Left unchecked, Japanese knotweed can destroy your home’s foundations, drainage systems, and walls.
It grows incredibly fast – by up to 20cm a day – and has deep, wide-ranging roots that can tunnel through tarmac and make it extremely difficult to kill. In short, it’s one plant you definitely don’t want in your garden.
Find out how to spot and tackle this vegetative threat to your property, and learn more about its impact on home insurance, mortgages, and house prices.
What is Japanese knotweed?
Japanese knotweed is an herbaceous plant with a bamboo-like stem and large green leaves. Introduced to the UK in 1825 as an ornamental plant, it spreads very quickly and can grow more than two metres high. Homeowners dread finding Japanese knotweed near their homes because its extremely strong and fast-growing roots can spread seven metres in all directions and cause massive damage to walls and buildings.
What does Japanese knotweed look like?
Japanese knotweed usually starts growing in March or April. The first sign is red or purple shoots sprouting up from the ground. These then quickly turn into hollow stems that look like bamboo with purple flecks and can shoot up by 20cm a day. Leaves are large, green, and heart-shaped, while the flowers are small and creamy white.
It’s easy to confuse Japanese knotweed with other types of plants, so ask an expert before starting to panic. You can check if you have Japanese knotweed by sending a photo to the Royal Horticultural Society.
Japanese knotweed features to look out for include:
- Red or purple shoots that appear in March or April
- A hollow, bamboo-like stem that shrinks and turns brown in the autumn months
- Large, smooth, heart-shaped leaves that zigzag up the stem
- Clusters of small, creamy white flowers that appear between July and October
How can Japanese knotweed damage my property?
Japanese knotweed can take over your garden and cause significant structural damage to your home. It spreads via its roots, which are extremely tough and can shoot out by up to seven metres in any direction. These roots penetrate tarmac and can therefore rip through drainage systems, cause walls to crumble, destroy outbuildings such as sheds and greenhouses, and even compromise the foundations of your home.
That’s why the Royal Institution of Chartered Surveyors views knotweed as a potential risk if found within seven metres of your house. The potential for damage is so severe that you could be sued if Japanese knotweed is found to have spread from your garden to your neighbour’s land. However, the good news is you can prevent Japanese knotweed causing damage by taking action to remove it.
How can I get rid of Japanese knotweed?
Japanese knotweed is notoriously difficult to destroy. It also dies back to ground level in the winter, meaning it can be hard to tell whether it’s really gone or not. But if you want to try to tackle it yourself, you can use a glyphosate-based weed killer designed to target Japanese knotweed (the Royal Horticultural Society suggests Roundup Tree Stump). Just be prepared to keep treating the plant for several years to kill it completely.
Other Japanese knotweed removal options include:
- Hiring a professional contractor – this is the easiest way to ensure you get rid of Japanese knotweed for good. It’s also the only way to get an insurance-backed guarantee or residential Japanese knotweed indemnity insurance policy, which you’ll probably need to get a mortgage if Japanese knotweed turns up on your property survey. Methods include excavation and spraying with professional-grade glyphosate weed killers, and costs range from about £3,000 to £10,000 or more.
- Digging it out yourself – this will be hard work and unlikely to provide a lasting solution. You’ll also have to dry it out and burn it, or dispose of it at a licensed landfill site because Japanese knotweed is classified as ‘controlled waste’ under the Environmental Protection Act 1990. If you choose to put it in a landfill site, you’ll have to pay the government’s landfill tax and use a specialist company to transport it. Again, this could cost several thousand pounds.
Does my home insurance cover Japanese knotweed?
Most buildings insurance policies exclude damage caused by Japanese knotweed. If the knotweed problem comes from your garden, any claim is likely to be refused because you’ve been negligent by failing to take measures to protect your home. If your home is affected by Japanese knotweed on adjacent land, your insurer will probably pursue the land’s owner for the related costs.
You’re also unlikely to be able to make a claim for treating or removing knotweed – unless you take out a specialist Japanese knotweed insurance policy prior to spotting the plant. But don’t delay getting rid of Japanese knotweed. As the plant grows, so will both the cost of treatment and the likelihood of bigger problems.
Do I need to tell my insurer I have Japanese knotweed?
You don’t have to tell your insurer there is Japanese knotweed on your property – unless it asks, in which case you must tell the truth. Otherwise, you could invalidate your entire home insurance policy.
Will Japanese knotweed increase my insurance premiums?
Most insurers will refuse to pay out for Japanese knotweed claims anyway, so having the plant growing on your land shouldn’t affect how much you pay for home insurance. However, your insurance company will expect you to take measures to protect your home – and other people’s property – by keeping the plant in check. That’s why removing it completely is the easiest option.
Will Japanese knotweed affect the value of my home?
Rampant Japanese knotweed will almost certainly reduce the value of your home. So if you’re hoping to sell, you have yet another reason to remove the plant from your land. Just cutting it back isn’t enough, as it will usually be spotted on the surveys conducted for potential buyers and their mortgage lenders; the only real solution is to get to the root of the problem.
Will Japanese knotweed affect my mortgage?
You may struggle to get a mortgage if you live in or are hoping to buy a property affected by Japanese knotweed. If, for example, the plant is spotted on the property survey you get done as part of your mortgage application process, your bank or building society will probably insist a professional attends to remove it before approving your mortgage. As a buyer, you could use this as a bargaining tool to reduce the price you pay for the property. Just remember to double check the plant has been totally removed before going ahead with the purchase.
Is it illegal to have Japanese knotweed in my garden?
It’s not illegal to have Japanese knotweed growing in your garden, but you could have problems if you fail to take action to control it. You might even be issued with an ASBO ordering you to ‘control or prevent the growth of Japanese knotweed’. Failing to act on this order would be a criminal offence and could result in a £2,500 fine.
If you’re caught spreading Japanese knotweed in the wild, meanwhile, you could face a £5,000 fine and up to two years in jail.
What happens if my Japanese knotweed damages my neighbour’s house?
You’ll be held responsible for any damage caused by Japanese knotweed that spreads to other people’s property from yours. So your neighbours can seek compensation for any damage caused by the weed – as long as they can prove it came from your property. These claims can be worth thousands of pounds, so it’s a good idea to act quickly if you spot Japanese knotweed in your garden.
If you have legal expenses cover alongside your home insurance, you may be able to claim for the legal costs involved in a neighbourly dispute about Japanese knotweed. Just make sure you check what’s covered before you start.
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