Despite the fact that Japanese knotweed is a garden pest, there is no legal obligation to monitor, eliminate, or even report it. Planting or causing Japanese knotweed to grow in the wild is, however, illegal. You could face fines of up to £5,000 as well as a two-year jail term if you do so. Furthermore, allowing Japanese knotweed to spread to private lands, such as your neighbour's garden, can result in legal action against you.