Warning as Japanese knotweed 'cluster' discovered in Plymouth Environment experts are urging local people to seek professional advice if they spot the plant on their property Updated Invasive and devastating perennial plant Japanese knotweed has been spotted in Plymouth, environment experts say. The plant, often labelled as one of the UK's most invasive plant species, has spread rapidly across Devon and the wider South West in recent years and can cause a serious problem for roadsides and riverbanks and increasingly, gardens and residential areas. While the plant is not harmful to humans, unlike Giant Hogweed, Japanese knotweed is seen as dangerous as it can cause significant damage to its surrounding environment, which can be very expensive to clear up.