Buying a house with Japanese Knotweed

Japanese Knotweed is an intense subject which can scare the living daylights out of people. But its risk has been overstated and recent guidance looks to take a more balanced approach. So whether you're buying a house with Japanese Knotweed or selling one, here’s everything you need. to know.

Angela Kerr Director, Editor

Should I avoid buying a house with Japanese Knotweed?

Buying a house with Japanese Knotweed and wondering if you should proceed? The presence of Japanese Knotweed is an increasingly common problem highlighted in surveyor reports.

According to research by invasive plant specialists, Environet, approximately 1% of properties in the UK have knotweed. If we also include those properties that share a border with affected properties and are at risk from encroachment the figure could be more like 5%. According to Office for National Statistics there are approximately 27.8 million dwellings in the UK. That’s a whopping 1.39 million homes that might be affected by Japanese knotweed.

So should you cut and run rather than consider buying a house with Japanese Knotweed? Let’s look at all the facts.

Why is Japanese Knotweed bad?

The impact on homeowners of having Japanese Knotweed on their properties can be significant. This is because it was believed that the plant’s deep roots meant it had potential, if unmanaged and found within 7 metres of the property, to cause major structural damage to properties. As a result, people buying a house with Japanese Knotweed often found it difficult to get a mortgage. And many home sellers faced the expense of paying for treatment plans. The invasive species’ damage doesn’t stop there; if found, it’s presence has been known to wipe up to 15% off a property’s value.

Buying a house with Japanese Knotweed in 2024

However, RICS published guidance in 2022 that abolished the ‘seven metre rule’ regarding Japanese Knotweed in favour of a more lenient approach that means surveyors can use their discretion when assessing the impact of the presence of knotweed.

RICS said the advice reflected “an improved understanding” of Japanese Knotweed, adding, “The so-called ‘seven metre rule’ focused more on what has been demonstrated to be an overstated risk of Japanese Knotweed to buildings, rather than its sometimes-serious impact on amenity.”

The body said research has demonstrated, and it’s now generally accepted, that Japanese Knotweed poses little or no risk of structural damage to robust buildings with substantial foundations such as dwellings, as opposed to less sturdy structures with shallow foundations, such as conservatories, garages or boundary walls.

RICS said it also recognises that when Japanese Knotweed is encountered, the most appropriate objective is to ‘ensure an appropriate level of control rather than to automatically strive for eradication.’

And the guidance also adds that in some cases, such as when construction is proposed, “proper control may involve physical removal but in many domestic situations effective control can be achieved by the managed application of herbicides”. This is welcomed news if you’re buying or selling a house with Japanese Knotweed

Need advice on your lenders approach to Japanese Knotweed? Speak to a fee-free mortgage broker today

Japanese Knotweed mortgages

In previous years, if the words Japanese Knotweed were in the surveyor’s report commissioned by the mortgage lender, that would be enough for some lenders to decline a mortgage application.

This position has evolved over time and lenders are now generally more open to lending in many cases. RICS has a sliding scale of category a to d when it comes to the severity and this broadly fits how lenders will view your Japanese Knotweed problem. These categories are:

• Management category A: Action. This is described as ‘significant impact, action required. Seek advice from remediation specialist on appropriate action. Recommend mortgage retention pending specialist report’.
• Management category B: Action. The is classified as ‘potentially significant impact, action required. Seek advice from remediation specialist on appropriate action. Recommend mortgage retention pending specialist report’
• Management category C: Manage. Described as ‘low impact, no action and no mortgage retention required for lending purposes.’
• Management category D: Report. If Japanese Knotweed is visible on adjoining land within 3m of the boundary. Described as limited impact. ‘No action and no mortgage retention required for lending purposes except in exceptional circumstances’. However, if there are concerns about the scale of an off-site infestation that may possibly affect the future value and saleability, this will be reported as a category D, even if it’s not within 3m of the boundary.

