Since Fallopia japonica var. japonica in Britain is exclusively female, the only way it can produce seed is by hybridisation with related taxa. This has happened to a great extent, and F. japonica var. japonica will hybridise readily with F. sachalinensis and the dwarf variety F. japonica var. compacta. However, outside of gardens, F japonica is only very rarely in contact with male-fertile Knotweeds and though the the hybrid F. x bohemica (japonica x sachalinsnis) is a very significant component of the Knotweed community - most of it has probably been produced and distributed by horticulturalists. In the wild Japanese Knotweed is generally pollinated by the hermaphrodite Russian Vine (Fallopia baldschuanica) and to a lesser by F. x bohemica.
Although most of the hybrids found so far have only been found as seed on open-pollinated plants in the wild, we do not understand the reasons for the poor rate of seed germination in the wild. This seed is usually very viable and present in considerable amounts. The figure shows the extent of hybridisation occurring between these taxa, but excludes the back-crossing that we know occurs.
Bailey (1999) The Japanese Knotweed invasion of Europe; the potential for further evolution in non-native regions
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