P. aviculare: annual; stems often 1-2 feet long; stipules white, scarious, becoming ragged at the edges; leaves small, narrow-oblong; flowers small, shortly stalked, clustered in the axils of the leaves. - Knotgrass. - Cultivated and waste places. Fl. May to September.
P. maritimum is distinguished by its thicker stems, larger thicker and more glaucous leaves, larger scarious stipules, and larger flowers.
P. amphibium: stems usually floating in water, rooting at the lower nodes; leaves oblong or lanceolate, spreading on the surface; spikes terminal, solitary, supported on short peduncles above the water, dense, cylindrical, rose-red. - Ponds and watery ditches. Fl. June to September. On dry ground it becomes a prostrate weed.
** Stems twining.
P. Convolvulus: annual; stems glabrous; stipules short; leaves stalked, heart-shaped or broadly sagittate, pointed; flowers in little loose clusters, the lower axillary, the upper forming loose irregular, terminal racemes. - Cultivated and waste places. Fl. July to September.
P. dumetorum is more luxuriant, and the three angles of the fruiting perianth are more or less expanded into white scarious wings.
*** Stems erect or ascending.
P. Bistorta: leaves long-stalked, ovate, subcordate; stems simple, erect, 1-2 feet high, terminating in a dense, oblong, or cylindrical spike of pretty pink flowers. - Bistort or Snakeweed. - Moist pastures and meadows. Fl. June.
P. Persicaria: annual; stems erect or spreading, branched, glabrous, 1-2 feet high; leaves lanceolate, often marked in the centre with a dark spot; stipules more or less fringed at the top with short fine bristles; spikes terminal, rather numerous, oblong or cylindrical, dense, the flowers reddish, or sometimes green. - Roadsides and waste places. Fl. June to September.
P. lapathifolium, which closely resembles this, is distinguished by the pedicels and perianths being dotted with small prominent glands, the plant being usually pale-green, and the stipules seldom fringed. - Waste damp places.
P. Hydropiper: annual; stems 1-2 feet high, often decumbent at the base; stipules fringed; leaves lanceolate, wavy; flowers in slender, drooping spikes, the clusters of flowers almost all distinct, the perianths dotted with small glands. - Ayater-pepper. - Wet ditches and edges of streams. Fl. August, September.