Japanese knotweed is a strong, fast growing, clump-forming perennial. It was originally introduced into the UK as an ornamental plant, thanks to its stems that resemble bamboos and its masses of flowers.
Once it is established in the garden, it becomes very difficult to control, because of its strong, deep penetrating rhizomes (creeping underground stems).
In accordance with The Anti-social Behaviour, Crime and Policing Act 2014, you must take action to remove and prevent the spread of Japanese knotweed.
Digging out the rhizomes creates a problem over disposal, since Japanese knotweed is classed as “controlled waste” under the Environmental Protection Act 1990. And under the provisions of the Wildlife and Countryside Act 1981, it is an offence to cause Japanese knotweed to grow in the wild, so it is illegal to simply dump it anywhere. Topsoil movement has caused much of its spread throughout the UK, for instance.
The Anti-social Behaviour, Crime and Policing Act 2014 includes Japanese knotweed. Allowing Japanese knotweed to become established in your garden could see you prosecuted as an act of anti-social behaviour.