A couple who bought a seaside property in Cornwall 15 years ago are suing the pensioner who sold it to them over a Japanese knotweed problem that has reduced the value of the property by £50,000.
Adam and Eleanor Smith, both 43, bought the three-bedroom detached home near Falmouth for £200,000 in 2002, unaware that the destructive weed was growing nearby.
Now the invasive plant, which can grow by up to 20cm a day, appears to be spreading on land next to the property, which was retained by the seller during the sale, as well as on the driveway of the couple's house.
Mr and Mrs Smith have launched legal action against 74-year-old Rosemary Line, claiming that the Japanese knotweed has reduced the value of their home, estimated at £500,000, by up to 10pc.
Ms Line insisted that she had done everything possible to stop the spread with herbicides and maintenance.
The plant is one of the world's worst invasive species and strong growth can damage concrete foundations, buildings, flood defences, roads, paving and retaining walls. It costs the economy around £166m every year in weed control and property devaluation.
Last month, Telegraph Money reported how an elderly homeowner whose bungalow lost half of its value thanks to the spread of Japanese knotweed had been embroiled in a legal battle with Network Rail over who will pay to eradicate the plant.
What to do if you find Japanese knotweed on your property
When the Victorians introduced Japanese knotweed to Britain, thinking it would be a pretty addition to the herbaceous border, they could not have predicted the damage its rapid, uncontrolled spread would eventually cause.
Today it can knock tens of thousands of pounds off house prices, and in the worst cases it can render a property impossible to sell.
The Government has begun a biological control plan which involves introducing thousands of bugs known as "plant suckers" to deal with the knotweed. According to the Royal Horticultural Society the insects are currently being trialled at a few test sites. If successful, the programme will be rolled out across Britain in the next five to 10 years.
In the meantime, homeowners who have an infestation have a choice of chemical or non-chemical controls. Both methods are expensive and can take as long as three years to fully take effect.
Non-chemical eradication involves the entire area being “dug out” – meaning the soil must be removed right down to the roots. The contaminated soil cannot be disposed of with normal household waste, however.
The chemical treatment uses a glyphosate-based weedkiller, which can be bought over the counter.
It’s only a weed - can’t I deal with it myself?
It can be tempting to try to tackle knotweed yourself. While you may be successful, Graham Ellis, a director at the Royal Institution of Chartered Surveyors, warned that DIY efforts to control the weed could go wrong.
“The most important thing is to get advice and to manage the situation properly,” he said. “People do try to cover it up or cut it back but the problem with Japanese knotweed is that it will continue to reappear. If it’s cut back it will only come back all the more vigorously.
"If you have any concerns it’s best just to get a specialist in. It may cost you money in the short term but in the long term it could save the value of your house.”
Digging out even what seems to be a small infestation is generally not successful: major roots from which re-growth inevitably appears can be as much as 7ft below ground. As with other far less serious but much-dreaded perennial weeds, such as bind weed and ground elder, every scrap of stem that remains in the soil is capable of becoming a new, invasive plant.
Cut stems or roots should never be put into green waste bins or domestic compost bins without being dried off thoroughly in the sun (and therefore killed) for days or even weeks beforehand.
The 1981 Wildlife & Countryside Act made it illegal to plant Japanese knotweed in the wild or to allow it to grow by carelessly disposing of unwanted cuttings or soil.