Is there Japanese Knotweed on your site? Identifying and dealing with this invader

Andrew WilliamsSurveying

As part of the planning process, most building sites have to be inspected by a qualified arborialist.

What if the arborialist’s report indicates that there is Japanese Knotweed lurking on part of the site?

Legal requirements

Environmental Protection Act 1990 Japanese Knotweed is classified as ‘controlled waste’ and as such must be disposed of safely at a licensed landfill site according to the Environmental Protection Act (Duty of Care) Regulations 1991. If soil contains rhizome material or is likely to contain such material it is regarded as contaminated and, if taken off a site, must be disposed of at a suitably licensed landfill site and buried to a depth of at least 5 m.

An offence under the Wildlife and Countryside Act can result in a criminal prosecution. An infringement under the Environmental Protection Act can result in enforcement action being taken by the Environment Agency which can result in an unlimited fine.

The offender can also be held liable for costs incurred from the spread of Knotweed into adjacent properties and for the disposal of infested soil off site during development which later leads to the spread of Knotweed onto another site.

The Costs

Reading the above, it should be obvious to any QS that having knotweed on potential site could have major cost implications. If it is removed, the excavated material has to be disposed of at licensed site. (and a premium paid.) It is also possible that uncontaminated fill will have to be imported to make up levels. If it the knotweed is treated in-situ it could take an experienced eradicator months of repeat chemical treatments before the weed is destroyed.

A prudent developer would also not start work until he was absolutely certain that the infestation really has been destroyed.

Delays cost money.

One of the reasons that eradication is so difficult is because the Japanese Knotweed rhizome network underground (a type of modified root system) may extend to a depth of up to 3-metres and 7-metres around the perimeter of the clump.

Just this month, a couple were reputedly forced to demolish their £300,000 four-bedroom home after it was invaded by the plant. According to news reports, they saw the value of their four-bedroom house drop from £305,000 to £50,000 because of the damage.

Although there had been no evidence of the weed when they bought the house, two years later it began forcing its way up though floorboards.

So the message is, if you have a site and do not eradicate the knotweed it will come back to bite you.

Identifying knotweed

Japanese Knotweed is a hardy bamboo like plant that was imported from Japan in the 1850’s and has become a major problem in a number of countries around the world because it has no natural predators in the host counties. Because the original plants were female, Japanese Knotweed in the UK does spread by seeding. It tends to spread by Vegetative reproduction. (Part of the mother plant breaks away or is transported by human agents and forms a new clump elsewhere.) It tends to spread along waterways and railways where it colonises bare land very rapidly. Japanese Knotweed is a tall perennial plant. This means that it dies back in winter and re-emerges in spring. The shoots start to emerge in late March / early April, are asparagus like and are a red-green colour. As it grows through the summer the red colour turns green. At full height it can reach up to 3m. The plant flowers in late summer and these consist of clusters of spiky stems covered in tiny creamy-white flowers.