Some lenders are happy to lend on category C and D properties, but if it’s a category A or B property they may require a specialist report and the provision of an insurance-backed treatment plan. But there is still no industry-wide standard when it comes to Japanese Knotweed and each lender has its own lending criteria.

Need advice on your lenders approach to Japanese Knotweed? Speak to a fee-free mortgage broker today

Selling a house with Japanese Knotweed

The TA6 form which your conveyancing solicitor will ask you to complete includes a section 7 on environmental matters, requiring you to tell prospective buyers whether your property has Japanese knotweed. You either need to answer yes, no or don’t know. If yes, you will also be asked to give details of any management and treatment plan in place and supply copies of any insurance policy you have. See below for more on this.

If you know your property has Japanese Knotweed you must declare it; if you knowingly misrepresent any answers on the TA6 form or in replies to further enquiries and the buyer relies on that information when buying the property, the buyer can sue.

What if my neighbour has Japanese Knotweed?

When it comes to mortgages, problems haven’t only arisen when Japanese Knotweed is on your property. It has also caused problems if it’s in a neighbouring property or public land nearby. However, the guidance RICS issued in 2022 doesn’t appear to solve this issue. The guidance recommends that “surveyors flag the presence of the weed if it’s visible within three metres of a property’s boundary, such as in a neighbouring garden”. However, it advised this would have “limited impact” and shouldn’t affect mortgage lending.

However, some lenders will decline to lend if there’s an infestation on neighbouring land that it deems to be sufficiently serious.

What does Japanese Knotweed look like?

Japanese Knotweed can be recognised for its lime-green bamboo-like stem, speckled purple and red. It has heart-shaped leaves – with sprouts having a reddish tinge and turning a lime green. In the summer, they produce clusters of creamy-white flowers. And the plant’s extensive roots can penetrate deep into the ground. Japanese Knotweed is fast growing and hard to eradicate.

How to get rid of Japanese Knotweed

There are various methods of removing Japanese Knotweed. These include:

Weedkiller control

• Perhaps the most effective and simplest method for the home gardener to tackle Japanese knotweed is with the glyphosate-based weedkiller Scotts Roundup Tree Stump & Rootkiller. This has label recommendation for controlling Japanese knotweed, instructing it to be applied to the cut canes. Bayer Garden Super Strength Weedkiller also has label control for this weed
• Alternatively, try other tough formulations of glyphosate, for example Scotts Roundup Ultra 3000, Scotts Tumbleweed, Bayer Tough Rootkill or Doff Maxi Strength Glyphosate Weedkiller.
• Glyphosate is usually applied to the foliage and is passed within the plant to the underground parts
• It is useful to cut away old stems during the previous winter to allow good access. The best time for spraying with glyphosate is at the flowering stage in late summer. However, it is difficult to spray at this stage, when the weed is 2.1m (7ft) or more high
• A more practical approach is to allow Japanese knotweed to grow to about 90cm (3ft), which will usually be reached in May, and spray then. There will be regrowth and consequently a second application in mid-summer is useful. Check during September and if it has grown once more, spray again before growth begins to die down in the autumn. Check again the following spring
• Avoid spray coming into contact with garden plants. Glyphosate-treated knotweed will often produce small-leaved, bushy regrowth 50-90cm (20in-3ft) in height the following spring. This is very different in appearance to the normal plant and it is essential that this regrowth is treated
• It usually takes at least three to four seasons to eradicate Japanese knotweed using glyphosate. Professional contractors, however, will have access to more powerful weedkillers that may reduce this period by half
• Also, if you attempt this yourself, you should consider the fact you won’t get an insurance backed guarantee. This could be important if you’re planning to sell your property soon or if a neighbour is threatening legal action regarding the spread of knotweed from your property

Cultural control

• Digging out is possible, but due to the depth that the rhizomes can penetrate, regrowth usually occurs. This method also creates problems over disposal as Japanese Knotweed is classed as ‘controlled waste’ under the Environmental Protection Act 1990. This requires disposal at licensed landfill sites. Specialist Japanese Knotweed contractors are usually licensed to safely remove the weed from site but check first before employing their services. Alternatively, it can be destroyed on site by allowing it to dry out before burning. On no account should Japanese knotweed be included with normal household waste or put out in green waste collection schemes
• If digging out is attempted, remove as much root as possible, then repeatedly destroy regrowth. In this way the energy reserves in the remaining underground parts will be gradually exhausted; a process which may, however, take several seasons
• Find out more from Royal Horticultural Society about how to deal with Japanese knotweed.

