You're legally required to keep any invasive vegetation on your land underneath control, this includes Japanese knotweed. Since the plant is labelled as controlled waste via the Wildlife and Countryside Act 1981 [eight], it is illegal to simply remove Japanese knotweed out of your land and dump it somewhere else. https://japanesecontrol86419.mybuzzblog.com/17872377/the-definitive-guide-to-japanese-control