Japanese knotweed

What is it?

Knotweed originated in harsh upland environments in the Far East and was introduced into this country by Victorian landscape designers. In our fertile, lowland environment it has become an invasive species that should be regarded as a form of contamination with implications for both the natural and built environments. The plant typically forms large clumps of persistent stems that crowd out the native flora and fauna: if left uncontrolled it can devastate the natural ecosystem in affected areas.

What are the potential problems?

Both the top growth and the extensive underground rhizome system represent a significant threat to buildings. Knotweed can grow through tarmac surfaces and weak structures and while it may not grow through concrete it is aggressive and will find and exploit weaknesses, so damaging the integrity of large structures.

What we can do

Environment Agency (EA) guidelines stipulate that knotweed should be controlled and place an obligation on developers to ensure that any knotweed found on their land is treated responsibly.

There are two basic approaches. The costly 'dig and dump' option has often been the preferred solution, where all potentially contaminated material is removed and deposited at a licensed landfill site. The second - usually cheaper - option is to treat and manage the contamination on site.

Although both approaches are in line with current government and EA policy, in our opinion it is impossible to guarantee complete eradication: small traces of knotweed can lie dormant in the ground for up to 10 years.

JP Associates advises developer clients not to rely on any one single control or mitigation measure and has devised a four-stage strategy for dealing with knotweed. We take a view of the whole project and can guide clients through initial control, help them with site management and advise on development design. One of our consultants specialises in Japanese knotweed issues and he will work closely with the client to manage the problem efficiently and effectively right through the development process.