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Japanese knotweed damage: How bad is it?

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 and 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 such as Tarmac. It grows through any gap it can find, affecting properties and structures.

Chances are, by the time you’ve spotted it, its canes are already over 2 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.

  • Himalayan balsam has much narrower, pointy leaves.

  • Giant hogweed’s leaves are narrow with spikes all the way down, a little like nettles.


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. 


How bad is Japanese knotweed?

It can cause huge problems. It grows fast and by the time you’ve spotted it, it’s established itself on your property.

But the real growth and damage Japanese knotweed can cause is underground.

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.

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, it takes time and it could be expensive.

Here are your options:


Removing Japanese knotweed 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.

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

First, here’s some kit you need:

  • Some overalls

  • 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're going to dispose of it yourself, call the Environment Agency first. For example, if you're planning on burning it, they need at least 1 week's notice.


What is the cost of removing Japanese knotweed if I hire someone to do it?

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

But despite the cost, this is the most recommended method for effective removal.

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

Treating the plant chemically 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 get at least 3 quotes.


How does Japanese knotweed spread?

It spreads via what are known as the rhizomes. These are underground stems that can spread 3 metres deep and 7 metres wide. Japanese knotweed grows 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 humans?

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.


How does Japanese knotweed affect my mortgage?

If you already have a mortgage and have noticed Japanese knotweed starting to grow, your home loan shouldn’t be affected. 

However, you still need to take action to stop it from encroaching onto neighbouring properties.

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

The survey 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 might want to know how close it is in metres to the property. If it’s closer than 7 metres, 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 too.

But it’s not all bad news. Research from Leeds university suggests that Japanese knotweed isn’t a reason to strangle mortgage lending. As a result, some mortgage providers are now more lenient.


Is Japanese knotweed insurance included as part of my home cover?

It’s unlikely that your home insurance covers 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.

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What happens if I have Japanese knotweed in my house?

You should take action straightaway.

It’s not a problem that will go away. Far from it, in fact.

It can grow quickly, so delaying the start of any treatment only makes it a bigger job – and a more expensive one, too.

There’s no legal obligation to remove it from your property or your land, but you are obliged to prevent it from spreading onto any neighbouring land. The longer you leave it untreated, the more chance there is of it affecting your neighbours. So that’s another reason to make immediate plans to tackle the problem.


What is the situation with selling a house with Japanese knotweed?

There’s no law against selling a property that has Japanese knotweed, but you shouldn’t attempt to hide its existence from prospective buyers.

In fact, if you fill out a TA6 form during the sale, you open yourself up to being the target of legal action if you lie. And you can’t claim you didn’t know about the need to mention the existence of Japanese knotweed, because a TA6 form has a dedicated section on it.

Let’s say you genuinely didn’t know there was Japanese knotweed on the property and the buyer paid for a HomeBuyer report that missed the weed’s existence. The buyer might have legal recourse against the surveyor instead of you.

Assuming the Japanese knotweed is known and you’ve passed that information on, any buyer might expect the price of the purchase to reflect the fact that work to remove the weed is needed.