Japanese knotweed is a phrase that strikes fear into land and property owners for many reasons, but largely because of some very negative press reporting about what can happen if you find Japanese knotweed in your garden and it is spreading 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 species of bamboo can also be very problematic. It is a perennial plant that comes back every year and spreads rapidly by its roots or rhizomes and stems, and it is very, 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, concrete hard standing and cause considerable damage.
The costs of removing knotweed and treating it can also be very substantial.
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 the landlord is not under a duty to disclose any information about the physical condition of the property. To deal with this, the buyer usually raises pre-contract enquiries to establish more details about the condition of the property, in addition to instructing a building surveyor.
In the context of a commercial property, the standard pre-contract enquiries for commercial property specifically ask for details of rising damp, rot, any fungal or other infection or any infestation and for details of 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, then you should make sure that your solicitor asks an additional question, namely, please confirm that there is no Japanese Knotweed on the property. You can even go further and ask whether there is any other invasive non-native plant on the property or whether the seller is aware of the presence of Japanese knotweed on adjoining land. Given the gravity of the consequences, if the seller gets this wrong, the seller will unlikely give a definitive answer. If a seller is not actually occupying the property and it is not inspected, then the likely response from a seller 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 the case that the seller has obtained a Japanese knotweed guarantee from a specialist company and that they can pass on the benefit of that to the buyer on the sale of the property, in which case the seller would have to reveal 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 leave you still asking questions about whether Japanese Knotweed is present on the property. If you are particularly concerned about the presence of knotweed, particularly if you have a plan to redevelop the site then 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 contains a management category assessment decision tree based on risk level to help valuers determine the appropriate management approach needed 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.
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 which 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 at £1,000), or the Local Authority can step in and undertake the necessary works and recover the costs of 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 the Japanese knotweed, which results in it spreading to the wild or being negligent or reckless about that occurring.
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 has to be substantial or unreasonable and could arise from either a single incident or an ongoing state of affairs. The leading case in this developing area is Network Rail v Williams from the Court of Appeal in 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 claimant's 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 had existed when the landowner bought their property at a time before an action in private nuisance had arisen, and that meant 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.
