I found few stems of this plant, which seems to be rare in Southern Germany, at the edge of a maize field at Weinheim an der Bergstraße, Southern Germany. It is a rather dry site, while wet sites like river sides are mentioned as more typical in literature.
Some publications like the "Farn- und Blütenpflanzen Baden-Württembergs" (i.e. Ferns and flowering plants of Baden-Württemberg, South-Western Germany, volume 1) treat it as own species, while other publications e.g. FloraWeb are treating it as subspecies of Persicaria lapathifolia (Polygonum lapathifolium) .