Japanese knotweed is a very considerable pest – 'indisputably the UK's most aggressive, destructive and invasive plant', according to the Environment Agency –as it can cause damage to buildings, spreads easily and is difficult to eradicate. As well as its potential for physical damage, its presence can also reduce the value of a property.
It is the responsibility of an owner of land which has knotweed growing on it to control it and to prevent it from spreading to adjacent land.
A recent case was brought by the owners of properties affected by knotweed because Network Rail had failed to stop it from spreading from land it owned. Whilst no actual damage was done to any building on the property owners' land, they sued Network Rail for the diminution in value of their properties caused by the knotweed.
Network Rail argued that there was no liability on its part because no actual damage had been done, but the Court of Appeal rejected that argument, concluding that 'nuisance' had been established. In order to show this in such cases, it is necessary that there has been an interference with the owner's legal rights – i.e. with the quiet enjoyment or amenity value of the land. This does not mean there has to be any physical damage.
Network Rail was aware of the presence of the knotweed and should have understood that a risk of damage to nearby property existed.