Japanese knotweed

Japanese knotweed (Fallopia japonica) belongs to the knotweed family (Polygonaceae) and is native to large parts of mainland China. The plant is a fast grower and spreads rapidly on nutrient-rich soils displacing existing vegetation. In urban environments, the aggressively growing roots of Japanese knotweed pose a threat to foundations and sewers. Japanese knotweed is therefore an invasive species. The species has spread across Europe from an imported specimen in the Hortus botanicus in Leiden. In winter, the plant dies off above ground. In March and April, new stems with light green leaves appear among the withered stems of the previous year. The stems are formed by hollow stem segments and are pliant. In August and September, Japanese knotweed blooms with small white flower spikes.

Location
Japanese knotweed occurs in nutrient-rich, humous, somewhat moist soils, usually in sunny to slightly shaded areas. Japanese knotweed is found at forest edges, along railroad embankments, in roadsides and along highway noise barriers, wasteland and gardens.

Control
Japanese knotweed are not attacked by insects and fungi in Europe; there are no natural enemies. Because of its vigor, the plant is difficult to control.
Effective is digging out the rhizomes where the stems have to be repeatedly cut off. This exhausts the Japanese knotweed to the point that it will eventually die. The plant remains should not be added to the kerbside garden / food waste bin to prevent spreading: each piece of rhizome can form a new plant. It can take up to five years for the plant to disappear completely; therefore, control is a long-term process.
As an experimental biological control agent, the Japanese knotweed psyllid (Aphalara itadori)—a non-native insect species—was introduced into the United Kingdom in 2010 to control Japanese knotweed. The first releases took place in southern England.