Japanese Knotweed - The neighbour from hell
Japanese Knotweed is a non-native invasive species and is the most invasive plant in the UK. It grows rapidly, the roots are extremely wide spreading and can penetrate foundations and concrete so can cause considerable physical damage to buildings and land. This will affect the value, marketability and insurability of property. It is well known that Japanese Knotweed can be very expensive and time-consuming to eradicate, remove, treat and dispose of.
There have been some recent County Court cases which have established that just having Japanese Knotweed growing on your land can open you up to potential liability for nuisance even if it does not spread and cause actual physical damage to neighbouring land.
For example Williams v Network Rail Infrastructure Ltd  UK CC, Waistell v Network Rail Infrastructure Ltd (both cases together called "Waistell") The Waistell cases and Smith & Another v Line.
The Waistell cases concerned houses adjoining Network Rail land. Japanese knotweed had existed on the adjoining Network Rail embankment next to the houses for more than 50 years. The claimants claimed damages for nuisance because of encroachment of Japanese Knotweed onto their land. The encroachment was proved but no physical damage was caused by the presence of the Japanese Knotweed, so the claimants were unable to claim damages.
Loss of amenity
The claimants also claimed damages for loss of amenity of their land because of the presence of Japanese Knotweed on Network Rail's land. The claimants succeeded on this point because the presence of Japanese knotweed on Network Rail's land reduced the value of the claimant's land (even if treated) by reason of the land being less marketable. Network Rail had also breached its duty of care to its neighbours to take reasonable steps to minimise this hazard. The claimants recovered damages.
Remember - Duty of care
In the Smith v Line case, the Judge found that although there was encroachment, again, no physical damage to the neighbour's land had occurred, but the owner of the land with the Japanese Knotweed (Mrs Line) had failed in her duty of care to take reasonable steps to prevent physical encroachment of the Japanese Knotweed. Mrs Line was held to have interfered with the claimant's enjoyment of their land and was ordered to pay the claimant's costs and a mandatory injunction was granted requiring her to treat the Japanese Knotweed on her land.
The Waistell case is currently being appealed, but subject to any Court of Appeal decision to the contrary it seems likely based on these cases, that any claim in nuisance based on encroachment will not succeed unless the claimant can show physical damage to their property. However, this leaves open the possibility of a claimant making a claim against neighbouring land owners based on diminution of value where there is Japanese Knotweed close to the boundary of their property, even where there is no actual encroachment.
At first glance this seems like a surprising decision, but no-body wants to discover Japanese Knotweed on their property and to find out, that as a result, their property is potentially unmortgageable. Remember you owe your neighbour a duty of care, so you should always take reasonable steps to minimise the risk of Japanese Knotweed encroaching onto neighbouring property, to avoid the risk of any claim.