Japanese Knotweed is an invasive non-native plant that forms thick, dense colonies that outcompete any other herbaceous species and is listed by the World Conservation Union as one of the world’s 100 worst invasive species. It also has the ability to grow through tarmac and concrete (in some cases within dwellings) and therefore must be cleared completely before starting to build or lay roads.
Japanese knotweed was intentionally introduced as an ornamental plant. The date of first introduction to Ireland is not know for certain, but it is believed that this plant arrived in the mid to late 1800’s. A native of Japan, Korea, Taiwan and China where both male and female plants are known, the species is now widespread in continental Europe and Britain but only female plants have been recorded to date.
Japanese Knotweed is capable of growing in a wide range of conditions. The success of the species has been partially attributed to its tolerance of a very wide range of soil types, pH and salinity. Its rhizomes can survive temperatures of −35C and can extend 7 metres horizontally and 3 metres deep, making removal by excavation extremely difficult. It is found near water sources, such as along river banks, low-lying and disturbed areas. It can also colonize coastal shores and islands.
It is a relatively large plant that can grow up to 2 – 3 m in height. It can form an extensive network of rhizomes (roots) which cause problems when managing this species. Small pieces of rhizomes are capable of rejuvenating the plant. Knotweed is known to out-compete native riparian vegetation which normally provide food and cover for fish and other aquatic species. The plant dies back in the autumn period exposing soil to erosion which is damaging to spawning fish (lamprey and salmon).
The leaves are shield or heart shaped, usually with a pale stripe down the middle. Flowers are creamy and arise from the tips of stems. Underground rhizomes are thick and woody with a knotty appearance, which when broken reveal a bright orange coloured centre.