This section is from the book "Wayside And Woodland Blossoms", by Edward Step. Also available from Amazon: Wayside And Woodland Blossoms: A Guide To British Wild-Flowers.
In the neighbourhood of manure-heaps and on the borders of cultivated ground one may come across this plant, which was formerly included in the British Flora, but is now known to be a mere waif of cultivation. Its home is in Central Asia, but it has been so long cultivated as a food-plant in Europe and in the United States that it has become naturalized in most places. In this country it is chiefly grown as a food for pheasants It is an annual, with a tall, slender, branched, reddish stem, and heart-shaped, almost arrow-headed leaves with entire margins. Flowers in panicles. The individual blossoms consist of five pale reddish sepals, no petals, eight stamens, and three styles. The flowers are of two forms, one with long stamens and short styles; the other with short stamens and long styles. The fruit is large, three-sided, solitary in a nut, very like beech-mast, whence its folk-name buck - or buck-wheat. It will be noted that at the base of the leaf-stalk is a pair of thin stipules, which sheathe the stem and mark the swollen nodes that give the knotted appearance so characteristic of the genus, and which has given it the name of many knees or joints (Greek polus and gonu). Buckwheat flowers during July and August. It is a valuable honey-plant, esteemed of bee-masters. There are a dozen British species; among them:
I. Bistort or Snake-root (P. bisiorta). Perennial, with large twisted rootstock, Radical leaves long, egg-shaped, the upper part of the leaf-stalk winged. Stem-leaves almost stalkless, broader near the stem. Flowers pink or white, producing honey; moist meadows. June to September.
II. Amphibious Buckwheat (P. amphibium). Perennial, rootstock sometimes creeping in the ground, at others floating in the water. If the plant is floating the leaves have long stalks; if growing on land they are almost stalkless. Stipules tubular, large, smooth in water, bristly on land. Stamens five, styles two. Flowers, rosy-red. July and August; margins of pools and in other wet places.
III. Spotted Knotweed (P. persicaria). Annual. Stem erect; leaves long, narrowly lance-shaped, with a black heart-shaped patch in the centre, downy beneath; the stipules fringed with a few long hairs. Flowers flesh-coloured ; stamens six, styles two. July to October, in moist places.
IV. Knotgrass (P. aviculare). Annual. Stems branching from the root, very slender and straggling, smooth. The leaves small and grassy, stipules small, white, torn-looking, red at the base. Flowers very small in the axils, pink. Stamens eight, styles three. Waste places and neglected gardens. May till October. The seeds are much esteemed by birds, and to the entomologist the fresh plant is invaluable as an almost universal food for the caterpillars of geometers.
V. Black Bindweed (P. convolvulus). Annual, with twining stems. The leaves are very similar to those of the true Convolvulus, the lobes more pointed; stipules short. Sepals green, with paler margins. Fields and wastes. July to September.