The Polygonaceae are a family of flowering plants known informally as the knotweed family or smartweed—buckwheat family in the United States. The name is based on the genus Polygonum, and was first used by Antoine Laurent de Jussieu in 1789 in his book, Genera Plantarum. The name refers to the many swollen nodes the stems of some species have. It is derived from Greek; poly means many and goni means knee or joint.
The Polygonaceae comprise about 1200 species distributed into about 48 genera. The largest genera are Eriogonum (240 species), Rumex (200 species), Coccoloba (120 species), Persicaria (100 species) and Calligonum (80 species). The family is present worldwide, but is most diverse in the North Temperate Zone.
A few species of Triplaris provide lumber. The fruit of the sea grape (Coccoloba uvifera) is eaten, and in Florida, jelly is made from it and sold commercially. The seeds of two species of Fagopyrum, known as buckwheat (sarrasin in French), provide grain (its dark flour is known as blé noir (black wheat) in France). The petioles of rhubarb (Rheum rhabarbarum and hybrids) are a food item. The leaves of the common sorrel (Rumex acetosa) are eaten in salads or as a leaf vegetable. Polygonaceae contain some of the worst weeds, including species of Persicaria, Emex, Rumex, and Polygonum, such as Japanese knotweed.
Most Polygonaceae are perennial herbaceous plants with swollen nodes, but trees, shrubs and vines are also present. The leaves of Polygonaceae are simple, and arranged alternately on the stems. Each leaf has a peculiar pair of fused, sheathing stipules known as an ochrea. Those species that do not have the nodal ocrea can be identified by their possession of involucrate flower heads. The flowers are normally bisexual, small, and actinomorphic, with a perianth of three to six sepals. After flowering, the sepals often become thickened and enlarged around the developing fruit. Flowers lack a corolla and in some, the sepals are petal-like and colorful. The androecium is composed of three to eight stamens that are normally free or united at the base. The ovary consists of three united carpels that form a single locule, which produces only one ovule. The ovary is superior with basal or free-central placentation. The gynoecium terminates in 1 to 3 styles, each of which ends in a single stigma.