Guide to Japanese Knotweed House Damage

Everything You Need to Know About Japanese Knotweed Damage

Japanese knotweed house damage

Japanese Knotweed Damage to Property

Japanese knotweed

Since being introduced to Britain in the early 19th century, Japanese knotweed has spread far and wide across the country, invading the majority of the land.

Labelled by the government as an ‘invasive plant’, it has become enshrined in Japanese knotweed law, and has plagued homeowners who are looking to sell their properties.

Able to grow up to 10cm each day during the warmer weather, Japanese knotweed’s root system spreads via an underground network of rhizomes which can remain dormant beneath the ground for years at a time.

In the past, mortgage lenders have kept Japanese knotweed at more than an arms’ length – with some even flat out refusing to lend on properties affected. However, today, getting a mortgage on a house with Japanese knotweed is much easier.

While selling a property with Japanese knotweed is no easy feat, it can be done – you just need to know what you’re dealing with. It’s important that you do your research and understand what Japanese knotweed is, why it is so bad, how it can damage your house and how to get rid of it.

In this guide to Japanese knotweed house damage, we’ll outline everything you need to know about this invasive plant and the effect it could have if you’re planning on selling your Japanese knotweed-infested property.

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What is Japanese Knotweed?

Japanese knotweed is an aggressive weed that spreads rapidly and can cause major damage to properties. It was brought to Britain from Japan as an ornamental garden plant in the mid-19th century.

However, over time it has become widespread in a range of habitats, such as roadsides, riverbanks and derelict land.

In fact, Japanese knotweed is now the most invasive plant species in the county and across the UK. It can grow more than a metre each month and is famed for pushing through tarmac, concrete and drains.

Japanese knotweed not only causes house damage, though, its effect on native species is also devastating. It out-competes indigenous species, enveloping large tracts of land to the exclusion of the native flora and associated fauna.

Where is Japanese Knotweed Found?

Japanese knotweed

Japanese knotweed tends to be found on land that has been left unattended for a long period of time. So, if your property is close to land that has been unattended for a while, you might want to check that Japanese knotweed isn’t causing house damage.

Your house is more likely to be at risk of Japanese knotweed if it is close to land that has a public watercourse, footpaths, railways, motorways, car parks, cleared sites, or commercial buildings.

The most commonly affected areas of a property, according to the RICS, are outdoor spaces and gardens. Japanese knotweed is known to exploit structural weaknesses, which puts drains, or buried services like gas pipes, as being significantly at risk. Meanwhile, patios, driveways and garden walls can often hide the presence of the plant until it causes the structure to fall over.

How Fast Does Japanese Knotweed Grow?

Japanese knotweed has a reputation for fast, aggressive growth. Not only can this invasive plant force itself through cracks in brickwork and concrete, it can do so with astonishing, and rather frightening, speed.

In fact, at its most prolific, Japanese knotweed can grow up to 10cm per day. Japanese knotweed roots can grow up to 3 metres into the ground and spread 7 metres in all directions, leading to house damage.

How Can I Identify Japanese Knotweed on My House?

japanese knotweed

Do you suspect that your house is being affected by Japanese knotweed? If so, it’s better to spot it early – before it causes too much house damage.

You can confirm whether your house has Japanese knotweed by finding the plant. Making a positive Japanese knotweed identification will allow you to understand to what extent the plant is causing house damage.

Identifying Japanese knotweed can be as easy as finding a patch of brown canes, or spotting a handful of red shoots coming up through the ground. But, there are many instances where Japanese knotweed identification can be tricky. For example, in cases where land has been left to overgrow it can be difficult to distinguish Japanese knotweed from other plants.

If you don’t feel confident identifying the plant yourself, you can get a second opinion from a professional. These professionals could be removal firms who have experience in identifying or treating Japanese knotweed, or they could be RICS approved surveyors.

Why is Japanese Knotweed a Problem?

Since its introduction to the UK, Japanese knotweed has impacted the country’s eco-systems and caused damage to buildings, walls, drainage systems and flood defences.

It is a plant that has cost landowners, local authorities, homeowners and building developers thousands of pounds each year in removal fees and project delays.

The risk of Japanese knotweed house damage along with the loss of amenity use can significantly diminish a property’s value.

Most mortgage lenders will only be willing to lend on a property affected by knotweed if there is a Knotweed Management Plan in place by a recognised eradication company.

As well as devaluing property prices and putting mortgage lenders off, Japanese knotweed can cause all kinds of other problems for homeowners, especially those looking to sell. For example:

  • – Japanese knotweed in neighbouring properties could put off would-be buyers
  • – Poorly managed Council-owned properties could cause sales to fall through
  • – Japanese knotweed could impact neighbouring organisations or business owning property
  • – It can cause problems establishing the responsibility of Japanese knotweed infestation and treatment/removal

Can You Sell a Property With Japanese Knotweed?

Yes, you can sell a property with Japanese knotweed, however, you’ll likely need to take some extra precautions to keep the potential buyers happy.

They will need to feel fully comfortable purchasing the house and confident that they will be able to get a mortgage from their bank or building society. These extra measures could involve either completely removing the plant from the property, or paying upfront for an insurance-backed treatment plan.

