three years to be fully effective.

A small percentage of the plants — the knotweed has spread up to 80 ft on either side of the river — will be sprayed, but most will be injected by hand to avoid poisoning the river. It is illegal to transport Japanese knotweed or to propagate it, so the contractors will stack the cut stems on polythene until they have dried out and then burn them on the site.

Simon Ford of the National Trust, said: “The knotweed in the valley is a major problem. It’s a nightmare. We’ve tried everything we can think of in the past to get rid of it. We’ve tried pulling it up, we’ve laid carpets of over it, all to no avail.

“The normal method of control is cutting it and coming along with major amounts of herbicide, but that feels like flinging Agent Orange about — there’s nothing left.

“That’s why for environmentallty sensitive areas like Kenidjack Valley we’ve developed this method of injecting herbicide in each of the hollow stems. It’s laborious but seems to work.”

Among the plants that have been driven out of the valley where the knotweed has become established are bluebells, thrift, sea campion, kidney vetch, yellow flag and purple loosestrife. Similarly, the knotweed monoculture drives out dippers, grey wagtails and Daubenton bats.

Mr Ford added: “When you have cleared away a patch of knotweed you find that pretty well everything else has been destroyed. Nothing can compete with it. It’s the most pernitious plant there is.”

The project to clear the valley is a partnership between the Trust, landowners, local authorities, South West Water, the Environment Agency, the Department for Environment, Food and Rural Affairs, and English Nature.

James Macfarlane, of the Cornwall Knotweed Forum, said the project builds on previous schemes to rid a neighbouring valley of knotweed. the initiative was successful and the area has now been reclaimed by native flora. “We hope that this lead will be followed on a wide scale elsewhere as it will prove of benefit both for landholders and the environment.”

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