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Japanese Knotweed – a knotty problem for property owners and insurance cover

On a bright, sunny day in late March, you step out into your garden. You breathe in the fresh air, take note of the tulips that are finally starting to blossom, the sun’s glare warm and tranquil. You take note of some small, reddish shoots sprouting from the soil.

Three months later, those same little shoots have grown powerful, bamboo-esque stems with large green leaves, and underground, their roots have spread several metres down and across, growing ever-closer to your home. They are possibly only days away from impacting your drain pipes, which they are more than capable of burrowing straight through or worse, causing subsidence to other structural issues with your homes.

That’ll be one hefty bill.

What is this nasty little plant, you might ask? The answer: Japanese Knotweed.

What is Japanese Knotweed?

Don’t let the name fool you. This isn’t any old weed. Japanese Knotweed can be extremely destructive, capable of causing no end of chaos in your own and/or your neighbours’ gardens and homes. It is widely regarded as one of the most damaging plants in the world! It grows rapidly, up to 20cm a day! And can even grow through tarmac, concrete, timber, and cracks in your plaster. Within one growing season it can grow as high as your roof gutters! Recent estimates suggest that it affects up to 1.45 million homes in the UK (Source 1 – Yahoo Finance).

Just take a look at these pictures to see some of the places this plant can grow:

What are the laws surrounding Japanese Knotweed?

Current regulations in the UK prevent any homeowner from allowing this plant to grow beyond the boundaries of their land. If you’re caught planting Japanese knotweed in the wild, you can expect a fine of up to £5000. It isn’t illegal however, to have the plant growing in your garden or on your private land.

How can I identify Japanese Knotweed?

There are several key identifying characteristics to look out for if you’re concerned that your garden has Japanese Knotweed:

  • Red/purple shoots initially appear in March/April which can look like asparagus spears.
  • A thick, bamboo-like stem that grows rapidly in a zig-zag pattern.
  • Creamy-coloured flowers that bloom in late summer.
  • Large green leaves which can grow up to 20 cm in length.
  • Above ground, Japanese Knotweed can grow up to 3 metres high.
  • Substantial roots that can spread underground in any direction, and up to 7 metres in length, creating a subsidence threat.
Japanese knotweed growth
Example of the growth stages of Japanese Knotweed.

These traits, however, can easily be confused with other non-destructive plants. If you’re unsure if you have Japanese knotweed growing, you should contact an expert. The Royal Horticultural Society (Source 3), for example, can help to identify your unwelcome undergrowth.

Can Japanese Knotweed decrease the value of my home?

Yes! Your property value is likely to be considerably reduced by as much as up to 20%, depending on the extent of the damage. In one particularly extreme case reported by the BBC, a Hertfordshire couple were told the value of their new-build home had dropped by £250,000 after it was invaded by Japanese knotweed. The situation was so bad that three metres of soil needed to be removed from beneath the foundations of the house in order to remove the plant!

You can get an idea of how much the value of the property you’re buying or selling might be affected by visiting an online calculator, which takes into account the size of the infestation and how different treatment methods will protect the property’s value.

How can I get rid of Japanese Knotweed?

This invasive, fast growing plant might sound scary but fortunately it can be treated, killed, or removed. You can try a DIY approach but attempts are rarely successful, to save the stress and additional costs in the long term, its best to bite the bullet and contact a specialist treatment firm who will also offer an insurance backed guarantee with their work.

Treatment options can include a full dig out and excavation – this is costly and disruptive but it does solve the problem quickly or chemical treatment to the plant itself which is cheaper but can take longer, possibly up to 5 years.

How much does it cost to remove Japanese Knotweed?

This really does depend on the severity of the growth of the Knotweed and the treatment you decide upon. It can range from £500 for a small area of chemical treatment to over £100,000 for large excavation.

This company provide a rough outline of costs on their website to give an idea.

Additional Japanese Knotweed Insurance Cover FAQs

Talk to Glowsure for advice on commercial property insurance cover for Japanese knotweed

Fear not – while a knotweed infestation can be dangerous, and you may be unsure what steps to take if you’re concerned about the possibility of Knotweed growing on your property, or if you’ve already identified it, Glowsure can advise you.

Contact us today to receive expert advice and support on how to make a Japanese knotweed insurance claim that can leave you stress-free, and make the knotweed fear you!


  1. Yahoo Finance
  2. Knotweed Help
  3. Royal Horticultural Society

Useful Links

Below are a series of helpful links that provide greater detail of defining characteristics of knotweed, its identifying features, and the type of damage it can do to your property:

Rebecca Haynes
Rebecca Haynes
Rebecca is the Operations Manager at Glowsure and a self confessed insurance nerd. She helps businesses and landlords to retain income and continue trading in the event of a claim.
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