Despite banks moving beyond their blank intolerance of homes affected by Japanese knotweed, there is still a lot of stigma surrounding these properties. So, in order to sell a house with Japanese knotweed, it’s important to prove that steps have been taken to control and, if possible, fully eradicate the plant from the property.

Given that the Japanese knotweed removal process can span a number of growing seasons, management plans usually cover 10 years and are insurance backed. That means treatment is guaranteed, even in the case where the original treatment firm closes.

Will Japanese Knotweed Devalue My House?

Japanese knotweed

Japanese knotweed can devalue a property between 5-15%. However, there have been severe cases where Japanese knotweed house damage has led to properties being completely devalued.

The extent to which a property is devalued will depend largely on the severity of the Japanese knotweed infestation and the proximity of the plant to your home. This devaluation will usually be equivalent to the cost of removing the plant and restoring the property to its original value.

How Much Does a Japanese Knotweed Survey Cost?

So, given that Japanese knotweed can significantly influence the sale price of a property, it’s a good idea to get a proper survey done by a professional.

A Japanese knotweed survey costs between £140 and £240 from an RICS surveyor. But some companies will offer a free informal Japanese knotweed survey that will help you identify whether you do or do not have an infestation on your property.

What is Included in a Japanese Knotweed Survey?

A Japanese knotweed survey should be carried out by a PCA-accredited surveyor. The survey report should then include the relevant proof of credentials from whoever conducted it.

It should also specifically address the presence, or absence, of Japanese knotweed on the property in question, along with the extent of the infestation.

Photographic evidence should be provided to support the statements made in the report, as well as a map highlighted to show which parts of the house are affected by the Japanese knotweed.

In addition, the report should outline the general condition of the land, its purpose and the nature of the plant infestation, including the presence of any rhizomes, roots or evidence of knotweed vegetation above ground.

It might identify the RICS Risk Category that the invasion falls under, and whether it has affected any buildings or structures on the property.

Finally, the report should outline recommended Japanese knotweed treatment, the associated costs and any precautions that the owners of the property should take to prevent further spread of the plant and house damage.

Should I Tell My Estate Agent About Japanese Knotweed on My Property?

Yes, you should notify your estate agent if you have Japanese knotweed on your property. This is because, in order to sell your home, they need to be able to give potential buyers an honest impression of the house that reflects its true value.

Choosing to hide or omit the presence of Japanese knotweed could lead to your sale falling through at a later date, which could have a knock-on effect on your house purchase.

Do Estate Agents Have to Declare Japanese Knotweed?

japanese knotweed

Yes, estate agents must declare Japanese knotweed in order to act within the Consumer Protection Regulations.

If an estate agent chooses to lie or misrepresent a property as being free of Japanese knotweed, they could be reported to the National Association of Estate Agents and could be banned from the profession.

How Do You Get Rid of Japanese Knotweed?

Small patches of Japanese knotweed are fairly straightforward to manage and can be easily removed by the homeowner themselves by digging or spraying with weedkiller. 

However, we would recommend hiring a qualified, professional company to control larger clumps of the plant.

The advantage of using a professional company is that they draw up risk reports and offer treatment plans with a guarantee on its complete eradication, which makes mortgage lenders more likely to accept. 

Also, by digging the plant out of the ground yourself, you risk causing more problems in the long run. This is because Japanese knotweed has the ability to regenerate from small pieces of root and it can be difficult to dispose of. 

It is possible to gradually weaken the plant by removing all leaves as soon as they grow – this stops the plant from photosynthesising.

However, this method can take many years to have an effect – no use if you are wanting to put your property on the market and sell it as soon as possible. 

Another alternative for Japanese knotweed removal is through the use of chemical controls. A glyphosate-based weedkiller is the best option – but bear in mind that it can take several applications, over up to four seasons, to completely eradicate Japanese knotweed.

How Much Does Japanese Knotweed Removal Cost?

If you want to opt for the safer option and hire a professional Japanese knotweed removal company, how much might it end up costing you?

The cost to remove Japanese knotweed from a property can vary greatly depending on the severity of the infestation and the area of land that the plant covers.

Removal firms tend to charge per square metre, but not all charge at the same rate. No matter how much you pay, though, Japanese knotweed removals should always come complete with a 10-year insurance-backed guarantee and an indemnity policy that ensures the treatment will be completed.

If you have just identified the plant on your land and are hoping to sell your property soon, you should be prepared to pay for your Japanese knotweed treatment plan in advance.

Most potential buyers will not want to purchase a house knowing that they’ll have to pay to get rid of it, and mortgage lenders will consider their security to be at risk if a treatment or removal plan isn’t already in place.

Preventing Japanese Knotweed House Damage

So, now you know – Japanese knotweed is no laughing matter! 

Not only can Japanese knotweed cause serious damage to properties and the structure of buildings, it can significantly devalue your house and make it difficult for potential buyers to get a mortgage. 

If you think you might have Japanese knotweed invading your house, make sure you get a survey done and professional removals company to come and eradicate it. 


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