Environment, Safety and Standards Property Management RPI

Japanese Knotweed: What do I need to know?

Sally Walmsley
Written by Sally Walmsley

Brought into Britain in the 19thCentury as an ornamental plant to adorn the gardens of the great and good, Japanese Knotweed now has a stranglehold on the country – and can cause huge headaches for landlords.

This is more than just a weed. It grows quickly, takes significant work – and cash – to remove and if left unchecked can wipe thousands off property prices.

The devastating effects of this plant are so extreme that you could even be refused a mortgage.

Not only that, but you could face a fine or even a jail sentence if you remove it, but do not dispose of it properly, causing it to spread. It could even see you landed with an ASBO.

And it is not just you as a landlord affected. The government estimates the total annual cost of Japanese Knotweed to the British economy is £166 million in treatment and home devaluations.

So how do you identify whether that pesky plant growing out of control in the garden is a harmless weed – or the dreaded knotweed? And once you have identified it, what do you do next?

Why is Japanese Knotweed such a threat?

Japanese Knotweed is extremely invasive. It can grow through concrete and tarmac and can have deep roots going down as far as four metres and spreading as wide as seven at a rate of up to a metre a month.

It can cause structural damage to buildings and paths, cause walls to collapse, with roots that can crack or block underground drains and affect patios, driveways and paving.

How to spot it?

Growing at up to 20cm a day knotweed, or Fallopia Japonica to give it its Latin name, starts to grow in early spring and by this month can reach around 1.5m high – growing to up to 3m in June, often in dense thickets.

Knotweed has:

  • A purple speckled stem
  • Large lush-green shield-shaped leaves with a flat base
  • Leaves that are arranged in a zig-zag pattern along the stem
  • a hollow stem with regular nodes, like bamboo
  • distinctive white flowers during summer months
How does it spread?

Japanese Knotweed can grow from minuscule fragments of ‘rhizomes’ – the underground network of stems and roots. This means it can spread very easily, which is why proper treatment and disposal is necessary.

What is the financial impact?

The presence of Japanese Knotweed can render homes unsaleable. In a recent case in Wales (see panel) two neighbours saw the value of their homes halved after knotweed spread from a nearby railway embankment.

The Council of Mortgage Lenders (CML) said lenders determine individual policies on knotweeds and take into account a range of factors when considering whether to lend.

Valuers who inspect property for mortgage purposes are instructed to report to lenders where knotweed is present. The pre-contract enquiries that conveyancers seek as part of the legal process also ask whether Japanese Knotweed is present.

The CML said the mortgage lenders will normally require evidence of treatment that will eradicate the plant as a condition of lending if knotweed is present or near the site of a property before they will mortgage a home.

However, some may refuse to lend altogether, if there has even so much as a report of knotweed on the site in the past.

Also, although it is not specifically excluded, most buildings insurance companies do not cover damage and problems caused by Japanese Knotweed.

According to the Royal Institution of Chartered Surveyors (RICS) a number of mortgage lenders claim they are unable to obtain insurance cover for property affected by Japanese Knotweed.

This in turn canleave a home buyer in a difficult situation where their preferred lender will not grant a mortgage unless the home buyer can secure a building insurance policy that covers damage caused by Japanese Knotweed; but the home buyer cannot get an insurance policy that does.

What to do next?

First things first – stay vigilant. Carry out regular inspections of your rental homes to identify any potential issues. And if you are buying a property to rent out factor in the cost of tackling the Japanese Knotweed problem, which can run to several thousands of pounds, even for a modest property. Treatments include chemical spraying and excavation.

Alex Dayes is managing director of Japanese Knotweed Solutions, said there are various ways of treating knotweed, but it is so difficult to eradicate it is worth getting a professional to do the job. 

He said: “You can treat the problem yourself, but there is an element of risk and unless you know what you are doing it is best to get an expert in.

“If you want to sell the house most mortgage lenders will want an insurance backed guarantee – and if you are doing it yourself you won’t be able to offer this.”

There are two trade bodies which regulate the marketplace, the Property Care Association and the Invasive Non-Native Specialists Association and both offer insurance-backed guarantees.

