Japanese knotweed (Fallopia japonica). Japanese knotweed is a tough, highly invasive perennial, that was introduced to this country by the Victorians. It is incredibly hard to kill, as less than one gram of living rhizome, a horizontal underground stem, is needed to regenerate a plant. There are few chemicals that can destroy the plant, and biological control, the use of introduced fungi or insects, is now being considered in the UK. Japanese knotweed grows to two to three metres high every spring, often crowding out native species as it competes with them for light, its root grow at least as deep. It is now illegal to plant this species in the UK.