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Japanese knotweed, a non-native plant introduced to the UK in the 19th century, is very difficult to eradicate and can grow up to 10cm a day in the summer months. The plant鈥檚 underground rhizome (root) system is extensive and can lay dormant for long periods, meaning that cutting the plant down or digging up the roots is not an effective method of treatment.

The public perception of the problems caused by knotweed over the last decade has developed into a stigma for properties with, or even near, untreated knotweed. While it rarely causes structural damage, the time and expense of treatment means that knotweed is offputting to prospective purchasers and can reduce property values by 5%-10%.

To calm fears and reframe the risks of knotweed, RICS revised its valuation guidance note with effect from March 2022 (reissued as a professional standard in October 2022) for use in residential property. It offers four categories of risk with recommendations based on the impact of the knotweed, rather than the mere presence of it. The categories range from management category A (significant impact causing visible material damage and requiring action) to management category D (visible knotweed on adjacent land within 3m of the boundary 鈥� unlikely to require action or impact value except in exceptional circumstances).

A neighbourly nuisance

If a developer is acquiring or redeveloping land with neighbouring knotweed, does it need to take any special precautions? You might think not, particularly if the visible stems of the knotweed are more than 3m from the boundary 鈥� extrapolating from the reasoning of RICS guidance. However, the rhizome system can spread many metres underground. Disturbing soil containing pieces of rhizome can spread the plant unintentionally. Small sections of rhizome less than 1cm long can regrow into a new plant. If contaminated soil is dispersed during development works, a developer could be faced with a claim in nuisance from neighbours whose land was previously free of the plant.

For example, in a recent Court of Appeal case, Davies vs Bridgend County Borough Council (2023), the council was found liable to Mr Davies for breaching its duty to take reasonable steps to prevent the spread of knotweed on Mr Davies鈥� land. Mr Davies did not need to prove the knotweed was a risk to buildings on his land, nor that it would interfere with any future development plans. Once it had been established that it had physically encroached onto Mr Davies鈥� land in a more than merely trivial way, there was as actionable nuisance.

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The court was clear that there would not be an actionable nuisance simply because the stigma of knotweed on a neighbouring property reduces the value of land. There must be physical damage or interference to the land, otherwise any reduction in value is a pure economic loss (which is not legally recoverable for policy reasons). However, once there is a non-trivial encroachment onto the land, damages are available including for any diminution in the value of the land.

It should be noted that knotweed had been growing on the council鈥檚 land for over 50 years and that the encroachment onto Mr Davies鈥� land probably happened more than 15 years before he first raised concerns. And while the knotweed had not been treated by the council, it also had not been disturbed to spread the rhizomes.

Criminal offences

It is not a criminal offence to have knotweed on your land, but it is an offence to allow it to spread, under the Wildlife and Countryside Act 1981. The Department for Environment, Food and Rural Affairs鈥� guidance on the law suggests that negligent or reckless behaviour, such as inappropriate disposal of contaminated soil that spreads the knotweed, would constitute an offence. Penalties include fines up to 拢5,000 and a maximum of two years鈥� imprisonment.

Also, any soil containing knotweed is designated as 鈥渃ontrolled waste鈥� under the Environmental Protection Act 1990, imposing statutory duties to take all reasonable measures to ensure it is properly dealt with. In practice, this means that contaminated soil must only be removed and disposed of by a licensed operator.

Nipping it in the bud

Ignorance of the perils of knotweed is not a defence, so the issue needs to be identified early and properly managed. Practical points include:

• Taking care when acquiring land for development, including inspecting neighbouring land (without committing trespass). Brownfield sites and areas with a history of fly-tipping may present a higher risk.
• Engaging specialists to survey the land and soil before breaking ground.
• Where identified, not cutting knotweed down but first engaging specialists to advise on appropriate treatment and management.
• Using properly licensed operators for the removal and disposal of contaminated soil.
• Particular care is needed where the acquisition is corporate 鈥� that is, where the landowner company does not change but the developer is buying the shares in that company. This will create the risk of liability for historic claims against that company.

Matthew Bonye is a partner and Shanna Davison is a professional support lawyer in the real estate dispute resolution team at Herbert Smith Freehills