Mr Dayes said: “You need a contractor from either PCA or INNSA. There is the initial cost for treatment and a warranty of two to five years, depending on the chemicals used. There is then typically a 10-year guarantee from the end of warranty period.

“Should the knotweed regenerate in this time then the lender has some form of comfort. The contractor will come back to spray the regrowth – and if the contractor has gone out of business one of the trade bodies will provide someone to do the work.”

Mr Dayes said that the best choice for people in an average home who do not want to extend is to go for a chemical control treatment – with the choice of chemicals depending on the other plants in the area and any watercourses nearby. Some can be injected into the plants, some are sprayed on.

Another option is to excavate the weed using a digger. Given the depth to which the plant grows this was once considered an extreme option, but is now more and more common as, once the infected soil has been removed by specialist the problem has been dealt with.

Mr Deyes said this method is favoured by those buying a property to do up and sell on.

He said: “For some it depends on the finances – a mechanical excavation is more expensive.

“Ball park figures would be about £2,500 for a two-year spraying and £3,000-£3,500 for a five year spraying. Mechanical excavation is £4,000-£5,000 upwards. The figures are about the same across the industry and whatever you sign up for, you should look closely at the insurance and the guarantee.

“If the knotweed is affecting more than one property in the neighbourhood – it often grows on boundaries – I would always try to get everybody on board. To successfully eradicate it you really need to treat it all simultaneously, unless you do this it is likely to migrate back over time.”

He said that, despite the scare stories discovering Japanese Knotweed is not the end of the world.

“If you are planning to do a property up to sell on then make sure you have your maths right and take into account how much the treatment will cost.

“There are a lot of scare stories out there. But the bottom line is that at the end of the day this is just a plant. Yes, it is a problem, but you can sort it out as long as you get sound advice and don’t cut corners.”

Crime and punishment

The issue of knotweed is so severe that the Home Office has reformed anti-social behaviour powers in a bid to tackle the issue.

  • You don’t have to remove these plants or control them on your land but you could be prosecuted or given a community protection notice for causing a nuisance if you allow Japanese knotweed to grow on anyone else’s property.
  • Community Protection Notices can be issued to people who do not tackle knotweed in their gardens. These orders can force people to address the issue with fine of up to £2,500 for non-compliance.
  • You must not plant Japanese Knotweed in the wild or cause it to grow there. This can include moving contaminated soil or plant cuttings. You can be fined up to £5,000 or sent to prison for up to two years.

Knotweed in the news

  • In 2011 the Daily Mail reported homeowners Matthew and Suzie Jones were told it would be cheaper to knock down and rebuild their £300,000 London home than to try to treat their knotweed problem after the weeds started to grow up through the floor of their home. According to the Mail and other newspapers the value of the property dropped from £305,000 to 50,000 as a result of the issue.
  • A man in the West Midlands killed his wife then himself back in 2013, claiming he was ‘driven mad’ by Japanese Knotweed spreading into his garden from a neighbouring golf course. At the inquest, it was revealed Kenneth McRae had become ‘obsessed’ with the weed preventing him from selling his house in future. No knotweed was found on their property although a patch was found nearby.
  • And earlier this year Network Rail was successfully sued for damages by two neighbours in Wales after knotweed from a railway embankment spread into the foundations of their homes – with the value dropping by half. It is thought the County Court judgement could have huge implications nationally opening the floodgates for claims potentially worth millions of pounds. It was reported Network Rail could appeal at the High Court.

About the author

Sally Walmsley

Sally Walmsley

Sally Walmsley is the Magazine and Digital Editor for the NRLA. With 20 years’ experience writing for regional and national newspapers and magazines she is responsible for editing our members' magazine 'Property', producing our articles for our news site, the weekly and monthly bulletins and editorial content for our media partners.


  • What rights do you have if your neighbour has it in their garden, right up against your extension (badly maintained tenanted house next door)? I can’t prove it’s causing damage to my house (yet), but I’m sure that if I remortgaged it would be picked up on. So how do I get them to remove it legally?

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