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The truth about Japanese knotweed

Updated 22 December 2022

5min read

Nick Green
Financial Journalist

It looks like bamboo, it tastes like sour rhubarb, and it can knock tens of thousands off a property’s value. But are the scare stories surrounding Japanese knotweed really justified? And what should you do if you find it? Article by Nick Green.

Japanese knotweed in flower

Blame Phillip von Siebold. Little did the German botanist know, when he sent an innocent-looking parcel to Kew Gardens in 1850, that he was to cause untold misery for thousands of homeowners in the coming centuries. Every outbreak of Japanese knotweed in Britain, Europe and North America is a direct offshoot from that little cutting he popped in the post. Yeah, thanks Phil.

So what happens if you’re buying or selling a home, and someone mentions the dreaded JK? There’s no doubt that your task has just become trickier and perhaps more expensive – but neither is it a reason to panic. Let’s prune back some of the myths surrounding this problematic plant.

What is Japanese knotweed?

Japanese knotweed (Fallopia japonica) is a herbaceous perennial plant that looks a bit like bamboo, with large green shovel-shaped leaves. Because it grows so fast in a wide variety of soil types, it can quickly spread, growing from underground roots (rhizomes). These rhizomes make it hard to get rid of, since a new plant can sprout from even a small fragment left in the soil.

Why is Japanese knotweed a problem?

Over many years, Japanese knotweed has acquired a reputation as one of the most invasive plants, and has been blamed for causing damage to properties. This, combined with its zombie-like refusal to die, has made it into a big green bogeyman for the housing industry. Mortgage lenders will often refuse a mortgage to people buying a property with live Japanese knotweed growing on the premises, making it hard to buy or sell such a home. Similarly, most buildings insurance won’t cover damage by Japanese knotweed. You don’t have to tell the insurer you have knotweed unless they ask you, but if you don’t, you may not be covered for other damage even if the knotweed didn’t cause it. And if you do tell the insurer, you can expect higher premiums.

Is it illegal to have Japanese knotweed in your garden?

It’s certainly not illegal to have Japanese knotweed in your garden – indeed, some brave gardeners have been known to cultivate it deliberately. However, it can be a cause of serious disputes with neighbours if you let it get out of hand, as it can invade the next-door garden by growing under the boundaries. You could then be liable for any removal costs or damage caused.

How do I get a mortgage on a home with Japanese knotweed?

A mortgage lender will be wary about lending money to buy a home where Japanese knotweed is growing, but it’s not a deal breaker. If you or the current owner can provide written evidence that the problem is being treated then there is no reason why you should not be offered the mortgage provided the rest of your application is satisfactory.

Once your offer on a property is accepted, you’ll receive a TA6 form from the seller. This form covers additional information about the property (e.g. details of any alterations to the building) and should also state if Japanese knotweed has been identified on the premises. The seller should then arrange professional treatment of the problem, preferably by a member of the Property Care Association invasive weed group, with a transferable insurance-backed guarantee (IBG) lasting 5 to 10 years. This should satisfy the lending policies of most mortgage providers.

How much does it cost to treat Japanese knotweed?

Treating Japanese knotweed is quite costly, but relative to the other high costs of buying a home it’s not prohibitively expensive. If treated with glyphosate herbicide alone (one effective solution), an area under 50m2 can cost around £1,000 to £3,000 to treat. This treatment is usually carried out over several years, but provided that your mortgage provider is aware that the treatment is ongoing, they should be willing to lend on this basis.

More rigorous treatment of Japanese knotweed is sometimes necessary. Fully digging out and disposing of a similar-sized area might cost up to £20,000 but is usually only necessary in extreme infestations. If you are the buyer, you can ask the seller to deduct the likely cost of treatment from their asking price.

Is Japanese knotweed really as bad as people say?

There turns out to be an ironic twist to Japanese knotweed, which is that it may not be as damaging as its reputation suggests. The most recent and extensive research to date, a 2018 study by global infrastructure services firm AECOM and the University of Leeds, found no evidence that Japanese knotweed is especially damaging when compared to other plants. AECOM’s principal ecologist, the aptly-named Dr Mark Fennell, said, ‘We found nothing to suggest that Japanese knotweed causes significant damage to buildings – even when it is growing in close proximity – and certainly no more damage than other species that are not subject to such strict lending policies.’

This is in stark contrast to the alarmist warnings in the public domain, which include claims such as that knotweed ‘can cause massive damage to house foundations’. Co-author Dr Karen Bacon of Leeds University is quick to refute this. ‘This plant poses less of a risk to buildings and other structures than many woody species, particularly trees. [It] is capable of damaging built structures, but where this occurs it is usually because an existing weakness or defect has been exacerbated.’

The 2018 findings tally with an earlier report in 2012 by the Royal Institution of Chartered Surveyors (RICS), which concluded that Japanese knotweed was ‘treatable and rarely [causes] severe damage to the property’.

The 2018 study also found that Japanese knotweed rhizomes rarely extend more than 4m from the visible plants, and usually spread less than 2.5m. This is much less than the 7 metres commonly cited as the risk zone.

This isn’t to suggest that Japanese knotweed doesn’t damage buildings – it can and it does. But a small clump growing some distance from your house should be a perfectly manageable problem.

Japanese knotweed – know the drill

In reality, it would appear that Japanese knotweed is usually more of an annoyance than a doomsday shrub. The problem is that mortgage lenders and home insurers will take some time to accept this (if they ever do). As long as lenders believe that knotweed could damage a property, this pesky plant will be an obstacle to lending and a drain on a property’s value. This means, unfortunately, having to treat it as if it will damage the house, even if it’s unlikely to do so.

You should therefore follow the Japanese knotweed drill to prevent unpleasant surprises.

  1. Learn to recognise the plant – several species look similar, so err on the side of caution.
  2. If you find it on your own property, don’t panic – just treat it immediately. A professional service is best. Don’t delay as it grows fast, and don’t simply cut it back – the rhizomes grow underground.
  3. If you find it on a property you’re viewing, talk to the seller about it and get written confirmation that they are treating it. If they won’t provide this, walk away.
  4. If you suspect it growing on a neighbour’s property, try to raise the issue with them diplomatically. If it’s growing on nearby common ground, alert the local authority.

In the end, Japanese knotweed should not prevent you from securing a mortgage or buying your dream home, provided you know how to deal with it. For more insights, talk to your mortgage adviser.

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About the author
Nick Green is a financial journalist writing for Unbiased.co.uk, the site that has helped over 10 million people find financial, business and legal advice. Nick has been writing professionally on money and business topics for over 15 years, and has previously written for leading accountancy firms PKF and BDO.