Japanese Knotweed

1 October, 2007

Under section 14(2)(a) the Wildlife & Countryside Act 1981 it is an offence to plant or otherwise cause to grow in the wild any plant included in Part II of Schedule 9 of that Act.  Japanese knotweed is a plant listed under Schedule 9.

Under Part II of the Environment Protection Act 1990, japanese knotweed is “controlled waste” and must be disposed of in accordance with the proper procedure.  Since the initial treatment of the weed requires herbicides, the Control of Substances Hazardous to Health Regulations 2002 must be followed….

As well as being a statutory offence to allow the plant to grow in the wild, it is a common law nuisance to allow japanese knotweed to grow onto neighbouring property.  In applying the general principles relating to nuisance claims, the courts have indicated in the past that they are likely to be guided by the cases relating to encroaching tree roots.  The case of Flannigan v Wigan Metropolitan Borough Council (1995) related to the invasion of japanese knotweed where the knotweed had invaded the claimant’s garden from the council’s neighbouring land.  The court ordered the council to treat a one metre strip of the plant along the boundary between the properties with glyphosate for three years and to install a reinforced concrete boundary.  The council was also ordered to contribute towards the owner’s costs of bringing the action.

Although the courts have looked to the cases relating to encroaching tree roots, it is our view that the expert intelligence surrounding Japanese knotweed has now developed to the extent that a more strict form of liability may be imposed by the court in the event that a large scale case were to be brought to trial today. In view of the fact that knotweed has been designated by the Environment Protection Act in a similar bracket to other contaminants, we consider that any landowner who allows the knotweed to spread onto neighbouring land could be strictly liable for all and any damage caused by such encroachment. Such damage could be significant where development is either delayed or prevented.

So what is the duty between neighbouring landowners?
A duty of care exists between all neighbouring landowners not to do anything that would cause an encroachment and/or physical damage to their neighbour’s land or building or vegetation grown upon it. This includes a positive duty to intervene where such encroachment has occurred or is likely to occur.

What is the extent of the landowner’s liability?
Assuming that the knotweed causes some damage to the adjoining land, the responsible landowner will be liable for all and any foreseeable damage so caused.  Depending on the layout of the adjoining land, consequential damage could be considerable.

What practical measures would the court expect the landowner to take in order to contain the problem?
The landowner would be expected to take all such steps as are necessary to eradicate the knotweed on both their and the neighbouring land. Depending on the urgency, it may be that immediate eradication is required and this is unlikely to be achieved by the application of herbicidal treatments. In such cases, it may be that the only other options are the installation of a root barrier (followed by herbicidal treatment to the knotweed on the affected side of the barrier) or the excavation of the knotweed and removal of the same to a licensed waste management site (the “dig and dump” method). 

How should the neighbouring landowner react to an invasion of knotweed on their land?
Once they discover the knotweed, a neighbouring landowner should invite the owner of the  ‘affected’ land to treat the knotweed on the adjoining land. If the owner of the ‘affected’ land refuses to act then the neighbour can seek injunctive relief from the courts to enforce such action. If the neighbour fails to do anything or refuses to allow the owner of the ‘affected’ land to treat the knotweed then the neighbouring owner may be in breach of their common law duty to mitigate their losses. At worst, they may find that they are liable for any further encroachment of the knotweed from their land onto other neighbouring land. A joint approach (as between all owners) is the way to deal with large scale invasions.

This is a complex area of the law and this note is only a brief summary of the legal issues. Further advice should be obtained in order to assess the full extent of the liabilities of the various landowners involved in any problem site.

Reviewed in 2015