Property sales across the country are falling through because some banks refuse to lend against homes that are blighted by Japanese knotweed.

Buyers are losing money on fees and surveys only to be told they cannot get a mortgage, while homeowners are facing huge bills to remove the invasive plant.

The weed can grow 3m (9ft) in just ten weeks, so it can quickly spread from neighbours’ homes, nearby wasteland, waterways, railway lines or footpaths. Its root system can push through tarmac and damage buildings and drains. A pair of secateurs is no match for it, as it will soon sprout back. It can take years to fully eradicate the menace and some lenders will not offer home loans until the plant has been removed completely.

Queries about Japanese knotweed to Natural England, the Government advisor, have more than doubled from 14 in June last year to 39 last month, but there may be many more sites that remain undetected. The weed was recently discovered to be taking hold in an area of Hampstead in northwest London near the homes of the actor Tom Conti, footballer Thierry Henry and the TV presenters Esther Rantzen and Melanie Sykes.

A family in Stockport revealed this week that the sale of their property fell through when their home was down-valued by £20,000 because of knotweed growing on neighbouring land. Another couple in Hertfordshire saw the value of their property plunge from £305,000 to £50,000 thanks to the destructive plant.

Dr Paul Beckett, of Phlorum, a company that treats Japanese knotweed, said: “We have had a massive increase in inquiries in the past 18 months.” He added that this might be down to the guidance that the Royal Institution of Chartered Surveyors (RICS) has written on spotting and dealing with the plant.

“Conveyancing surveyors are now much more clued-up on knotweed as an issue to look for on properties.”

The RICS and Council of Mortgage Lenders (CML) worked together on guidelines for surveyors on reporting knotweed, and the Property Care Association (PCA) has developed a code of practice for those treating the plant to help to enable banks to lend. Yet many remain extremely wary. Barclays, through its mortgage arm Woolwich, says that it will automatically decline any properties that currently have knotweed as well as those where it has been treated due to uncertainty over whether the weed might return.

Halifax says that it would consider offering mortgages on these properties, but some borrowers such as Natalie Waterworth (see case study) are still being knocked back if the problem is deemed to be too severe.

Lenders such as Coventry Building Society, Saffron BS and Precise Mortgages will not consider lending until the knotweed has been removed by an approved company, while others such as Santander and Skipton Building Society may lend if an insurance-backed treatment plan is under way. NatWest and Nationwide Building Society say that they take a case-by-case approach.

Stuart Gregory, of Lentune Mortgage Consultancy, says: “There seems to be a communication problem, as I have often been given a blanket ‘no’ by lenders on knotweed cases, although many now say that they are starting to consider these properties. It is a grey area where there needs to be a clearer policy. It can be very unsettling for borrowers.”

If you want to ensure that your property remains mortgageable you would be wise to choose a reputable contractor that is a member of the PCA.

Philip Santo, RICS spokesman on knotweed, said: “It is not a good idea to try and get rid of it yourself. The chemicals that you can buy from the garden centre are not the same strength as those used by professionals with the right qualifications.”

It is also possible to get warranties from PCA members on their treatment plans to give lenders greater assurance that the plant will not return. There are strict rules about which treatments may be used near waterways and where you can dispose of invasive plants. Sourcing a company through the PCA will also help you to stay safe from rogue traders who might take your money without treating the area effectively.

Treating a domestic property using herbicides over a treatment programme of two to four years, with a 10-year insurance-backed guarantee via the PCA, costs from £2,000 to £3,000.

If you catch sight of the plant on your property it is vital that you do not bury your head in the sand (or soil). If it spreads to neighbours’ homes they could take civil action against you.

The CML says that its work with the RICS is helping to improve the situation for homeowners. “Better information and treatment to eradicate knotweed are making it easier for lenders to agree to lend on property where knotweed is present but being treated,” it says. Camden Council, where the Hampstead outbreak has occurred, says it takes swift action where knotweed is found.

Case study: ‘It strangled my purchase’

Natalie Waterworth, 29, the co-founder of a careers website, lost out on the flat she was trying to buy in Islington after knotweed was found.

The problem was revealed in the property information form that the seller had to complete. At first she was willing to proceed with the purchase, as her broker told her that some lenders were willing to consider properties with knotweed problems. She was turned down by Woolwich but Halifax offered her a mortgage in principle.

Ms Waterworth was quoted £4,000 to treat the knotweed on her prospective home and that of the neighbours. Halifax then decided that the risk to the property was too great and turned her down.

She has already given notice on her current home. Ms Waterworth says: “It has had such an impact on my life. I hope that other borrowers do not fall into this trap.”

Both lenders have now refunded the mortgage fees, but she is unlikely to get back the £2,500 she has spent on legal work, searches and surveys. Halifax has also offered a £100 goodwill payment for the inconvenience caused.

Blame the Victorians

Japanese knotweed, with its shovel-shaped leaves, was made popular across Europe as an ornamental plant by the physician and traveller Philipp von Siebold, who brought it to his nursery in Holland in the mid-19th century.

It was included in a parcel of plants that he sent to the Royal Botanic Gardens, Kew, in 1850 and it was also sent to the Royal Botanic Garden in Edinburgh four years later.

By then it was being sold by commercial nurseries across Victorian Britain. It was later discovered that the Horticultural Society had been growing a Chinese variant of the plant in its gardens in Chiswick from as early as 1825.

Source: University of Leicester