A knotty problem concerning liability

According to the government, Japanese knotweed (JKW) is the most common UK invasive knotweed species. It spreads directly through rhizome (root) growth and a new plant can grow from root pieces as small as 1cm. The government indicates that while ‘you do not legally have to remove Japanese knotweed from your land unless it’s causing a nuisance’, nevertheless ‘you must stop Japanese knotweed on your land from spreading off your property.’ 

In terms of civil liability, private nuisance is an unlawful interference with an owner’s use or enjoyment of their land. A recent decision of the Supreme Court concerning knotweed liability was Davies v Bridgend County Borough Council [2024] UKSC 15. This considered an important issue of causation in the ‘but for’ test when determining whether the defendant’s breach of duty in private nuisance was responsible for the loss suffered. Judgment was given on 8 May by Lord Stephens (with whom Lord Reed, Lord Lloyd-Jones and Lady Simler agreed) and Lord Burrows delivered a concurring judgment.

At some date ‘well before 2004’, JKW had encroached on to the claimant’s land from that of the defendant council’s adjoining land. While at the date of encroachment there was no actionable tort of private nuisance against the defendant, an actionable private nuisance arose in 2013 when the defendant was, or ought to have been, aware of the risk of damage and loss of amenity to the claimant’s land as a result of publicly available information about JKW at the time, and failed to implement a reasonable and effective JKW treatment programme when it knew or ought to have known JKW was growing on its land. It was only in 2018 that the defendant implemented a reasonable and effective programme.

Consequently, it was held at first instance that the defendant was in continuing breach of the relevant duty in private nuisance between 2013 and 2018. Per Jalla and another v Shell International Trading and Shipping Co Ltd and another [2023] UKSC 16 (at paragraph 26): ‘In principle, and in general terms, a continuing nuisance is one where, outside the claimant’s land and usually on the defendant’s land, there is repeated activity by the defendant or an ongoing state of affairs for which the defendant is responsible which causes continuing undue interference with the use and enjoyment of the claimant’s land.’

Per Network Rail Infrastructure Ltd v Williams and another [2018] EWCA Civ 1514 (at paragraph 48): ‘The purpose of the tort of nuisance is not to protect the value of property as an investment or a financial asset. Its purpose is to protect the owner of land (or a person entitled to exclusive possession) in their use and enjoyment of the land as such as a facet of the right of ownership or right to exclusive possession.’ Consequently, at first instance the claim was held irrecoverable for pure economic loss. The claimant also similarly failed before the circuit judge. The Court of Appeal, however, dismissed the defendant’s submission that the diminution in value of the claimant’s land was not caused by the nuisance and considered that, given the physical manner of the JKW encroachment, the claimant’s loss was not purely economic.

In considering the ‘but for’ causation test, Lord Stephens in the Supreme Court said that: ‘In the tort of private nuisance involving encroachment of JKW rhizomes from the defendant’s land on to the claimant’s land, the claimant is required to establish that the defendant’s breach of duty did in fact cause the loss suffered.’ The purpose of this test is to eliminate irrelevant causative factors. In the present context the ‘but for’ test asks: ‘Would the diminution in value of which the claimant complains have occurred “but for” the breach of duty of the defendant between 2013 and 2018?’. For: ‘If the diminution in value would have occurred in any event, then the defendant’s breach of duty is eliminated as a cause of the diminution in value so that there would be no causal link, as a matter of factual causation, between the defendant’s breach of duty and the diminution in value.’

Lord Stephens considered that ‘the answer to the “but for” question is simply that the diminution in value had occurred long before any breach by the defendant of the relevant duty in private nuisance first occurred in 2013’. Consequently: ‘The application of the “but for” test in this case eliminates the defendant’s subsequent breach of duty as a causative factor.’ For the diminution in value would have occurred in any event and there was therefore no causal link between the defendant’s breach of duty and the diminution in value claimed.

The claimant had raised a further issue, namely that if the defendant council had commenced treatment of the JKW in 2013 instead of in 2018, then the clock would have started to run earlier so that the stigma and resultant amount of diminution in value would have decreased by 2018.

However, Lord Stephens said that: ‘Quite simply, the claimant’s further issue was not pleaded in the particulars of claim’, and ‘there was not one word of evidence to support it’. The appeal was consequently allowed, and no damages were awarded.

In his concurring judgment, Lord Burrows said that ‘the breach of duty from 2013 did not factually cause the residual diminution in value of the land’. For ‘the JKW was already present on the claimant’s land before 2013 so that the residual diminution in value had already been brought about by the natural, non-actionable, encroachment of the JKW’.

Nicholas Dobson writes on local government, public law and governance