Japanese Knotweed is an invasive perennial plant. It may look attractive - but hiding behind the veneer is a tough, resilient, hard nosed invader that can cause considerable problems for those whose land is affected.
Japanese Knotweed is named under the Wildlife and Countryside Act 1981, Schedule 9; it is an offence "knowingly to introduce Japanese Knotweed into the Wild or to cause the spread either intentionally or accidentally".
Japanese Knotweed was originally introduced as an ornamental plant. However, now, it is an aggressive weed. It can thrive in a wide variety of habitats but essentially grows along riverbanks, footpaths, road verges and railway lines, graveyards, in gardens and on brownfield sites.
Description of Japanese Knotweed
Japanese Knotweed is a hardy, herbaceous, rhizomatous perennial. It can colonise a wide variety of habitats but requires high light environments. It can attain a height of up to 3 metres and its stems form dense cane-like clumps. The deep root system, or rhizomes, allows Japanese Knotweed to tap nutrients and water that other plants cannot access.
The leaves are bright green, shield or heart-shaped leaves, with a flat base. Their size can be up to 12 cm long. In September to October, little white flowers appear primarily at the top of the plant.
There is no male Japanese Knotweed plant in the UK and hence the female form never sets seed in the country. However, Japanese Knotweed has an amazing ability to regenerate from a tiny piece of rhizome, stem or even leaf material. Vegetative propagation, as it is known, allows Japanese Knotweed to spread very quickly especially in areas where land disturbance or plant material is moved around. Typically this means Japanese Knotweed has been able to colonise vast areas around rivers, railways, roads and brownfield site.
Impacts of Japanese Knotweed
Japanese Knotweed causes a range of problems including restriction of access along paths and riverbanks, blocking sight-lines on roads, reduction in land values and increased risk of flooding and erosion on riverbanks. It has adventitious roots and shoots that seek out and expand the tiniest of cracks and weaknesses causing damage to archaeological sites, hard surfaces, buildings and flood defence structures. It increases erosion when the bare ground is exposed during the winter and the risk of soil erosion and bank instability following removal of established stands in riparian areas.
Japanese Knotweed forms dense stands of stems that become impenetrable by other plants once well-established. The rapid growth of new shoots and leaves in the spring shades out any vegetation below, suppressing the growth of other plants, including established native species. It competitively excludes all other vegetation and often access to humans and animals. It has effects on soil decomposition, invertebrate soil communities, detritivorous insects and respective food chains. The monocultures that often form following Japanese Knotweed invasions contribute to reductions in native biodiversity
The construction industry has been hit hard by Japanese Knotweed. Due to the damage that it can cause to underground services, foundations and hard surfaces, there have been various planning conditions implemented to guarantee that developers deal with this contamination issue responsibly.