Evidence of hybridisation between Himalayan knotweed and Dwarf Japanese knotweed (above). Plant on left is 'pure' Himalayan knotweed, whilst plant on right has developed shield and heart-shaped deformities on some leaves.
Hybridisation
DNA studies have shown that all the Japanese knotweed plants growing throughout Europe and the UK have been cloned from a single female plant imported sometime during the Nineteenth Century. Consequently all these plants are female and reproduce purely by vegetative means. However, they are not completely sterile, merely lacking the presence of a male plant to fertilise the seed. Knotweed species other than Fallopia japonica are present in both male and female forms in the UK, though, meaning they are all capable of producing seeds. This can lead to hybridisation, not only with each other, but also with Japanese knotweed.
Studies conducted in Belgium (reported by M S Tiébré, S Vanderhoven, L Saad and G Mahy in 2007) confirmed the sterility of Japanese knotweed, but noted the potential for the restoration of sexual reproduction through hybridisation. Japanese knotweed was discovered to be capable of producing viable seed when presented with the pollen of other species. Giant knotweed, ‘Bohemica’ and Russian vine all produce viable pollen, making them all potential sources of hybridisation with Japanese knotweed. Hybrids have also been discovered forming between Japanese knotweed and Dwarf Japanese knotweed. ‘Bohemica’ is itself a hybrid of Japanese knotweed and Giant knotweed, and a strain of ‘Bohemica’ has been discovered to be produced by the interaction of Giant knotweed and Dwarf Japanese knotweed.
Knotweed in its many forms has already taken over most of the UK. If these hybrids enable Japanese knotweed to produce long-lasting viable seeds then the already considerable scale of spread could see a dramatic increase.