The growing threat of Japanese knotweed

Back in 1850, visitors to the Royal Botanical Gardens, Kew, could admire a new species Japanese knotweed. Favoured by Victorian gardeners, partly because it looked like bamboo, its use spread rapidly.

However, what we now know is that Japanese knotweed is one of the world's most invasive species, according to the World Conservation Union. Once established, it can grow up to 10cm per day from spring to autumn. Its roots can grow to three metres in depth and seven metres laterally. If even the smallest piece of root remains after treatment, it can re-infest a site. It can penetrate foundations, concrete hardstanding and walls.

So what are the implications for property and, in particular, developers? And how do you know if a site may be affected? Brownfield land, and sites near railways and watercourses by which the plant may have accidentally been transported – are a particular risk. But any prospective development site could be affected.

Developers should, of course, make all necessary due diligence enquiries – caveat emptor applies and the seller is under no obligation to reveal its existence on site. However, even a desktop environmental search may not be enough, and a specialist survey may be necessary if the site is thought to be at risk.

Assuming knotweed is found, look to specialist firms to give a guarantee that it has been fully eradicated from the site – and negotiate for such guarantee to be assignable.

The costs of treatment and removal can be substantial. However, Land Remediation Relief may be available for expenditure incurred in removing knotweed from land in a contaminated state – for example, for off-site treatment. However, it is not available for disposal of knotweed to landfill.

In addition, there are a raft of potential civil and criminal fines and penalties for breach of environmental legislation. These are outside the scope of this article – but, suffice to say, the key is to stay on top of the issue and deal with it appropriately.

Public awareness is growing and court judgments are widening liability. For example, in April 2017, a court held that knotweed found on railway land was an actionable nuisance before it caused physical damage on neighbouring land - because it affected the land's amenity value. Not surprisingly, given the size and nature of its estate, Network Rail is appealing this decision.

And in a case only last month, Cornish homeowners Adam and Eleanor Smith succeeded in their claim for loss of property value against neighbour Rosemary Line – on the basis that Ms Line had allowed the plant to spread to their garden.

Both were county court decisions, but the warning is there – the threat from Japanese knotweed is growing.

If you require further information on anything covered in this briefing please contact Andrew Wade or your usual contact at the firm on 020 3375 7000.

This publication is a general summary of the law. It should not replace legal advice tailored to your specific circumstances.

© Farrer & Co LLP, March 2018