Japanese knotweed: how to identify and manage this invasive plant
Everything you need to know about Japanese knotweed, the fast-growing plant nobody wants in their garden.
Everything you need to know about Japanese knotweed, the fast-growing plant nobody wants in their garden.
Japanese knotweed is notorious. Homeowners fear it, lenders steer clear of properties where it’s left untreated, and court cases dealing with its presence have seen people forced to pay out large sums.
It can be tricky to cut through the noise surrounding Japanese knotweed. Frightening media headlines have included a case of a London accountant landed with a huge legal bill after selling his home – because Japanese knotweed was found behind the shed.
So how do you identify Japanese knotweed through the seasons? And how it can affect your home insurance? What do you do if you’re buying or selling a home and think the invasive plant might be growing there?
And most importantly, what solutions are there for managing Japanese knotweed, and how do they work?
We'll answer all these questions for you, and more.
With homeowners nervous about what finding Japanese knotweed in their gardens could bring, we teamed up with Dr Paul Beckett to learn the facts.
An expert witness in several legal cases, Beckett has an academic background in plants and ecology. He has also founded Phlorum, a consultancy specialising in the detection and treatment of Japanese knotweed.
Japanese knotweed is an invasive species with a deep, far-reaching system of roots (which are technically not roots but rhizomes – creeping underground stems).
It’s a perennial, meaning it lives for several years, with the dense stem growth seen above ground appearing rapidly in spring then dying back each winter.
Japanese knotweed has been growing in the UK since the late 1800s, when it was introduced from Japan by German botanist Philipp Franz von Siebold, who admired its impressive structure. It wasn’t long, though, before the Victorians regretted his infatuation.
“It grows on volcanic soil in Japan,” says Beckett.
“This is free-draining and strongly alkaline soil, so knotweed is drought-tolerant and will grow on anything.”
Tasty and nutritious
There’s no denying that Japanese knotweed is a nuisance in the UK, but did you know that it is edible?
The young stems tastes a little like a lemony rhubarb, according to Environet, and feature in sweet and savoury recipes from jams to soups and wine. It is a source of vitamins A and C and contains zinc and potassium, as well as an antioxidant called resveratrol – hence its centuries-long use in Japanese medicine.
Don’t forage Japanese knotweed unless you are sure it hasn’t been treated with herbicides, and don’t do anything that could help to spread it (some sources recommend heating any discarded trimmings in the microwave to kill them, and definitely don’t add them to your compost heap).
Japanese knotweed is known for its ability to damage buildings, a characteristic made worse by how quickly it grows.
According to the UK government, knotweed grows in two ways: from its rhizome spreading underground, or by new plant growth from fragments of rhizome.
The latter is highly problematic, as whole new plants can grow from pieces of root as small as 1cm (a third of an inch).
“There’s evidence that if the rhizomes aren’t killed, they can go into a state of dormancy,” says Beckett.
“And that dormancy can last a very long time. You may think it’s stopped growing because it’s not above ground, but then if you start digging the ground as part of landscaping or building works, then you could be reawakening that rhizome.”
"If you start digging the ground as part of landscaping or building works, then you could be reawakening the rhizome”
This can lead to soil over a wide area being contaminated with the rhizome or its fragments. Beckett’s website states that most reproductive knotweed material is found less than 1m (more than 3ft) underground, though the rhizome can grow at depths of 3m (10ft) and with a radius of more than 7m (23ft) from where the plant is visible above ground.
Such a large potentially affected area means, of course, that removing all fragments of the plant is no easy task. And traditional chemical-free methods for how to get rid of weeds won't usually work.
But there are ways to address Japanese knotweed growth, as we’ll explain.
The presence of Japanese knotweed in a garden or on nearby land doesn’t necessarily spell the complete destruction of houses and structures nearby.
“This is a misconception about Japanese knotweed,” says Beckett.
“It will only grow and cause damage to a structure if it can grow into a gap – the same way that trees and other plants that cause damage do. It won’t punch a hole in solid objects. But saying that, once it gets into a crack, it can exert incredible force sideways, jacking that crack wide open over time.
"It will only grow and cause damage to a structure if it can grow into a gap"
He adds that when Japanese knotweed does cause damage to a building, it’s because it has been there a long time and been allowed to grow into these cracks.
Improved recognition of Japanese knotweed has led to many people recording sightings around the UK. Though it may feel like the plant is a growing problem, Beckett says it’s probably a case of greater awareness rather than increasing prevalence.
You can see just how widespread Japanese knotweed is around the UK with this online map.
This is where the issue of Japanese knotweed gets a little sticky. Beckett told us the vast majority of Japanese knotweed growing on people’s properties is not causing any physical damage to structures.
But before updated guidance on Japanese knotweed was issued by the Royal Institute of Chartered Surveyors (RICS) in 2022, lenders used a distance measure of 7m (23ft) to assess the risk of contamination, blighting properties even with no apparent problem.
That changed when the RICS updated guidance replaced the distance measure with an impact assessment. Yet house prices are still being affected.
