Japanese knotweed (Fallopia japonica) is not the only knotweed. In fact, there is both a dwarf variety (F. Japonica var. compacta) and a separate species - Giant knotweed (Fallopia sachalinensis).
In addition there are some established hybrids: One is a cross between Giant and dwarf knotweed; and the other, a cross between Giant and standard Japanese knotweed. Both of these hybrids are called Fallopia x bohemica. There is also the rare hybrid between Japanese knotweed and Russian vine known as F. x conollyana.
The figure below shows the known extent of hybridisation. Many of these hybrids are only known about from the seeds collected from knotweed plants. So hybrids are being produced but for some reason are not generally becoming established. Any large knotweed plant that is “male” and is not Giant knotweed (F. sachalinenisis) is likely to be the hybrid F.x bohemica. This can be confirmed by an examination of the underside of the larger leaves, where a good hand-lens or low power microscope will reveal numerous short pointed hairs.
Leicester University keeps records of all known knotweed hybrids and would greatly appreciate reports of suspected hybrids. To help their research, Leicester University would like you to send a dried leaf sample of the suspected hybrid and where possible, flowers from the plant. To do this, you can take a single large leaf and small piece of the flowering structure and simply place it inside a telephone directory for a couple of days.
Please send any specimens and details to Dr John Bailey, Biology Department, University of Leicester, LEICESTER LE1 7RH