Buying or selling a property affected by Japanese knotweed?

We asked Nic Seal, Environmental Scientist and Managing Director of Environet UK to provide advice to buyers and sellers of property affected by Japanese knotweed.

It’s the UK’s most invasive plant and awareness among the general public has grown significantly in recent years, but why exactly is knotweed such a cause for concern among homeowners?

Destruction – If left unchecked, Japanese knotweed can cause damage to property. While some claims are exaggerated (no, it can’t grow through concrete), it will exploit weaknesses such as cracks in concrete or brick walls and it can grow directly through asphalt as well as into drains, resulting in costly repairs. It will also travel laterally to emerge at the side of a concrete structure that has been built on top of it.

Dominance – Japanese knotweed has been spreading steadily across the UK since the 1850s and is now thought to be present in nearly every 10 sq km of the UK. Approximately 4 – 5 per cent of homes are now affected either directly or indirectly (i.e. neighbouring a property with knotweed). If you want to find out how prevalent Japanese knotweed is where you live, visit Exposed, the Japanese knotweed heatmap and enter your postcode to discover the number of infestations near your home.

Survival – It’s extremely difficult to kill. DIY methods such as covering to deny sunlight, mowing, burning or treating with over the counter weed killer, simply won’t work and in fact can make things worse. Japanese knotweed requires professional treatment, either with herbicide over two to three years or by excavation. Heavy machinery is required and the plant can regrow from a piece of rhizome as small as a fingernail!

That’s why the Environment Agency describe Japanese knotweed as “indisputably the UK’s most aggressive, destructive and invasive plant”. And yes, it can have a drastic effect on residential property sales so buyers and sellers of knotweed affected property should read on, or watch a short video at

Japanese Knotweed and the property market

Knotweed can have a drastic effect on residential property sales. Mortgage lenders won’t lend on an affected property unless there is a professional treatment plan in place with an insurance-backed guarantee, so there’s really no option but to confront the problem.

Because of the costs associated with treating knotweed and the stigma attached to the plant it can impact property values, often by as much as 10-15 per cent. 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.

The presence of knotweed on a property does not need to be a deal breaker but you do need to understand and be prepared for what you are taking on.

What a buyer ought to know when buying a property with Japanese knotweed

Environet’s latest research with YouGov showed that a third (32 per cent) of people would still go ahead and buy a property with knotweed, as long as they could negotiate a suitable discount on the price. It doesn’t need to be a deal breaker, but you do need to understand what you are taking on and satisfy yourself that it’s reflected in the price you are paying.

1) Get a professional Japanese Knotweed Survey and Management Plan report done

A professional Japanese Knotweed Survey and Management Plan report should highlight all the issues and the costs. It will also determine the likelihood of the plant having caused damage to the building, which may not be immediately obvious since it can damage underground elements such as drains.

It should identify if “encroachment”, the spread of knotweed from one property to another, is an issue. If it is, then the first discussion you have with your new neighbours may not be as friendly as you had hoped.

2) Understand the liabilities Japanese knotweed presents

Make sure you understand the liabilities the knotweed presents, because it will be your responsibility as the new owner. If you’re still comfortable to proceed with the purchase you should be able to negotiate a discount on the price to reflect the cost of treatment and the “knotweed stigma”.

3) Don’t let the seller ‘sort out’ the Japanese knotweed problem

Don’t fall into the trap of letting the seller “sort out” the knotweed problem. A cheap attempt at eradication with inadequate guarantees is unlikely to work. Insist that the work is carried out by a firm that you trust will do the job properly, otherwise walk away.

You will need an insurance-backed guarantee to satisfy your mortgage lender, so check out their policy and requirements first.

4) Japanese knotweed does not have to scupper your deal

The presence of Japanese knotweed on a property does not need to be a deal breaker, as it can be successfully removed using the right techniques.

Make sure you fully understand what you’re taking on, but you don’t necessarily have to walk away from your dream home.

What a seller ought to know when selling a property with Japanese knotweed

A property infested with Japanese knotweed can make it difficult to sell. Buyers would much prefer to buy a knotweed-free property than have to fork out considerable sums of cash to eradicate this highly invasive species.

This is where you as the seller need to make the most out of the situation, in order to make your property attractive to potential buyers.

1) What should you do when it comes to selling a property with Japanese knotweed?

A property infested with Japanese knotweed can be difficult to sell. Most buyers would much prefer to buy a knotweed-free property than have to fork out considerable sums of cash to eradicate this highly invasive species.

Sellers are legally required to disclose if their property is, or has been, affected by the plant when they complete the Law Society’s TA6 form as part of the standard conveyancing process. You must answer honestly.

If you fail to disclose its presence and it’s discovered after the sale has completed, professionals will be able to determine the age of the plant and whether it was there prior to the sale, which could land you in court facing an expensive misrepresentation claim.

Even if you have the knotweed treated professionally prior to marketing the property, you’re still legally required to disclose that the property was affected.

This so-called “Japanese knotweed stigma” can continue to impact the price buyers are prepared to pay, even after the plant itself is long gone.

2) What is the correct way to go about removing Japanese knotweed and improving the chance of selling?

It’s best to engage the services of a reputable knotweed specialist who can eradicate the knotweed and provide suitable insurance backed guarantees which will be acceptable to all lenders. Don’t fall into the trap of DIY attempts.

3) Why the estate agent must know about your Japanese knotweed problem

Be completely upfront with the estate agent and make sure they inform any potential buyers early on, so you don’t waste time with deals falling through further down the line.

A new way to detect Japanese knotweed

There are several ways buyers can collect information about whether a property is affected by Japanese knotweed. Firstly, sellers are legally obliged to declare it if they know knotweed is present on the property. Secondly, if it’s visibly growing above ground, a surveyor should identify it during a survey.

But what if the seller is dishonest and fails to declare it, or they genuinely don’t know that the property is affected?

What if it’s winter and the knotweed has died back, it’s been deliberately concealed by the seller or has been induced into temporary dormancy by previous herbicide treatment, meaning the surveyor sees no above-ground growth?

There is a solution in the form of knotweed sniffer dogs. A dog detection survey, recently launched by Environet UK, helps solve this problem and brings peace of mind to homebuyers that a property is free from knotweed.

Two sniffer dogs, Mick and Mack, have been specially trained to detect knotweed in a garden or development site, even if it’s hidden beneath the ground.

The pair of Fox Red Labrador Retrievers can cover a garden in a matter of minutes and will indicate by ‘freezing’, or staying completely still, when Japanese knotweed is detected.

Dog detection surveys will also be useful to sellers of property in locations badly affected by knotweed, who may wish to reassure potential buyers from the outset that the plant is not present. Where knotweed is not detected, a five-year insurance-backed guarantee is provided.

Content provided by is for information purposes only. Independent and professional advice should be taken before buying, selling, letting or renting property, or buying financial products.