Japanese knotweed: how to identify and manage this invasive plant

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.” 

Japanese knotweed is known for its ability to damage buildings, a characteristic made worse by how quickly it grows. 

According to the UK government website, 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 (10 ft) 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.  

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) last year, lenders used a crude distance measure of 7m (23ft) to assess the risk of contamination, blighting properties even with no apparent problem.  

That changed in March 2022 when the Royal Instititute of Chartered Surveyors’ 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: 

• Small purple shoots that appear suddenly from the ground 
• Clumps of dead-looking, hollow stems 
• Early Japanese knotweed canes and stems are bamboo-like 
• Japanese knotweed leaves are described as shield, shovel or heart-shaped 
• Incredibly rapid rates of growth 
• Encroachment of invasive plant vegetation from neighbouring property 
• Damage caused to structures from plant growth  

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.  

For a full guide on formal identification, head to phlorum.com

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 told us. “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.” 

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 – or perhaps even a consideration – for the majority of property owners, there’s no harm in remaining vigilant. 

Beckett has shared his top tips for staying aware if you’re buying or selling a property: 

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. 

Digging out can cost around £10,000 for a relatively small amount of Japanese knotweed.

“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.

Curious to learn more about how knotweed grew into such a problem for homeowners in the first place? Beckett wrote this piece for Inside Ecology.