Bohemian Knotweed (Fallopia x bohemica)


Photographed September 9.

Father Pitt is about to make a botanical pronouncement that is likely to be controversial, and even likely to be wrong. He believes that most of the “Japanese knotweed” that has become such a plague around Pittsburgh is actually Bohemian Knotweed, a hybrid between Fallopia japonica and Fallopia sachalinensis. He has two mean reasons:

1. The leaves are usually intermediate between F. japonica and F. sachalinensis, with bases that are not quite heart-shaped like the leaves of F. sachalinensis, but more shovel-like than the leaves of F. japonica.

2. The rapid spread of the weed can only be accounted for by seed dispersal, and reliable authorities tell us that the hybrid is much more likely to produce viable seeds than either of its parents. Thus, once the hybrid gains a foothold in the area, it is likely to become the most common of the three in a very short time.

If any botanists out there can help him verify or correct this identification, Father Pitt would be very grateful.

These plants were growing along a street in Beechview, where shady woods did not discourage them at all.

Taxonomically the Japanese monster knotweeds are in a mess. The Flora of North America puts them in Fallopia. Wikipedia, following many current botanists, places them in Reynoutria, explaining, “As with many species in the family Polygonaceae, the taxonomic boundaries of Reynoutria have been much confused; in particular, it has been repeatedly merged with and separated from Fallopia.” The USDA PLANTS Database lumps everything together in the giant genus Polygonum. Father Pitt had to pick one of those possibilities almost at random.


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