Few plants invoke the level of dread that Japanese knotweed inspires in developers, homeowners, and homebuyers alike. This invasive plant has developed a reputation for being an aggressive blight across the country, with the potential to seriously damage structures.
It’s true that Japanese knotweed is a voracious species that will spread relentlessly if left unchecked, but it doesn’t have to be quite so fearsome. It’s possible to get Japanese knotweed under control and prevent it from damaging your property before it’s too late.
Here’s what you need to know about Japanese knotweed, from how it can damage your property to how it can affect you financially – and, most importantly, what you should do if you find it.
What is Japanese knotweed?
This bamboo-like weed was introduced to the UK back in the 1800s as an ornamental plant. German botanist Philipp von Siebold brought Fallopia japonica (now Reynoutria japonica) to Kew Gardens around 1850, but by the 2000s it would be recognised as one of the most invasive weeds in Britain.
Japanese knotweed dies back in the winter, but in the warmer months, it can grow up to 10 feet tall at a rate of several centimetres a day. Not only does it grow upwards, but Japanese knotweed has a deep root system that can reach up to 23 feet horizontally.
These underground roots, known as rhizomes, are responsible for Japanese knotweed’s voracity – it can grow back from even the smallest fragment left in the soil, even after you think all of the roots have been removed. Standard weed killers aren’t enough to tackle this resilient species.
To be able to spot the weed, you’ll need to know what Japanese knotweed looks like. Red or purple shoots will first begin sprouting from the ground in March or April, which will then become hollow stems that look a bit like bamboo with purple flecks.
These will grow quickly and begin sprouting large heart-shaped leaves, up to 12cm in size. Feathery white flowers may also grow on the plant between July and October. By the end of autumn, the leaves and stems will turn brown and shrink, but the canes will still be visible above the ground.
How does Japanese knotweed damage property?
Since Japanese knotweed is strong and grows fast, its roots can find their way through all kinds of cracks and joints in search of moisture. The pressure of the expanding weed can damage other underground systems, like cables and pipes, and push up through otherwise solid surfaces.
As a general rule, if water can get in through a tiny fissure, then so can Japanese knotweed. This problematic plant is known to cause damage to the following property features:
- Patios, paths, and driveways – stalks can grow between bricks or push up from underneath paving slabs, disrupting and cracking them.
- Drains and gutters – densely packed knotweed can clog drainpipes and ground drains, blocking the flow of water and leading to damp, mould, or flooding.
- Landscaped gardens – the fast-spreading plant will quickly take over any open green space, blocking light and preventing other plants from growing.
- Outbuildings – if left to multiply, the abundant weed can overwhelm lightweight structures like garden sheds and greenhouses.
- Retaining walls and fences – increased pressure from a dense mass of knotweed can cause old or poorly constructed walls and fences to collapse.
- Foundations – though rare, it’s possible for Japanese knotweed to spread underneath houses and grow upwards, disrupting the flooring.
Japanese knotweed can’t break through solid concrete or brick on its own, but it does exploit any weaknesses it may find. So, if there are existing cracks, the weed can penetrate the material and force the cracks to expand until the slab or block eventually breaks apart.
Once Japanese knotweed finds its way into your infrastructure, it will only continue to cause more damage as it spreads, so it’s imperative to tackle it as soon as you find it.
Does Japanese knotweed affect property values?
Though it can and does happen, cases of Japanese knotweed actually growing inside homes aren’t common. One of the more drastic and unavoidable ways that this weed harms your home is by damaging the property value.
Depending on how severe the infestation may be and how close the outbreak is growing to the main house, Japanese knotweed can knock 5–19% off a property’s value. The stigma associated with this invasive plant makes it difficult to sell the land and property without removing it first.
As we’ve mentioned, removing Japanese knotweed entirely is extremely difficult. There are treatment plans that can beat the knotweed back and eventually eradicate it through excavation, but these can not only be costly and disruptive, but also take several months or even years.
If you’re on the other end of the problem, hoping to buy a property that has had a Japanese knotweed infestation, you’ll also find it difficult to secure a mortgage. Most lenders won’t be willing to offer a mortgage on a property with evidence of Japanese knotweed, unless there is a fully paid treatment plan underway that will come with a 10-year guarantee on completion.
What should you do if you find Japanese knotweed?
