Japanese knotweed disputes and the law

Japanese knotweed is a phrase that strikes fear into land and property owners for many reasons, but largely because of negative press reporting about what can happen if you find Japanese knotweed in your garden and it spreads elsewhere.

Common concerns about Japanese knotweed and other invasive non-native species are that they:

• can cause physical damage to buildings and land, which can affect the value of a property as well as its marketability and insurability;
• can be very expensive and time-consuming to remove or treat;
• can result in criminal and civil liabilities for owners, occupiers and persons handling Japanese knotweed if they do not do so in accordance with the law.

What is Japanese knotweed?

Japanese knotweed was introduced into Great Britain in the 19th Century as an ornamental plant. It is now one of the most invasive plants in the UK, although various bamboo species can also be very problematic. It is a perennial plant that comes back every year and spreads rapidly by its roots (rhizomes) and stems, and it is very difficult to eradicate from land.

In the growing season between April and October, it can grow up to 10 centimetres a day. Below ground, the roots can extend to a depth of three metres and up to 7 metres laterally. Even if a small piece of root or stem is left in the ground, it can re-infest the land, and the vigorous roots and top growth can penetrate building foundations and concrete hard standing, causing considerable damage.

The costs of removing knotweed and treating it can also be very substantial.

It is a problem if it encroaches onto your land because it would have to be disclosed if you were selling your property, and mortgage lenders can be difficult about lending if it is present. In many instances, however, a lender will have no issue with lending on it if it can be shown that a management care programme is in place to stop the spread.

How do you identify Japanese knotweed?

Seek advice from someone with specialist plant knowledge to determine whether it is knotweed and whether it is contained or spreading. There are specialist arboricultural firms that can help to identify plant species to give you some clarity about whether it is Japanese knotweed that you have spotted in your garden.

Japanese knotweed when buying a property

You may be familiar with the term buyer beware (in Latin, caveat emptor), which means that when you purchase a property, the seller or landlord is not under a duty to disclose any information about the property's physical condition. To address this, the buyer usually raises pre-contract enquiries to obtain more details about the property's condition, in addition to instructing a building surveyor.

In the context of a commercial property, the standard pre-contract enquiries specifically ask for details of rising damp, rot, any fungal or other infection, any infestation, and hazardous substances or contaminative materials.

One of the enquiries also asks for details of any environmental problems relating to the property or land in the vicinity that might adversely affect the property, its use or enjoyment, or give rise to any significant liability or cost on the part of the owner or occupier. Arguably, asking these questions should reveal whether or not Japanese knotweed is present, although it is unclear whether Japanese knotweed is a contaminative or potentially contaminative material.

If you are particularly worried about Japanese knotweed as a buyer, you should ask your solicitor an additional question: please confirm that there is no Japanese knotweed on the property. You can also ask whether there are any other invasive non-native plants on the property, or whether the seller is aware of Japanese knotweed on adjoining land. Given the gravity of the consequences, if the seller gets this wrong, they are unlikely to give a definitive answer. If a seller is not actually occupying the property and it is not inspected, the likely response will be “the buyer should rely on its own inspection and survey”.

If the seller replies “not so far as the seller is aware” without making reasonable efforts to check the position, it could subsequently be held liable for misrepresentation if it turns out that Japanese knotweed was present at the property on the date of the answer. The seller can, therefore, reduce this risk by making a statement such as “not so far as the seller is aware, but it has not inspected the property since X date and gives no warranty as to the absence of Japanese knotweed. The buyer should make its own enquiries and inspect.” It could also be that the seller has obtained a Japanese knotweed guarantee from a specialist company and can pass on the benefit to the buyer on the sale of the property, in which case the seller would have to disclose the existence of the guarantee.

In addition to pre-contract enquiries, obtaining a building surveyor’s report on the property is good practice and, if your budget allows, commissioning an environmental desktop report from an environmental consultancy. You should, however, be careful with these types of reports because they are based on publicly available information and do not include a site visit, so you may still be asking questions about whether Japanese knotweed is present on the property. If you are particularly concerned about the presence of knotweed, especially if you plan to redevelop the site, you should commission a knotweed survey and assessment from an environmental consultant or specialist knotweed consultant.

Is it safe to rely on a standard building survey?

From March 2022, the Royal Institution of Chartered Surveyors Professional Standard, Japanese Knotweed and Residential Property First Edition applies. This reflects an improved understanding of Japanese knotweed and sets out an assessment method. It confirms that, in general, three metres from the property boundary is the distance within which Japanese knotweed should be reported to a lender.

The 2022 standard includes a management category assessment decision tree, based on risk level, to help valuers determine the appropriate management approach and how it should be reported. As a buyer, you need to take particular care as to the instructions that are given to your building surveyor because not all surveyors will, as part of their instruction, consider the 2022 standard; i.e. it will not be taken into account if the surveyor is simply asked to provide a valuation without having inspected the property.

Should you buy a house with Japanese knotweed?

There is no issue with buying a house where knotweed might be present in the garden, so long as it is being treated under a care plan and carefully monitored. It is no longer to be feared as it once was.

Local Authority role when Japanese knotweed is present

A Local Authority does have the power to serve a notice on an occupier of land requiring it to remedy the condition within a specified period where in the Local Authority’s opinion, the amenity of an area or adjoining area is adversely affected. This power could be exercised for land invasion by Japanese knotweed if it is at risk of spreading into adjoining land.

