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Japanese knotweed – a new old problem?

The words “Japanese knotweed” will send a chill through even the most seasoned developer, and it is, indeed, a headache to remove from site, what with the transport and disposal regulations and the seemingly endless rounds of herbicide.

But is it really the subsidence-causing, foundation-destroying shrubbery that some expect it to be?

What’s all the fuss about?

According to a 2019 report by the House of Commons Science and Technology Select Committee’s this may not be the case. Latest research suggests that the physical damage caused by Japanese knotweed may be no greater than native trees or shrubbery. It is, however, still notoriously difficult to get rid of as it is incredibly resilient and capable of returning many years down the line if its rhizomes or fragments of its vegetation remain left in the soil.

So, if it isn’t as dangerous to our foundations and concrete hardstanding as perhaps expected, why should we be so concerned about it?

Like any plants and trees, it can cause damage to foundations but, unlike others, it is subject to a host of regulation – so remediation isn’t a simple process. Further to this, and the subject of the article, a recent Court of Appeal case (Davies v. Bridgend County Borough Council [2023]) has held that in the case of continuing nuisance, those who have ignored the nuisance of Japanese knotweed will be responsible for any residual diminution in the value of affected neighbouring land. This has not previously been seen, as prior cases regarding Japanese knotweed and the diminution in value of neighbouring property ruled that such loss was pure economic loss only and was therefore generally irrecoverable. In a legal shake-up, however, Davies is distinguished from that notion. 

What happened in Davies?

In brief, the defendant (a local authority) had Japanese Knotweed on their land and the knotweed had encroached, unnoticed, onto the claimant’s neighbouring land, stretching its rhizomes underground across the boundaries between the two. The claimant was then put on notice that they could make a claim for such an encroachment and first did so in 2019. In this first instance, the judge dismissed the claim for diminution in value of the property as it was held to be pure economic loss and therefore irrecoverable, as it is a well-known legal concept that the tort of nuisance does not protect purely economic interests.

However, the Court of Appeal disagreed. They based this decision on the fact that the original judge had used the case of Williams & Waistell v Network Rail Infrastructure Limited [2019] to show that diminution in value of the property was pure economic loss. In Williams, there was no active encroachment of Japanese knotweed, rather it was being argued that the mere presence of knotweed next door devalued the property. It was held as such that diminution in value of the property was pure economic loss and therefore irrecoverable.

Here, the Court of Appeal distinguished Davies from Williams as there was clear encroachment of the knotweed and therefore a physical interference with the claimant’s property resulting in consequential losses – the diminution in value of the property. Those losses were, therefore, not pure economic loss and were recoverable. The cost of treating the knotweed, however, was not recoverable because treating the knotweed would have been required regardless of whether there had been a breach of duty on behalf of the defendant or not.

There was also an important point to note that limitation periods didn’t really apply in the traditional sense to such Japanese Knotweed nuisance claims. Encroaching knotweed was a continuing, persistent nuisance that affected the owner’s quiet enjoyment and amenity of the property, caused by a breach of duty on the part of the defendant. This means that there is, theoretically, no limitation period for encroachment claims, opening everyone, even new purchasers of a property, to them.

What relevance does this have to construction?

Japanese knotweed is not, per se, a construction law issue; but the ramifications of mishandling it could be, as Davies shows, widespread and costly. If a local authority (often hard to win cases against as courts can seem reluctant to rule against governmental bodies for public policy reasons) can be considered in breach of duty for allowing the continuing nuisance of knotweed as in Davies, then it could open the doors for developers being at risk of similar claims, regardless of when they bought the site.

Although there haven’t been any precedents set (yet!) by negligent developers in their handling of Japanese knotweed, there are some obvious potential issues for those working under standard construction contracts:

  • Firstly, it is clear that any contract with a contractor or consultant hired to be responsible for the removal or management of Japanese Knotweed needs to be robust in addressing nuisance. The JCT suite is silent on the subject of nuisance, and notably any indemnity protecting the employer in the event of nuisance is also absent. Consideration should go into the inclusion of an indemnity or protection for the employer, and contractors themselves may have to amend their pricing if they are expected to take on such an indemnity. This is further compounded by ground risk – who is taking it and, if it is the contractor, are they able to rely on any reports that may or may not tell them of Japanese knotweed contamination? If a contractor is expected to take on ground risk next to a railway for example (where the rate of Japanese knotweed existing may be higher) then the employer should also expect that the contractor will be pricing for this risk.  
  • This then feeds into the second point of liability – not only have we seen that physical encroachment by Japanese knotweed can lead to successful claims, putting the owner of the site at risk, but the mishandling of knotweed can even result in criminal conviction. Knotweed is so heavily regulated that, for peace of mind, the employer may not want to push full responsibility of knotweed onto the contractor and instead approach it more collaboratively. Letters of reliance or warranties from any knotweed specialists become an absolute must-have, and there may come a point where a local authority becomes aware of knotweed contamination, resulting in specialist orders being issued. If this is mid-remediation and the contractor is working on-site elsewhere, then consideration may have to be given towards cooperation and collaboration clauses with external third parties and those statutory requirements clauses take on a whole new importance.
  • Thirdly, the owner of the brand-new site, ready for construction but now slapped with the problem of knotweed, may want to consider its own claims against a seller. Were there any misrepresentations in the sale? When buying, developers may want to consider an extra query regarding the presence of Japanese knotweed.  Conversely, when selling completed sites that have been contaminated with knotweed, even when treated, it may be wise to make any buyer aware of such contamination as to not be accused of misrepresentation.


At its core, Davies highlights the need to be vigilant regarding knotweed, regardless of how long someone has owned a site for and whether they were the cause of any ongoing nuisance or not. If a disgruntled neighbour finds knotweed in their garden and it seems to be coming from the developer’s site, Davies suggests that the new site owner could be liable for breach of duty and a failure to mitigate any ongoing nuisance. Added to the existing problems that Japanese knotweed can cause a development, the need for comprehensive site investigations and careful allocation of risk in construction contracts has never been more important.

This article originally featured in June 2023’s edition of our Aggregate newsletter: to read the complete edition, click here.

Sophie Bennett headshot

About the author

Sophie is a Trainee Solicitor, specialising in non-contentious construction matters. Read more about her here.

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