Japanese knotweed (Fallopia japonica) growing rapidly after being cut back. Japanese knotweed is a tough, highly invasive perennial, that was introduced to this country by the Victorians. It grows to two to three metres high every spring, often crowding out native species, its roots grow at least as deep. 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. It is now illegal to plant Japanese knotweed in the UK.