Japanese knotweed (Fallopia japonica, syn. Reynoutria japonica) foliage. This plant was introduced to Europe from Asia in the mid-19th century as an ornamental and fodder plant. It has since become an invasive species in the UK. The plant has an extensive rhizome (system that can reach a depth of 3 metres beneath the ground. Very tiny pieces of rhizome are capable of producing roots. In Europe, the plants are all female and so reproduction is asexual.