Japanese Knotweed
Japanese Knotweed - Fallopia japonica - is an introduced weed which is rapidly colonising river banks and areas of waste land. It is a perennial plant which grows from rhizomes in the spring to a height of about 3m in mid summer with stiff, bamboo-like stems. In the Autumn, the leaves and stems die but remain stiff and erect. The plant produces large masses of white flowers in summer but it is not thought to produce viable seeds. The plant overwinters in the form of rhizomes which grow rapidly and have been known to grow through cracks in rock, concrete and tarmac. Cut or broken stems lying on damp soil can root and form new plants. The spread of this plant to new sites may be caused by the transport of contaminated topsoil because fragments of rhizome as small as 1 gram can produce new plants. Spread along river banks may also occur when the shoots are cut and allowed to float downstream and are washed onto damp soil beside the river. Japanese Knotweed forms dense stands which severely impede access to river banks and shade out native species leaving the banks bare and liable to erosion in winter. The rhizomes can penetrate, damage and displace stone and concrete embankments and structures.
Control
Mechanical control - DO NOT FLAIL MOW this plant. Cutting the shoots in spring or summer has no long-term control and will encourage
the spread of this weed unless the shoots are collected and burnt. The rhizomes can survive for many years, even if shoots are cut regularly.
Transport of soil contaminated with this plant should be avoided and green shoots should not be
cut and allowed to float away from any infected area where engineering works are being
undertaken but should be collected, dried and burnt.
Sieving or screening of excavated topsoil on site through a 1-2 cm mesh may alleviate the problem
of contaminated spoil, and will allow re-use of the excavated soil on site.
Complete excavation of all the root system under expert supervision can be provided by our qualified team.
Chemical control - The eradication of this plant with the use of approved herbicides is a specialist operation. Aquatic Solutions UK can provide such a service with our qualified operators.
Biological control - None known.
Environmental control - Once the plant has become established, there is no environmental method of controlling it. However, the plant appears to be spread in two ways. First by the importation of contaminated soil which contains fragments of rhizome and it is advisable to ensure that any soil imported to a site is free of contamination. Secondly, the plant is spread by cut or broken stems floating downstream and washing onto bare muddy banks. These fragments are less likely to be able to establish on banks protected by a fringe of reed, covered by a dense grass sward which will hold the stems above the soil surface and allow them to dry out, or banks which have hard surfaces of stone, concrete or steel shuttering.
On river banks, seeds may be washed downstream from colonies along the river bank. Any attempt to eradicate this plant is unlikely to succeed unless control is exercised along the whole river system.