Diario de frontyardscientist

Since all species of the genus like to hybridize so much, I suspect there exists an undescribed hybrid between R. compacta and R. japonica, with an expected ploidy level of 2n=66 chromosomes like R. x bohemica. As such they cannot not be reliably differentiated based on chromosome count without knowing their parents, and they probably can hybridize with each other as well. At least R. x moravica, the hybrid of R. sachalinensis and R. compacta, normally has a ploidy level of 2n=44 like its parents and differs from them morphologically, so is probably easier to identify

Also, this article on the history of knotweeds in North America features a scan of the earliest known herbarium specimen of "R. japonica" from North America, on page 11. To me, the leaves more closely resemble those of R. compacta than R. japonica, and the black-and-white scan obsfurcates the flowers' colour, so they may very well be pink or whitish pink. The same article includes quotes from various horticulturists that sold "R. japonica" in North America, and most mention "rose-colored fruits" that hint at R. compacta. My hypothesis is that R. compacta was introduced to North America around the same time as R. japonica

Ploidy levels of the various species and hybrids

R. japonica - 2n = 88 chromosomes
R. sachalinensis - 2n = 44 chromosomes
R. compacta - 2n = 44 chromosomes
R. x bohemica - 2n = 66 chromosome
R. compacta x japonica (undescribed) - 2n = 66 chromosomes
R. x moravica - 2n = 44 chromosomes

Key

1a. At least some cordate to hastate leaf bases on upper leaves of main and/or secondary stems; broadly acuminate, broadly acute and/or narrowly acute leaf apices; often, but not always, undulate leaf margins... 2
1b. No cordate leaf bases; hastate leaf bases only on the lowest leaves of main and/or secondary stems; short acuminate and/or cuspidate leaf apices... 4
2a. Leaves much longer than wide; only cordate leaf bases; unspotted glaucous stems; long abaxial trichomes... R. sachalinensis
2c. Leaves longer than wide; mix of cordate to hastate leaf bases and sagittate to cuneate leaf bases... 3
3a. Densely spotted main stems; inflorescences shorter than subtending leaves; short abaxial trichomes... R. x bohemica
3b. Sparsely spotted main stems; occasionally only entire leaf margins... R. x moravica or R. compacta x japonica (don't know yet)
4a. Leaves as long as wide; leaves orbicular to deltate; undulate leaf margins, often tinged with red; whitish pink to red flowers/flower buds... R. compacta
4b. Leaves longer than wide (rarely as long as wide); leaves rhomboid to deltate; entire leaf margins; white to cream flowers/flower buds; inflorescences as long or slightly shorter than subtending leaves; no abaxial trichomes... R. japonica