Hybridization between two introduced plant species can influence the invasion capabilities of the exotic taxa, but the role of hybridization will likely differ in different invasions, even of the same species. Until now, studies concerning the ploidy of Japanese knotweed, giant knotweed, and their hybrids have been conducted in Europe or native ranges in Asia. Here, we assess the role of hybridization and ploidy in a U.S. invasion. We use flow cytometry to characterize DNA content in (1) established families in a common garden, (2) seedlings grown from common garden parents, and (3) wild populations. We also measured fertility in the garden and the field and vegetative growth traits in the garden. Although the majority of our parental and hybrid samples had ploidy levels previously documented in Europe (4X and 8X for parental species; 6X for hybrids), we found a wider range of knotweed cytotypes established in our garden (4X, 6X, 7X, 8X, 9X, and 10X) and additionally detected 5X, 11X, 12X, and possibly 14X ploidy levels in progeny from garden seed parents. The unexpected cytotypes were not confined to the greenhouse or common garden, in that all < 11X ploidy levels were also found in field populations in Massachusetts. In several cases, these data contradicted our expectations on the basis of morphological and molecular analysis, suggesting both significant introgression and the introduction of multiple cytotypes from Asia. With one exception (14X), we found all cytotypes were capable of strong vegetative growth, seed set, and the production of viable pollen. Without barriers to sexual reproduction, introgression is expected to progress, creating a progressively more diverse swarm of invasive genotypes.

Nomenclature: Japanese knotweed, Fallopia japonica (Houtt.) Ronse Decraene giant knotweed, Fallopia sachalinensis (F. Schmidt ex Maxim.) Ronse Decraenep; Fallopia × bohemica (Chrtek & Chrtková) J. P. Bailey