The Local Authority has to give a minimum of 28 days’ notice for the Japanese knotweed to be remedied or removed. During this time, the landowner can appeal to the Magistrates Court against complying in limited circumstances. If the occupier fails to comply with the notice, then the Local Authority can prosecute the landowner in the Magistrates Court.

On conviction, the landowner could be liable for a fine not exceeding Level 3 on the standard scale (currently £1,000), or the Local Authority can step in, undertake the necessary works, and recover the costs from the occupier.

Landowner obligations

Surprisingly, the owner or occupier of land is not obliged to control, remove, eradicate, or treat Japanese knotweed. The issues come to the fore when the landowner fails to take reasonable measures to control Japanese knotweed, resulting in it spreading into the wild, or is negligent or reckless about that occurring. This can amount to the offence of causing it to grow in the wild under section 14 of the Wildlife and Countryside Act 1981.

Therefore, it is prudent to take land management action to ensure there is no spread of Japanese knotweed. Neither is an owner or occupier of land obliged to report to the Environment Agency, Local Authority, or any other body that Japanese knotweed is present on the land.

Landowners can run into difficulty if they allow the spread onto neighbouring land because the owner of the adjoining land could theoretically issue common law private nuisance proceedings for compensation for loss of enjoyment or amenity (which may amount to the diminution in value of the property) or the costs of removal or a continuing injunction against reinfestation and/or requiring action to control the knotweed.

The nuisance must be substantial or unreasonable and can arise from either a single incident or an ongoing state of affairs. The leading case in this developing area is Network Rail v Williams (Court of Appeal, 2018). The claimants were adjoining freehold owners of two semi-detached bungalows in South Wales. Both of their bungalows abutted a railway embankment and access path, which the defendant, Network Rail Infrastructure Limited, owned. The railway embankment and path had been infested with Japanese knotweed for over 50 years, and it had persistently spread to the claimants’ land. In its judgment, the Court restated the principles of nuisance but also made the following findings:

• The proposition that damage was always an essential requirement of the cause of action was not entirely correct. The concept of damage was elastic so that rhizome contamination (from Japanese knotweed) could be enough.
• Nuisance could be caused by inaction or omission as well as a positive activity. An occupier could be liable for a continuing nuisance created by another person if, with reasonable presumed knowledge of its existence, they failed to use reasonable means to bring it to an end when they had had sufficient time to do so. An occupier could also be liable if they fail to act with reasonable speed to remove a hazard from their land of which they were aware and where it was foreseeable that it would damage and spread to the neighbour’s land.

A further case came before the Court in 2023 and was heard by the Supreme Court in 2024, looking at Japanese knotweed encroachment. In Davies v Bridgend CBC, the landowner sued for damages in nuisance for the residual decrease in value of their property caused by Japanese knotweed spreading from the Local Authority’s neighbouring land.

The Court found that the trespass existed when the landowner bought the property, before any action in private nuisance had arisen, and that the neighbour’s failure to treat the Japanese knotweed had not, in fact, materially contributed to the decrease in value.

The application of the “but for” test eliminated the neighbour’s subsequent breach of duty as a causative factor. The fact that the diminution in value would have occurred in any event meant that there was no causal link between the breach of duty and the diminution in value that was claimed.

The Court of Appeal found that if the value of the neighbouring property had decreased as a result of an interference with the quiet enjoyment or amenity due to the Japanese knotweed, then damages in nuisance for the decrease in value would be available. However, the Supreme Court overturned this decision.

How to make a Japanese knotweed claim

The first thing to do is get specialist advice on whether the plant you are concerned about is actually Japanese knotweed.

Once you have an expert opinion that it is Japanese knotweed that is growing in your garden, and an expert view that it is not originating in your garden but spreading from elsewhere, then you should in the first instance try and have a conversation with your neighbour to see if they recognise that they have Japanese knotweed on their land, and to enquire if they are willing to pay to take steps to remove the knotweed from their own land and your land. If you are concerned that damage has been caused to your land or property by the knotweed, you should also ask whether they are prepared to accept responsibility for remedying any damage caused to your land.

Hopefully, you will be able to have a cooperative dialogue with your neighbour in many instances. Once they realise Japanese knotweed is present, they will be keen to remove it as soon as possible, not only from your land but also from their own.

If they are unwilling to engage with you or are uncooperative, it may be necessary to seek specialist advice from a property dispute solicitor on what you can do next to force the neighbour to deal with the issue. Your solicitor will likely recommend sending a legal letter before action to your neighbour, relying on legal nuisance as the basis on which you are entitled to take action against them.

In the very worst cases, if your neighbour ignores the legal letter before action, it may be necessary to issue Court proceedings against them to force them to take the issue seriously.

Defending Japanese knotweed claims

If you find yourself on the receiving end of a complaint from your neighbour or a ‘letter before action’ asserting that Japanese knotweed has spread from your land onto neighbouring land, you should, in the first instance, seek expert advice on whether Japanese knotweed is present. Once you are clear on whether Japanese knotweed is growing in your garden, you should also ask the expert to advise whether the knotweed originated in your garden or the garden next door.

Even if you are advised that your garden is the source of the knotweed, you may still have defences if Court proceedings for legal nuisance are brought against you.

We can help you bring a claim against a neighbouring landowner if you find that Japanese knotweed is spreading from their land onto yours. Alternatively, we have extensive experience defending claims if you find yourself in the unfortunate position of a neighbour claiming that Japanese knotweed is spreading from your land onto theirs.