Many Knotweed plats are being poisoned one at a time
A project to clear Japanese Knotweed along a 7-mile (10km) stretch of a valley in west Cornwall is under way.
It is set to take three years to clear the plant, but the National Trust is tackling the problem by poisoning many plants individually by injections.
The Knotweed is an invasive plant, which can grow through concrete and up to 12ft (3m) high.
The weed will be tackled along the Tregeseal River and tributary streams that run through the Kenidjack Valley.
The weed is a major environmental problem in Europe and is thriving in Cornwall, where it covers a combined area the size of a nature reserve (250 hectares) and poses a serious threat to wildlife, the National Trust said.
Simon Ford, nature conservation adviser for the trust, said the weed had spread at "an alarming rate".
He said: "Without significant work, many areas we value will be lost under a 'sea' of stems.
"Managing knotweed on a catchment scale as the trust is proposing to do in west Penwith is the most effective way of protecting and enhancing the site and its wildlife."
The invasive plant can even grow through concrete and tarmac
Many plants on the watercourse will be eradicated by being individually injected with a herbicide in their stems, the trust said.
The trust added that the herbicide it used was approved by the Environment Agency as safe for use next to watercourses and that some spraying would take place too.
The Victorians introduced the plant to the UK from Japan but many dug it up and threw it out. It then spread in the wild after starting to overtake gardens.
Planting it or dumping it can lead to two years in prison and/or a large fine under the Wildlife and Countryside Act 1981.
The £20,000 Tregeseal Valley Japanese Knotweed Project also involves Cornwall County Council, South West Water, the Environment Agency, Defra, English Nature, as well as other local councils and landowners.