“You could have two properties that are identical in every way, but one has Japanese knotweed,” says Beckett.
“If you say to a willing buyer, ‘Which one do you want?’, they’ll ask for the one without it.”
People may consider buying a house with Japanese knotweed at a reduced price, but the fear is they may struggle to find a buyer should they later want to move.
Beckett estimates that Japanese knotweed could knock anywhere up to 7% – and sometimes more – off the value of a house because people don’t want to take on the risk.
Spotting the presence of Japanese knotweed sooner rather than later is key to minimising any potential damage to your home. Beckett reports the following common early signs:
Japanese knotweed shoots appear in March and April and grow fast. The shoots resemble young asparagus plants at first but can grow to more than a metre in height within weeks.
In summer, the plant can reach more than 3 metres (10 feet) tall, and you’ll see a thick canopy of shield-shaped green leaves supported in a zig-zag pattern by the hollow bamboo-like stems. Small white flowers will also appear between August and September.
Flowers and seeds are shed by autumn, and the leaves turn yellow and then brown by the end of October. The stems will likely turn red-brown and become brittle.
This continues into winter, when the leaves fall and stalks die, becoming straw-coloured and hollow.
Japanese knotweed has clusters of small, creamy–white flowers that appear on thin spikes around 10cm (4in) long.
Individual knotweed flowers on each spike are around 0.5cm (0.2in) wide.
The flowers appear in late summer and early autumn and attract bees.
The light green Japanese knotweed leaves are a large shield-like shape, around 15cm x 10cm (6in x 4in).
They have smooth edges and a flat base, with each leaf arranged alternately along the stem.
Some hybrids may have leaves that make them look more heart-shaped.
Japanese knotweed’s perennial root system (rhizome) grows underground.
The outside of the root is brown, and the inside is an orangey yellow.
The crowns, from which the stem grows, can be large, often around 40cm (16in) in diameter, with thick 3cm (1in) rhizomes growing in all directions.
Knotweed stems on mature plants are tall and can reach a height of around 3 metres (10 feet). They grow in clumps that look like separate stems from where the roots poke out just above ground.
The base of the stems can be quite thick, around 5cm (2in) and are light green with purple speckles. Rings, or nodes, around the stems resemble bamboo canes, but unlike bamboo, knotweed’s stems are hollow and fairly easy to snap.
It’s easy to confuse other plants with Japanese knotweed. Beckett has created this list to help people distinguish between a genuine knotweed problem and lookalike plants.
We know Japanese knotweed can impact house prices, but what about home insurance?
We spoke to the Association of British Insurers (ABI), which said that though most residential home insurance policies do not specifically mention Japanese knotweed in their wordings, it is unlikely to be covered.
“Most building insurance policies do not cover the cost of removing Japanese knotweed and will not cover gradual damage caused by knotweed growth,” the spokesperson said.
“This is because damage that’s caused gradually, rather than due to an insured event (fire, flood, storm), tends to be excluded.
"Most building insurance policies do not cover the cost of removing Japanese knotweed
“Home insurance may provide cover for subsidence or damage to drains caused by Japanese knotweed, but this will vary from policy to policy. Where cover is provided, this would not cover damage caused before the insurance was purchased.
"Often insurers will only provide cover in cases where you have taken the necessary steps to remove or control the knotweed.”
Yes, you should still be able to get cover if you have Japanese knotweed on your property, but you should be aware that gradual damage caused by Japanese knotweed is not generally covered by home insurers.
However, some insurers may assess this risk specifically and take into account the extent of growth and how it has been treated. After this assessment, insurers may be willing to provide cover for risks like subsidence, especially if you already have a comprehensive buildings insurance policy in place.
The best advice is to read your insurance policy carefully to understand what is and isn’t covered.
There is no requirement to tell your insurer about the presence of Japanese knotweed on your property unless they specifically ask about it.
As with all insurance policies, it is important to answer all questions fully and honestly, so if they do ask about knotweed then you must disclose it.
Some insurers provide legal expenses cover – usually as an optional add-on to standard home policies – in the event of needing to start legal proceedings or defend against any case made against you. Whether a claim can be made in relation to Japanese knotweed in a nuisance or negligence case will depend on the specific details of the legal proceedings in question, and so will be dealt with by insurers on a case-by-case basis.
It is also possible to purchase a specialist residential Japanese knotweed indemnity insurance policy. This can provide protection for home buyers if knotweed is discovered, and will cover the cost of a survey report to confirm the presence of knotweed, the cost of treatment and the repair of any damage.
It could extend to the cost of defending legal proceedings should any third party take action against you. These kinds of policies would not be included within standard home policies and so would need to be purchased separately.
Anyone who has ever bought or sold a home will know it’s a stressful process. Though the presence Japanese knotweed isn’t a deal-breaker for some buyers, you must tell them.
Saga Home Insurance expert Anna Thunstrom said: “Japanese knotweed is particularly notorious for causing structural damage to buildings, potentially impacting the property’s value.
"As a result, sellers are obliged to disclose its presence."
