The Japanese knotweed con exposed – and the plants that should really concern you

The very mention of Japanese knotweed can strike fear into homeowners and gardeners alike – with discovery of the invasive and highly resilient plant considered to be a death sentence for house prices.

We’ve all heard horror stories of neighbours forced to pay thousands after the plant spread under the fence and of homeowners sued for neglecting to mention an infestation when selling up.

But property and gardening experts say the fear of the weed is often misplaced and in fact, knotweed is no worse than garden favourite buddleia. 

Until last year, property surveyors had to assume that knotweed roots could creep as far as seven metres underground. This has rendered many homes with knotweed-infested gardens either unmortgageable, or vulnerable to substantial down valuations without serious money spent on the weed’s removal.

This is in spite of reports published in 2018 and 2019, which say the weed rarely extends more than four metres – in most cases it only extends by two – and that there is no evidence it causes more damage to buildings than other, less policed, plant species.

Leading knotweed expert Dr Paul Beckett, of environmental consultancy Phlorum, wrote this year in a journal published by the Royal Institution of Chartered Surveyors (Rics) that knotweed is in fact no more damaging than buddleia.

He also warned that “unscrupulous claimants or ‘ambulance-chasing’ claims firms” should get their facts straight before attempting to issue court proceedings.

Homeowners who discover knotweed spilling into their garden from their neighbour’s backyard are well within their right to file a private nuisance claim.

But Dr Beckett, who is also a member of the Expert Witness Institute, has suggested it is getting harder to win these cases and that solicitors blinded by “false confidence” could be in for a nasty shock.

He cited two failed claims which went to trial last year in London trying to argue that knotweed had encroached from one residential property to another.

Dr Beckett added: “A few years ago, it was easy for claims firms to spring up and churn out claims. We know firms have flyered people with gardens backing onto railways [following a famous case Network Rail lost to a homeowner in 2018].

“They knock on people’s doors saying there is knotweed in the area and tell homeowners they can claim if they’ve been affected.

“But now, the courts are a lot more savvy and defendants such as Network Rail are realising they do have a good defence against weak claims. As a result, I’m seeing claimants dropping out weeks before trial and facing hefty legal bills.”

‘They sent sniffer dogs round’

However the hysteria around knotweed has still caused homeowners to part with thousands to rid their gardens of the plant.

Caroline Cadoret was quoted nearly £100,000 for the removal of Japanese knotweed – along with 60 tonnes of soil – from her garden.

The 59-year-old said following her “extortionate” ordeal, she now feels as though the plant’s presence in her back garden was “not as big of an issue as it was made out to be” – but that she had to get rid of it so she could sleep at night.

Her biggest fear was that the plant might prevent someone from buying her house with a mortgage in the future.

Ms Cadoret first discovered the invasive plant on a bank holiday weekend in May 2021, just five months after she bought the house.

The plant can easily grow over the heads of humans, and is identified by its bamboo-like hollow stems and the purple speckles on its leaves.

Ms Cadoret said: “It was very hot weather, so I went out to water the garden. As I was watering, I saw it. I knew instantly. I thought ‘oh my goodness no, that’s Japanese knotweed’.”

The plant she had found in her West Sussex home was just two feet high. But she soon found a second area where it was growing. Both patches containing the weed were uncomfortably close to her home.

She said: “I was horrified. I feared I’d wiped thousands from the value of my property. It sent me into a downward spiral. I didn’t sleep for a week. This was my pension. I’d had a survey too which didn’t flag it. It was a real shock.”

The mere presence of the plant – even in small amounts – is said to diminish a property’s value by anywhere between 5pc and 15pc.

Trying to treat the plant with weedkiller just before a sale to hide the weed can backfire. 

In a case earlier this year, a property seller had to pay out £32,000 in damages and a further £95,000 for misrepresentation after a judge agreed with an expert that the seller had – despite saying knotweed was not present – treated the plant with herbicides.

Dr Beckett, who worked on this case as an expert witness, said he often uses Google Earth’s street view to check for evidence of the plant.

He added: “It has huge coverage in London especially. You can go through years of archives. It’s a very valuable tool.”

When Ms Cadoret discovered the weed in her sloped garden, she was told by two separate companies that they would need to dig down 2.5 metres everywhere they found it. With no side access to get a digger through and a river at the bottom, she felt forced to pay for a hand digging operation.

After a year of waiting, the three weeks of work commenced. Five companies came out in total, removing 60 tonnes of soil, backfilling it and then even sending in sniffer dogs to scout for any root remains.

She said: “I needed to be sure I had gotten rid of it and that I wouldn’t lose value on the house because of it. Now I can say it isn’t there and I have a ten-year warranty on it.”

Knotweed removal firms offer insurance policies which cover you for anywhere between five and 10 years. If it comes back during that time, then you can make a claim.

