Japanese Knotweed originates in Japan and was introduced to Britain as an ornamental plant in parks and gardens. It is very persistent and even small discarded fragments of root can re-establish themselves in the wild. It is mainly a problem in urban areas where it causes damage to infrastructure but can be spread along rivers during flooding when parts of roots are transported downstream. It forms dense thickets which can damage native species and may block routes used by animals for dispersal. It can also reduce the holding capacity of channels for floodwater. There have been a small number of cases recorded in the mid to upper catchment of the River Thame.