Fallopia x bohemica

This is the hybrid produced by crosses between F. japonica and F. sachalinensis. To understand what is going on, it is important to know something about the cytology of these taxa.  F. sachalinensis has 44 chromosomes, F. japonica var. japonica 88, and F. japonica var. compacta 44.  As F. japonica var. japonica occurs only as a female clone, it is only possible for it to act as the female parent in crosses with F. sachalinensis which give the hexaploid (2n=66) cytotype of F. x bohemica.  Both var. compacta and F. sachalinensis occur as both sexes, so each can act as the male or female parent to produce the tetraploid F. x bohemica.  It can be seen that there are different ratios of japonica:sachalinensis chromosomes in these two types of F. x bohemica, though I have yet to find a method of distinguishing the two chromosome levels by morphological means.

F. x bohemica is an important and increasing component of the Japanese Knotweed populations in Europe. The hexaploid is the one most frequently found in Europe the tetraploids are much more restricted, and mention should also be made to the very rare octoploid plants from Wales and parts of continetal Europe.  There is every indication that F. x bohemica is more vigorous and persistant than either of its parents, and since it is often male-fertile it is often able to backcross with either of its parents.

The current distribution of this taxon is much greater than can be explained by hybridisation and seed establishment in the wild, and it is thought that the hybrid may well have been distributed by commercial nurseries under the name of F. japonica.

This information and much more is available in Bailey et al 1996 

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