An appropriate report from a Royal Institute of Chartered Surveyors-approved (RICS) surveyor should look for obvious signs of knotweed and provide appropriate advice accordingly.
The advice might be to get a knotweed specialist in.
RICS surveyors will usually focus on potential building defects, so they might not be confident in identifying knotweed (especially if it has been partially treated, disturbed or even hidden).
If you get a professional survey done, you might be protected from legal action if the buyers later find knotweed on the property and try to blame you for not disclosing it.
Capture images of the outside areas of your property so that you have a record of what’s there in case a buyer tries to blame you for knotweed that appears in the future.
It’s important to do this in the growing season, as knotweed might not be visible in winter when the stems die back.
If you find knotweed on your property, it is important that you get it treated professionally by an accredited member of the Property Care Association, which works with the government and can offer the appropriate guarantees.
If you try to hide the problem or treat it yourself without a guarantee, you are potentially opening yourself up to expensive legal action.
Our staff have worked on many legal cases where we have been able to prove that knotweed on a property that appeared years after the property was sold would have been present and likely known to the vendor at the time they sold it.
Most property sales are accompanied by a completed Law Society TA6 Property Information Form.
If the vendor answers “No” or “I don’t know” to the question on the presence of knotweed at the property, make sure you take steps to satisfy yourself that knotweed isn’t present. And if the answer is “Yes”, then make sure you go into the sale with your eyes open to the risk and take measures to deal with the knotweed effectively.
If you are concerned about knotweed risk, such as where a property has a very large or overgrown garden, employ a knotweed professional to undertake a survey.
New–build properties do not require a completed TA6 form, so make sure the issue of knotweed is covered by the house builder’s documentation.
If it isn’t, ensure appropriate assurances around knotweed and property risk are included.
If knotweed is found, discuss the treatment options with the vendor. This could provide an opportunity for negotiating a reduction in the sale price.
If you have bought a property and knotweed springs up several years later, talk to a specialist law firm that deals with knotweed claims.
It might be that you’ll have a good case for claiming the costs of treatment and the reduction in value of your property caused by the knotweed.
This can depend on how likely it might be that the vendor lied to you, or if a surveyor might have been negligent in not noticing that knotweed was present when they surveyed it for you.
Since 2013, it has been a legal requirement for all home sellers to declare cases of Japanese knotweed when completing their TA6 form during the conveyancing process says Colby Short, co founder and CEO of estate agents comparison site Getagent.co.uk
“Where knotweed is present, it’s the seller’s responsibility to seek professional help to eradicate it,” he says.
“As a buyer, you should be fully aware of any knotweed before purchasing a property and your mortgage lender will require assurances that it will be taken care off before agreeing funds. The seller’s eradication plan via a professional company is usually sufficient when backed by a transferable guarantee [this means their services pass over to the new owners].
"You should be fully aware of any knotweed before purchasing a property"
“As a seller, failing to declare knotweed can result in delays to your transaction timeline as well as cost incurred to rectify the issue,”
Colby adds. If, as a buyer, you purchase a home and find knotweed is present, you have cause to launch a possible misrepresentation claim if the sale has already completed.
“While it’s not illegal to have Japanese knotweed in your garden, you could be prosecuted for failing to contain it,” says Colby.
“Amendments to the Anti-social Behaviour, Crime and Policing Act 2014 have been made to include the spread of invasive non-native plants such as knotweed.”
He says it’s strongly advised that even in the most minor case, you employ a specialist to treat the knotweed.
The good news is that it is possible to tackle Japanese knotweed if you’ve found it on your property. The plant can either be completely dug out and disposed of, or it can be treated in situ with herbicide.
Though using chemicals is cheaper, treatment can take a long time, potentially several years. It also limits how you can use the land afterwards because you must avoid awakening any dormant pieces of rhizome.
“Because it’s got that very deep rhizome system, it’s hard to get all the chemical absorbed through the limited leaf area into this large, iceberg-like root volume,” says Beckett.
“The herbicide is diluted into that large volume.
“The only other alternative is to dig it out. However, because it’s got a very deep root system, this leaves you with a lot of soil that is very expensive to get rid of. Not surprisingly, nobody wants soil contaminated with Japanese knotweed.”
Digging out can cost around £10,000 for a relatively small amount of Japanese knotweed on a domestic property, according to Beckett. This sum will increase in multiples of £10,000 if there’s more.
Some people also use synthetic root barrier sheets to contain Japanese knotweed growth. But this option might not protect neighbouring properties.
Despite the cost of removal, Beckett notes: “Damage being caused and the cost to sort out knotweed might pale in comparison to the legal fees and litigation surrounding any associated legal case.”
For more information on Japanese knotweed, contact your local council or visit the Government’s website.
The HomeOwners Alliance also has an updated advice page explaining the latest guidance from RICS. This will help people selling or buying homes blighted with Japanese knotweed.
Hero image credit: Shutterstock/Gabriel12
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Everything you need to know about Japanese knotweed, the fast-growing plant nobody wants in their garden.
