A 5 year, in-situ, herbicide management plan is essentially a ‘control’ method which puts Japanese Knotweed into dormancy. As the rhizomes are not being removed from the soil, if the soil were to be disturbed, the Japanese Knotweed would become viable and start to grow again. Disturbing the ground could also fragment the rhizome system, leading to spread and regrowth of the Japanese Knotweed.
Always get proper factual information from a qualified, certificated and professional firm. Japanese Knotweed Agency can help assess your situation and put you in contact with a panel firm that knows their onions, (and Japanese Knotweed too!).