An invasion of Japanese knotweed?

The dangers of Japanese knotweed have been widely publicised in recent years and it is no surprise that claims against property owners and property professionals have been rising. Last year’s Court of Appeal decision in Network Rail v Williams (2018) will only fan the flames.

In addition to potential loss of property value, owners of land on which Japanese knotweed is growing can face prosecution or civil claims in nuisance if they allow it to spread to another property. Bringing a claim against a surveyor for failing to spot Japanese knotweed has been seen as a potential line of recovery. All too frequently, however, these claims can lack the necessary evidential proof and coherency.

In the Network Rail case, the claimants’ houses were plagued by knotweed from a railway embankment at the foot of their gardens. They succeeded in establishing a private nuisance as a result of Network Rail’s apparent inability to prevent an ongoing encroachment and were awarded £10,000 and £10,500 for residual diminution (the stigma attaching to the risk of knotweed returning, despite treatment, and the ongoing presence of the weed on Network Rail’s adjoining land). The decision was upheld by the Court of Appeal but, critically, only because it refused to permit Network Rail to introduce new expert evidence to argue that knotweed had been present in the area for many years and had not adversely impacted on values in the locality.

Claims against surveyors will be difficult to prove. Expert evidence will be needed to establish that the knotweed could and, therefore should, have been identified when the original inspection was carried out. Often the expert evidence is limited to asserting that knotweed must have been present when the property was inspected, rather than whether it should have been spotted by a reasonably competent professional. A surveyor does not have a duty to go digging around in the garden, so if knotweed has been cut down and/or covered, then it is quite possible that the plant was not visible.

The Network Rail case is being cited in claims against surveyors as authority to recover diminution in value losses. However, many claims appear to be highly over-inflated with assertions that a property with knotweed is not mortgageable and is therefore blighted. This is incorrect. Lenders will usually not have a problem, as long as an insurance-backed treatment plan is in place. Accordingly, there may well be no diminution beyond the cost of treatment; a fact explicitly recognised in knotweed guidance produced by RICS.

Is this public fear of Japanese knotweed justified? There is an ever-growing consensus that knotweed is not as dangerous as believed. Extensive research by Leeds University and global infrastructure services firm AECOM concluded:

(i) there was no evidence to suggest that knotweed causes significant damage to buildings, even where growing in close proximity to a property
(ii) knotweed poses less of a risk to buildings than many woody species, particularly trees;
(iii) knotweed is capable of damaging buildings but this usually occurs due to an existing, inherent weakness or defect that has been exacerbated.

Effective treatment is widely available and relatively cheap and can be backed by insurance. Knotweed can be damaging, but the threat is often exaggerated. As the above research suggests, on its own, the plant will NOT damage buildings any more than other plant/tree interference.