Two people packing up a cardboard box with a dog lying next to them

The dangers of Japanese knotweed

by Leeds Building Society


Cracked concrete, broken brickwork and muddled mortgage applications - so many of these home-buying horror stories have a single, unlikely villain: Japanese knotweed.

What’s Japanese knotweed?

It’s a weed, as you might expect. In the summer, it can grow up to 20cm a day. It’s a perennial root, so just a little bit of leftover root is enough for it to grow all the way back up again.

What does it have to do with my house?

The roots of Japanese Knotweed can grow through concrete foundations and brick walls.

Properties with Japanese Knotweed are really hard to buy and sell, because mortgage lenders generally won’t lend on a property if the survey reveals that it’s growing.

It’s also notoriously hard to get rid of, and it can lie dormant for as long as 20 years before coming back to life.

Can I get a mortgage with Japanese Knotweed?

Most lenders will be reluctant to lend on a property that has Japanese Knotweed growing on it. There's a lot of risk that the Knotweed will damage the property, which will affect its resale value.

But that doesn't mean it's impossible. Different lenders will have different criteria about Japanese Knotweed. Check your lender's criteria guide to find out their policy.

What does it look like?

Japanese knotweed can be tricky to identify, because unfortunately it looks a lot like any other weed. Even seasoned gardeners can struggle to spot it.

It looks like this:

The green-fingered amongst you will have spotted that it looks an awful lot like other weeds. But there are a few visual clues to spotting Japanese Knotweed:

  • A large, heart-shaped leaf
  • The stems have rings round them, making them look like bamboo canes
  • An alternating zig-zag leaf pattern
  • Hollow stems with purple speckles

How do I get rid of it?

You can get rid of it yourself, but it’s a constant headache. The roots of Japanese Knotweed can be dormant for 20 years before they start to grow, so it’s easy to miss.

It’s not a good idea to just cut off the bits you can see. If you leave even the tiniest amount it can grow back. And cutting it can often stimulate quicker growth. It’s the rhizomes underground that are the problem, and cutting it won’t do anything to get rid of them.

You can get certain sprays. The most effective type is a glyphosate-based weed killer. You can get brands that are specifically designed to kill off Japanese knotweed.

You might find it easier to hire someone to get rid of it for you – these professionals are the shiny heroic knights of this terrifying story. They’ll work their magic to get rid of the Japanese Knotweed permanently. If you use a professional, you can also get a guarantee. If you have a guarantee, the knotweed shouldn’t stop you from selling your house.

It's not illegal to have Japanese knotweed growing in your garden, but you do have to stop it from spreading to other peoples gardens, or generally being a nuisance in your neighbourhood. In other words, you need to keep it under control.

How much does it cost to get rid of Japanese knotweed?

The cost of getting Japanese knotweed removed varies depending on the height of the weed and how far it has spread, but it can often mount up to thousands of pounds.

If you do get a professional to remove it, make sure you get a guarantee, so that they have to come back and get rid of it again if the Japanese knotweed returns.

Is it covered by buildings insurance?

Most buildings insurance policies don’t cover Japanese Knotweed, but it's a little more complicated than a yes or no answer.

Depending on how much the knotweed has grown, you may or may not need to let your insurer know about it. But if you don't, and you have to make a claim because of it, your insurer might refuse to pay out if they don't think you've done enough to stop the plant from damaging your property.

You should look for independent advice if you're not sure whether or not to let your insurer know that you have Japanese knotweed growing on your property.

Why is it such a bad thing to have?

Left untreated, Japanese Knotweed can seriously damage the structure of your house. It can cause small structures like sheds to collapse altogether.

And it grows and grows underground, covering the entire underside of your house. It’s a creeping menace, like something out of a ‘70s horror film.

So keep an eye for Japanese knotweed growing on your property. The earlier you spot it, the cheaper and easier it may be to manage and remove it.

Will Japanese Knotweed affect the value of my house?

Usually, yes, especially if you don’t have a professional remove it and implement a plan to make sure it doesn’t come back.

If you’re trying to sell your property without getting rid of the Japanese knotweed, you’ll probably only be able to sell to cash buyers, because they’re unlikely to be able to find a lender who will lend on the property.

Will a house survey flag it up?

If you get a surveyor to look at your house, they should spot Japanese knotweed and record it as part of the survey.

But bear in mind that the basic mortgage valuation carried out by your lender probably won’t point it out. That valuation is done for the benefit of the lender to check that the house will recover the loan if you can’t pay back your mortgage. It’s not to help you identify problems with the property.

To check for Japanese knotweed (and any other problems with the property), get a chartered surveyor to carry out a full survey on the house.

My neighbour has it. What can I expect them to do about it?

Legally, they have an obligation not to let it spread onto your property. But they don’t have to get rid of it on their own property.

If you’re trying to sell your home, this can pose a problem, because a lender may not lend on your property until your neighbour’s knotweed is taken care of and there’s a plan in place to stop it growing back. But they don’t have any obligation to do that, as long as it isn’t creeping onto your property.

Although it shouldn’t really be up to you to take care of it in this case, the person trying to sell their house often has more of a reason to get rid of it than the person whose property it’s growing on, so they may end up footing the bill.

The best tactic might be to scare them with horror stories about Japanese knotweed. Show them this article. Then they’ll want to get rid of it.

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This guide is intended as a summary only and does not constitute legal or financial advice given by Leeds Building Society. No reliance should be placed on this guide. We recommend that you seek independent legal and/or financial advice if you have any questions or queries.