Ignoring Japanese knotweed is the worst thing you could do – it won’t go away on its own. Similarly, you can’t just cut the plant back aboveground and hope for the best, because the roots will continue spreading underground until there’s knotweed everywhere.
If you try to dig it out yourself, not only will it be hard work, but you’ll also have to figure out how to dispose of it safely. There are laws controlling the disposal of the plant and any soil containing fragments of it under the Environmental Protection Act 1990, so you could end up breaking the law – only for the weeds to grow back, anyway.
The best thing to do is to contact a Japanese knotweed specialist, who can conduct a survey and assess the best method for treating the infestation. It’s the only way to ensure professional, thorough treatment, backed by an insurance policy.
It’s recommended to follow the Invasive Non-Native Specialists Association (INNSA) Code of Practice for managing Japanese knotweed, which sets out the best ways to treat Japanese knotweed and safely prevent it from spreading further.
These include both non-chemical and chemical methods of disposal, from the use of herbicides to mechanical remediation (removing and disposing of contaminated soil). Treatment plans can include several years of monitoring to make sure the infestation doesn’t return.
Do insurance policies cover Japanese knotweed?
Unless you purchase a specific policy regarding Japanese knotweed from a specialist provider before finding evidence of the plant on your property, it’s extremely unlikely that any insurance you already have will cover anything to do with Japanese knotweed.
Most building insurance policies will exclude damage caused by this plant, likely refusing claims on the grounds that the spread of the weed is due to your negligence as the homeowner, as you should have noticed the knotweed and made attempts to treat it before it damaged your property.
In the unlikely event that you did make a successful insurance claim to cover the costs of removing Japanese knotweed, your premiums would only increase after that, and you would find it difficult to get other types of cover that may be affected by the invasive plant.
Due to the persistence of the plant, there are no 100% guarantees against Japanese knotweed. This includes any structural warranty that may come with the property – though this type of building warranty offers financial protection for instances such as subsidence or cracking walls, you can only make a claim if they are a result of faulty materials or workmanship. It would be difficult to prove that the developers knew they were building on land contaminated by Japanese knotweed.
In fact, while sellers of secondhand homes are required to declare Japanese knotweed on the TA6 Law Society Property Information Form during the conveyancing process, it’s a known issue that new build developers sometimes fail to disclose Japanese knotweed because they aren’t legally required to complete that form.
Most issues covered by the TA6 form are covered by the building warranty that the developer typically passes to the buyer with the new build property, but Japanese knotweed simply isn’t discussed. So, if you don’t explicitly ask about it, they technically don’t have to tell you.
If you’re going to be buying a new build home, the best way to avoid this from happening to you is to get a full building survey from a chartered surveyor, rather than relying on a Homebuyer’s Report.
Is Japanese knotweed really that bad?
Despite the alarm surrounding Japanese knotweed, the most recent research suggests that it’s not as harmful as rumours would have it. A 2018 study found that Japanese knotweed causes‘no more damage than other species that are not subject to such strict lending policies.’
As such, the Royal Institution of Chartered Surveyors (RICS)has since released updated guidance for 2022, stating that Japanese knotweed:
- “Rarely causes structural damage to substantial buildings such as dwellings.”
- “Is not typically associated with major issues such as subsidence, heave or impact damage.”
- “Is less capable of causing significant structural damage than trees or many woody plant species such as buddleia.”
However, this more relaxed attitude to Japanese knotweed is yet to be reflected by property valuers and surveyors, mortgage lenders, or UK law.
As the owner/occupier, if you find Japanese knotweed on your site, you’re still legally obligated to take action to prevent it from spreading. It’s not illegal to have the weed growing on your property, but if it spreads to adjacent land, legal claims could be made against you.
If you are found responsible for spreading Japanese knotweed, you could still be fined thousands of pounds, or even prosecuted and sentenced to several months in prison, under the following laws:
- Wildlife And Countryside Act 1981
- Environmental Protection Act 1990
- Anti-social Behaviour, Crime and Policing Act 2014
While Japanese knotweed isn’t poisonous, and damage to large structures is rare, it still poses an environmental threat – as does its treatment.
It may not be the doomsday weed it’s often made out to be, but that doesn’t mean you should take Japanese knotweed lightly.
Be sure to consult a specialist as soon as possible if it begins to grow on your property – and always remember to ask the seller about Japanese knotweed before committing to buying a new property.
