Japanese knotweed: A question of timing and causation

Japanese knotweed can present landowners with a tangle of practical and legal problems, particularly where it spreads across property boundaries. The recent County Court appeal decision, Sateeshkumar v Wigan Metropolitan Borough Council [2026] EWCC 39, provides a useful reminder that the timing of encroachment, knowledge and action can be critical when responsibility for treatment costs is disputed.

The case concerned Japanese knotweed that had spread from a council-owned park onto neighbouring residential land before 2018. Although the Council knew, or ought reasonably to have known, about the infestation by 2018, the court found that the knotweed had already encroached onto the claimant’s land by that time. The first-instance judge had awarded £2,635 for treatment costs, but that award was overturned on appeal. The claimant could not establish that those costs were caused by the Council’s subsequent breach of duty: once established on the claimant’s land, the knotweed was sustaining itself independently of the infestation on the Council’s land.

While the decision is not binding on other courts, it nevertheless provides useful guidance on the application of ordinary causation principles to Japanese knotweed claims. It also follows the Supreme Court’s decision in Davies v Bridgend County Borough Council [2024] UKSC 15, which upheld the importance of establishing a causal link between the defendant’s breach and the loss claimed.]

A clear paper trail

Where a landowner knows, or ought reasonably to know, that Japanese knotweed is present and poses a risk of spreading to neighbouring land, a failure to take reasonable steps to prevent its spread may give rise to a claim in nuisance. The timing of knowledge, encroachment and remedial action may therefore become crucial evidence in any subsequent dispute.

Good record keeping is therefore important. Farmers and rural landowners should consider retaining dated photographs, survey reports, correspondence with neighbours, records of when the infestation was first identified, contractor reports and treatment records. Where neighbouring land is affected, it may also be sensible to record what was communicated to the adjoining owner and when.

Records will also be valuable where there is tenanted land. At the end of a tenancy, questions may arise over whether an infestation predated the tenancy, spread during the tenant’s occupation or resulted from a failure to comply with the tenancy terms. A clear record of the condition of the holding, when knotweed was identified and any treatment undertaken, together with the terms of the tenancy itself, can help establish where responsibility lies.

Effective early treatment

Simply having Japanese knotweed on private land is not, in itself, generally an offence and there is no general legal requirement to eradicate it. However, Japanese knotweed is listed in Schedule 9 to the Wildlife and Countryside Act 1981, making it a criminal offence to cause it to grow in the wild. Furthermore, the Environment Agency also warns that soil or plant material contaminated with invasive non-native plants may constitute controlled waste. This makes its removal potentially problematic, and so specialist advice should always be obtained before disturbing an infestation and disposing of contaminated soil.

Treatment is rarely an overnight exercise. Professional herbicide programmes can run for several years and may require subsequent monitoring. The appropriate approach will depend on the site and the extent of the infestation; in some circumstances effective management and control may be more realistic than complete eradication.

Contractor documentation should therefore be examined carefully. In particular, landowners should understand the proposed treatment period, payment stages, monitoring arrangements, responsibility for regrowth and the scope and transferability of any warranty or guarantee.

These matters can become particularly important when land is sold or refinanced. The Royal Institution of Chartered Surveyors (RICS) guidance adopts a risk-based approach to Japanese knotweed and recognises the reassurance that Management Plans, five- or tenyear warranties or guarantees and insurance-backed protection can provide to lenders and subsequent owners.

Don’t let the problem take root

The practical message from Sateeshkumar v Wigan is simple: act early, prevent further spread and keep a clear documentary record. If Japanese knotweed is identified, specialist advice should be sought before disturbing it and any treatment plan should be properly documented.

Early action may not eliminate every legal or financial issue, but it is usually far less costly than determining responsibility after the event. For farmers and rural landowners, regular monitoring of affected areas and prompt action can help protect neighbouring land, preserve evidence and minimise the potential for expensive disputes further down the line.