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02 Mar 2020
Adam Bate

Everything you need to know about Japanese knotweed


Japanese knotweed

It’s causing havoc across the UK. But what is Japanese knotweed and how do you get rid of it?

What is Japanese knotweed?

Japanese knotweed is one of the most dreaded, despised and potent species of plants you can find in the UK.

It grows strongly and quickly, with its roots penetrating as far as 3 metres deep and 7 metres across.

Left unchecked, it can cause massive damage to house foundations, drainage systems and walls.

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What does Japanese knotweed look like?

It usually starts growing from early spring and can reach up to 3 metres by June.

And don’t be fooled over the winter season. It may look like it has died during these months, but it'll be back again in March the following year.

Signs to help you identify Japanese knotweed:

  • Red/purple shoots when it breaks through the ground, usually between March and April.

  • Large, heart-shaped green leaves usually develop between May and June.

  • A hollow, bamboo-like stem which could grow up to 2.1 metres tall in spring and summer.

  • Leaves arranged in a zig-zag pattern along the stem.

  • Clusters of creamy-white flower tassels appear between July and October.

  • When uninhibited, it can grow in dense clumps that could reach several metres in length.

  • It’ll also leave brown stems when it dies back between September and November.

If you're in any doubt, you can send a photo of the weed to RHS Gardening Advice

Japanese knotweed

What are the common problems?

What makes knotweed particularly problematic is the struggle of removing it. It costs the UK around £150 million a year in treatment.

It grows at a ridiculous rate, it’s nearly impossible to get rid of, and when you do, it comes with a hefty price tag.

One particular case in London saw that it would’ve been cheaper to knock down and rebuild a £300,000 home than treat the knotweed problem alone.

It even affected the London 2012 Olympics to the extent it took four years to control the weed.

The most common areas that the Japanese knotweed affect include:

  • gardens

  • drains and other buried services

  • patios, paths and drives

  • retaining walls

  • outbuildings like sheds, greenhouses and poorly-built garages

  • conservatories

You could also be sued if it spreads to your neighbours’ land. So it’s definitely worth checking your garden every now and then to make sure you’re not a victim of the knotweed.

Read more: Subsidence insurance

How do I get rid of Japanese knotweed?

The RICS suggests that knotweed is a potential risk if found within 7 metres of your house.

If you think this monster weed is lurking in your garden or you’re a potential buyer checking the house out, it’s best to call a specialist to check.

Here are some treatments you can try get rid of this nasty intruder:

Dig it out

Digging out knotweed is possible, but due to its rapid growth and strength, it’s likely to come back.

There’s also a problem with disposal, as Japanese knotweed is classified as ‘controlled waste’ under the Environmental Protection Act 1990.

So even if you get shot of it, you can only dispose of at licensed landfill sites.

Off-site disposal cost: £10,000

Use chemicals

Chemical treatment is usually the cheapest and most effective option. A glyphosate-based weed killer is a popular choice for most gardeners.

This method could take more than three years to be effective, though. Professional contractors will have access to stronger weed killers, which may be quicker.

Cost: £3,000 - £5,000

Feed it to bugs

A plant-sucking bug is being trialled in the UK as a biological control for knotweed.

With a bit of luck, it’ll become available to gardeners if it works.

Read more: Lock types explained

Japanese knotweed

What do mortgage lenders say?

It’s one of a potential buyer’s worst nightmares.

Mortgage lenders have been known to refuse mortgages on buildings that are affected by Japanese knotweed.

If it’s spotted on a property survey, then a lender is likely to insist on hiring a professional to handle the problem before granting a loan.

Homeowners affected by Japanese knotweed are also likely see a considerable drop in value of the house.

Read more: How to calculate the rebuild cost of your home

What do insurers think of it?

Most buildings insurance policies don’t cover damage caused by Japanese knotweed.

Though you don’t necessarily need to tell your insurer if you have Japanese knotweed unless asked, you are obliged to do everything you can to prevent any damage to your home.

So if you ignore the presence of the plant, any future claim for structural damage might be turned down.

If your neighbour has knotweed and your house is affected by it, most insurers are likely to pursue others for the cost of the damage caused to your building.


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