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Everything you need to know about Japanese knotweed damage

It’s the quick-growing weed that’s estimated to cost the UK economy tens of millions of pounds a year - Japanese knotweed.


It could hamper mortgage applications, affect property values, and if it’s on your land you’re legally obliged to control it.

Here’s what you need to know about the pervasive plant.


What is Japanese knotweed?

Japanese knotweed (Fallopia japonica) is an invasive, non-native plant. As the name suggests, it originally hails from Japan. It’s also native in other countries in East Asia, including China.

Sprouts emerge from the ground in spring and the plant flowers by late summer. The weed remains dormant in winter.


What does Japanese knotweed look like?

Japanese knotweed is distinct looking. It has reddish bamboo-like stems, large leaves, and cream flowers. When it first emerges, its appearance is akin to asparagus tips.

It grows at a quick pace, up to 10cm a day in the summer months. Japanese knotweed damage occurs when its shoots exploit gaps and weaknesses in tough materials like tarmac. It’ll grow through any gap it can find, affecting properties and structures.

Chances are, by the time you’ve spotted it, its canes are already over two metres high.


How to identify Japanese knotweed

It’s often confused with other plants – including Himalayan balsam and giant hogweed. Knowing key differences between these plants can help you correctly identify Japanese knotweed.

Each has pretty distinctive leaves, and that’s the easiest way to tell them apart.

  • Japanese knotweed has spade shaped leaves, with a point at the end – so they’re quite broad leaves.
  • Himalayan balsam has much narrower, pointy leaves.
  • Giant hogweed’s leaves are narrow with spikes all the way down, a little like nettles.


How does Japanese knotweed spread?

It spreads via what are known as the rhizomes. These are underground stems that can spread three metres deep and seven metres wide. Japanese knotweed will grow through most materials, other than concrete and brick.

If there are any cracks in foundations, it can grow through them. It might also take advantage of weaknesses in underground pipes and compromise them too.

It’s a hardy plant that thrives in difficult circumstances.


Is Japanese knotweed dangerous to handle?

No. It’s not toxic to humans. However, it is a danger to local habitats. If a small fragment of it escapes on your clothing and is deposited elsewhere, there’s a good chance it’ll grow there too.

That makes handling it dangerous as it could lead to more destruction.


Japanese knotweed damage

Japanese knotweed can do extensive damage if left untreated.

While it can’t grow through bricks and concrete, it can work its way through small cracks and gaps.

It can cause damage to foundations, patios, walls and more if ignored. This is why mortgage lenders take it seriously.

It could also damage your bank balance and reputation.

If the plant exists within your property’s boundaries, it’s your duty to control it.

Otherwise, you might find it a challenge to sell your property in future. And you could even be prosecuted if it spreads to neighbouring properties.

If you allow the spread of Japanese knotweed into the wild – and it spreads easily – you could be fined or imprisoned.

Japanese knotweed can grow as much as 1ft a week during the summer months.

But the real growth and damage Japanese knotweed can cause is underground. If it’s outside your property boundaries, call your local council to see if they can eradicate it.


How can I get rid of Japanese knotweed?

After properly identifying the plant, and understanding the risks posed, next on the agenda for homeowners is looking at how to kill Japanese knotweed.

It’s not easy, takes time, and could be expensive.

Here are your options:

Doing it yourself 

While it’s possible to remove it yourself, we recommend hiring a professional to do it. Especially if you want it to stay gone.

If you’re going to do it yourself, you need determination and patience.

First off, here’s some kit you’ll need:

  • Some coveralls
  • A visor or face shield
  • Face mask to P3 standards
  • Disposable shoe/boot covers
  • Glyphosate weed killer
  • A knapsack style sprayer
  • Some rubber gloves.

We also recommend researching the DIY removal process as you have to be diligent. We found job-prices.co.uk lists the process clearly and step-by-step.

Here are a few tips to keep in mind if you’re thinking of removing it yourself:

  • It could take 3-5 years to completely remove it.
  • Don’t put any part of the knotweed plant, dead or alive, in your compost or normal waste.
  • It must be disposed of in a licensed waste disposal centre (including the soil dug up with the knotweed) and only registered waste carriers can transport it.

If you are going to dispose of it yourself, call the Environment Agency first. For example, if you're planning on burning it, they'll need at least one week's notice.


Paying someone else to do it 

This is the most recommended method for effective removal of Japanese knotweed. But it might not be cheap.

Removal costs roughly £1,000 per square metre. The overall cost depends on how big the job is and the provider you choose.

You also need to ensure professionals you hire have the right insurance and guarantees that go alongside their work.

Chemically treating Japanese knotweed is one of the cheaper options.

Whereas having the area excavated to remove all rhizomes tends to be pricier.

The specialist you appoint should explain your options and recommend the best one for you. We recommend you to get at least three quotes.


Will it affect my mortgage?

If you already have a mortgage and have noticed Japanese knotweed start to grow, your mortgage is already in place, so don’t worry.

However, you‘ll still need to take action to stop it from encroaching on neighbouring properties.

If you’re seeking a new mortgage, your options are more limited. Some providers might decline, others might want a survey done and evidence of a treatment or removal plan.

The surveyor should most likely be by a Property Care Association (PCA)-approved surveyor. Lenders are likely to want the removal specialist to be PCA-approved too.

They’ll want to know how close it is in metres to the property, specifically if it’s more or less than 7 metres away. If it’s close, they’ll likely want it removed by an approved specialist.

Sellers of an affected property might need to provide details on a TA6 form for their buyers.

But it’s not all bad news. In 2018 further research came out suggesting Japanese knotweed isn’t a reason to strangle mortgage lending. As a result, some mortgage providers are now more lenient.


Does home insurance cover Japanese knotweed?

It’s unlikely that your home insurance will cover the cost of removing Japanese knotweed, or repairs for damage arising because of it.

You should still be able to get home insurance cover generally.

If you’re asked about the presence of Japanese knotweed when you buy home insurance it’s important to be honest.

Otherwise, you risk future claims being rejected, even ones unrelated to the plant.

Successfully claiming for damage caused to your property by the weed depends on your efforts to control it and how comprehensive your policy is.

But ultimately you’re responsible for removal costs. And for any damage it causes to neighbouring properties.

If you have legal expenses cover as part of your home insurance, you might be able to use that to get legal advice.