buckwheat, common name for certain members of the Polygonaceae, a family of herbs and shrubs found chiefly in north temperate areas and having a characteristic pungent juice containing oxalic acid. Species native to the United States are most common in the West. The largest genus of the family, Polygonum (or Persicaria), contains the knotweeds and the smartweeds, found in many parts of the world. The common smartweed (P. hydropiper) is an annual sometimes called water pepper for its acrid quality. Some knotweeds are in the genus Fallopia, such as the Japanese knotweed (F. japonica), a bamboolike perennial that can exceed 15 ft (4.5 m) in height; introduced to the United States as a landscape plant, it has become an invasive species. Several species of the dock genus (Rumex) are sorrels (the common name used also for the similarly acrid but unrelated oxalis). The garden, or green, sorrel (R. acetosa) and the sheep, red, or field sorrel (R. acetosella) have long been used in Europe for salads and greens. Among the plants used as potherbs are the patience or spinach dock (R. patientia) and the tanner's dock (R. hymenosepalus); the latter is the source of canaigre, a substance used for tanning. Economically the important members of the family are of the rhubarb genus (Rheum) and the buckwheat genus (Fagopyrum), both native to Asia. Most of the rhubarb cultivated for the edible thick, fleshy leafstalks is R. rhaponticum, called also pieplant and wine plant. Medicinal rhubarb is obtained from this and other species of the genus. The cultivated buckwheat (F. esculentum) has been grown in the Old World since the Middle Ages as a honey plant and for its characteristic three-cornered grain, which is utilized for poultry and stock feed. Buckwheat flour is used in the United States, Japan, and eastern Europe; the plant is sown as a cover crop and is a food staple. The genus Eriogonum includes the wild, or yellow, buckwheat (E. alleni), restricted to the Appalachian shale barrens, and many Western species, e.g., the desert trumpet (E. inflatum), a desert flower of arid plains and plateaus. The interesting genus Koenigia has only one species, but it is found in the Arctic, in the Himalayas, and in Tierra del Fuego. Buckwheat is classified in the division Magnoliophyta, class Magnoliopsida, order Polygonales, family Polygonaceae.

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