Hybrids and related species
Giant Knotweed (Fallopia sachalinensis) occurs in Devon. Both the leaves and the stems are much larger than Japanese Knotweed with stands growing to 5 metres (Japanese Knotweed grows to 3 metres). The undersides of the leaves have scattered stiff hairs.
Himalayan Knotweed (Polygonum polystachum) also occurs in Devon, it has slightly hairy stems, the leaves are narrower and stands are not as tall as Japanese Knotweed, growing to around 1.8 metres.
There is also a hybrid between Japanese Knotweed and Giant Knotweed called Fallopia x bohemica which occurs in Devon. This hybrid is particularly worrying as it may be capable of producing viable seed. Stands grow to 2.5 – 4 metres. Leaves are intermediate between Japanese and Giant Knotweed and have scattered stiff hairs on the undersides.
There are other recorded hybrids and the resulting plants have a combination of characteristics making identification ambiguous.