Narrowleaf Knotweed (Polygonum bellardii All.)

USDA PLANTS lumps several taxa, including the introduced species narrow-leaf knotweed (Polygonum rurivagum), and the introduced subspecies prostrate knotweed (Polygonum aviculare) ssp. neglectum (but not the other two subspecies of that species in our treatment) in with this species. As such USDA PLANTS reports the species by this name as occurring throughout the US and Canada, introduced in the US; we are not sure why they report it as native to Canada as our research suggested that P. rurivagum and P. aviculare ssp. neglectum are also introduced in Canada. We follow POWO's treatment which separates both of these species. Under this treatment, all but a few Massachusetts records belong to other species. Note that not only this species, but most related taxa are treated inconsistently in different sources.