Quality Assured National Warranties is the main insurer behind these policies. Its full year accounts show it made a £1m profit last year – up from a £667,657 profit the year before. Shareholder funds also jumped from £754,686 to £1.7m.

‘If it’s a small amount you can treat it yourself’, says RHS

The Royal Horticultural Society will advise homeowners to treat knotweed themselves if the plant only amounts to a “little bit” or a “small isolated clump”.

Once dug out and with herbicides poured on the soil, a homeowner can pay a company to take the weed, classed as ‘controlled waste’, to a licensed landfill site.

Guy Barter, chief horticulturist at the RHS, said: “You can burn it in situ, but for urban garden owners this would be very antisocial behaviour.”

A homeowner then needs to make sure they have two clear years of regrowth before being able to say they have successfully treated it. The knotweed will either be dead or dormant and remain that way unless disturbed.

But a seller still needs to declare this if they fill out a TA6 form, which most sellers fill out. If they do not, there is an onus on the buyer to check off the presence of knotweed.

Mr Barter said: “It takes at least two years of treatment to kill it. The first year, you can set it back. Then the second year is a coup de grâce.”

Another method is to cover it with heavy duty fabric to suffocate it – however, the plant can also grow sideways and find its way out.

Developers of London’s Olympic Park got around this by spending £70m on digging five-metre-deep trenches to bury the weed and starve it of oxygen, after finding 10 acres of land they were trying to build on was infested with it.

While he admitted the plant can “go berserk” where natural enemies are not present to keep it at bay, Mr Barter said it is never a guarantee that the plant will spread.

“Clumps can stay the same over 50 years. One does sometimes wonder whether removal companies exaggerate problems to drum up business.

“Knotweed is more of a problem in larger gardens where it’s been allowed to run amuck.

“It’s a rather handsome plant, really. It was awarded a gold medal [by the Utrecht Society of Agriculture and Horticulture in 1847] when it was first introduced. It makes fantastic pea shooters.”

‘On an individual basis, people may have overspent’

Yet those who work within the knotweed removal industry are adamant it is not one where chancers looking for a quick buck would have much luck.

This is predominantly because of the long-term liability removal companies have to take on, which can last up to 10 years.

Steve Hodgson, of the Property Care Association, got involved with the market 12 years ago. He describes it as “mature” and “highly competitive” with a large number of firms which bid for work on a daily basis.

Mr Hodgson said: “We’re not talking about playing around in a pick up truck and spraying some glyphosate around. These companies aren’t going bust. The market is not a profiteering Wild West.

“No other country I’m aware of in the world has taken a pernicious plant like this and got it under control without primary legislation. It’s a triumph that taxpayers have not had to pay for this weed which has run riot. We don’t have an exponential growth curve of this weed like we did 10 years ago.”

The PCA oversees some 110 firms responsible for removing knotweed. The trade body got involved in an effort to assure mortgage lenders that the firms removing the weed were credible, and to start lending again on homes affected by the weed.

In response to Ms Cadoret’s near six-figure quote, Mr Hodgson said he “recoils a little bit at these numbers” and “can’t believe that was just excavation and removal of contamination of soil”. 

He added: “On an individual basis, people may have overspent.”

But he also pointed out that whether it was necessary or not, removal is an option and that clients are almost always presented with multiple options.

“Herbicide treatment takes a long time. It’s not quick. Removal, on the other hand, is.”

Mr Hodgson blames articles written about the plant in recent decades for why so many homeowners confronted with the plant feel as though their only option is removal.

He said: “Go back 10 years, and journalists were claiming it made your house fall down. This was far more responsible for what we see now than the industry.

“Knotweed is a dormant plant. While in many areas it should not be downplayed, in many other areas it should.”

Is bamboo the next knotweed?

Another plant proving hard to control that is still regularly sold in gardening centres is bamboo.

There are some 1,000 different species, and the same laws apply to it as they do to knotweed – if it invades a boundary your neighbour can make a private nuisance complaint.

Mr Seal said: “The number of cases we’ve seen where it’s got out of hand is already more than knotweed.

“The runners spread further laterally and far quicker. We’ve seen gardens seven metres wide where it has spread into the neighbour’s land

“It’s more damaging than knotweed too. It can come up through the concrete floor. And yet awareness of bamboo is far, far lower.”

In one Hampshire property, Mr Seal’s firm discovered hundreds and hundreds of metres of bamboo under the floor. They had to send a digger into the homeowner’s living room.

Back in 2010, TV gardener Alan Titchmarch wrote a piece for the Daily Express in which he described bamboo as “great garden plants with bags of character”.

Mr Hodgson said gardeners who have promoted the plant – particularly as one to plant in grass – have a lot to answer for.

He said: “Great swathes of people have planted it as a result.”

But he is reluctant to pit one invasive plant against another. “Bamboo has long-running roots that are difficult to kill, and knotweed has hard-to-kill characteristics too. They’re both just big cats.”