Japanese Knotweed removal cost

The cost of removing Japanese Knotweed will vary on factors such as what method of removal you choose to use and the size of the treatment area. According to Checkatrade, the starting cost for herbicide treatment is around £950 for an area of around two square feet. While the cost of digging out and removing it from a site typically starts at £4,000. But this will vary depending on the extent of the problem.

Do I need to remove Japanese knotweed?

When it comes to Japanese Knotweed, there is more to consider than the ability to buy or sell your home. Most notably, if you allow it to spread off your land you could be prosecuted.

This is because an amendment to the Anti-Social Behaviour, Crime and Policing Act 2014 includes invasive non-native plants including Japanese knotweed.

• So it’s not illegal to have Japanese Knotweed in your garden. But you should aim to control it to prevent it becoming a problem in your neighbourhood. And if it has a “detrimental effect of a persistent or continuing nature on the quality of life of those in the locality”, this legislation could be used in order to enforce its control. And as a property owner, you may be prosecuted. The first known prosecution was in 2018 when a Bristol property owner was fined £18,000 plus costs for failing to control the spread of Japanese Knotweed. Plus they were ordered to secure a plan within a month from a specialist company detailing how the issue would be resolved.
• However, if you encounter problems with Japanese Knotweed in neighbouring gardens, the best course of action is to speak directly to your neighbours first. They may already be seeking action to control it.

For more information, see the guidance from The Environment Agency

5 other plants and trees to look out for when buying property

When buying a house, it’s a good idea to look out for:

1. Bamboo: Bamboo spreads easily and can become a nuisance quickly. While you’re free to plant bamboo if you wish, if it spreads to a neighbour’s land it’s possible they could take legal action. Bamboo isn’t classed as invasive so you shouldn’t have an issue getting a mortgage.
2. Oak, poplar and willow trees: If you have trees or large shrubs planted too close to your home they can cause subsidence as the plant drains the moisture from the soil causing it to dry out and sink. Estimates suggest that around 70% of all subsidence cases are a result of tree roots absorbing all the moisture out of soil. If a home has subsidence it can cause problems getting a mortgage. Also, roots can cause issues by spreading and breaking through concrete and tarmac. Read our guide Subsidence: What is it and how to prevent it.
3. Giant Hogweed: If sap from this invasive plant comes into contact with your skin and it’s then exposed to the sun it can blister badly. By law, you must not plant or intentionally grow it. If you have Giant Hogweed in your property and it’s considered a serious problem likely to affect how much buyers would be willing to pay for your house, you may have issues getting a mortgage. It is possible to remove Giant Hogweed yourself but you’ll need to be very careful and protect your skin and eyes. Check the place where it was growing for several years as it could come back. And you must destroy the waste property with by burning or in a licensed landfill. You may feel more comfortable getting an expert in to remove it for you.
4. Himalayan Balsam: This can quickly take over gardens. While you mustn’t encourage its growth you’re not legally obligated to remove it. But if it spreads to a neighbour’s property they might sue. However, Himalayan Balsam is unlikely to affect your mortgage chances, unless it has taken over your property to the extent that it affects its value.
5. English & common ivy: While ivy can look attractive, when it grows on a building it can penetrate and expand cracks, lift roof tiles and damage guttering. It’s unlikely to cause issues with mortgages, unless it has worked its way into the loft space. And while you can remove it yourself, be aware that it’s an offence to damage or destroy wild birds’ nests while they are in use or being built. So check for any nests.