Plants often purplish tinged, homophyllous, rarely subsuc-culent. Stems prostrate, 5-30 cm. Leaves: ocrea 2.5-5 mm; petiole 2-5 mm; blade purple tinged, narrowly obovate or oblanceolate, 8-30 × 4-12(-15) mm, apex rounded to obtuse; middle stem leaves 1.1-1.8(-2.2) times as long as adjacent branch leaves. Cymes 2-7(-10)-flowered. Pedicels 1-1.5 mm. Perianths (2.2-)2.5-3.3(-3.5) mm. Achenes ovate-lanceolate, (2-)3-gonous, (1.8-)2-2.5(-3.1) mm, faces often unequal, ± flat to concave, roughened, rarely obscurely tubercled.

Flowering Jul-Sep. Gravelly seashores; 0-30 m; Man., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., Que..

Wolf and McNeill described Hudsonian knotweed as a subspecies of Polygonum caurianum (P. humifusum subsp. caurianum in this treatment). In raising it to the rank of species, H. R. Hinds (1995) noted that 'separation between P. fowleri and P. hudsonianum, however, is more difficult.' M. Costea and F. J. Tardif (2003) concluded that the taxon is best treated as a subspecies of P. fowleri, based on the existence of intermediate plants.