Polygonum is a genus in the buckwheat family Polygonaceae. Common names of polygonum species include knotweed, knotgrass, bistort, tear-thumb, mile-a-minute, and several others. In the Middle English glossary of herbs "Alphita" (ca. 1400-1425), it was known as ars-smerte. There have been various opinions about how broadly the genus should be defined. Buckwheat for example has sometimes been included in the genus.
The genus primarily grows in northern temperate regions. They vary widely from prostrate herbaceous annual plants under 5 cm high, others erect herbaceous perennial plants growing to 3–4 m tall, and yet others perennial woody vines growing to 20–30 m high in trees; several are also aquatic, growing as floating plants in ponds. The smooth-edged leaves vary greatly in shape between species, and can be narrow lanceolate, oval, broad triangular, heart shaped or arrowhead shaped; they range from 1–30 cm long. The stems are often reddish or red-speckled. The flowers are small, pink, white, or greenish, forming in summer in dense clusters from the leaf joints or stem apices.
Polygonum species are used as food plants by the larvae of some Lepidoptera species - see list of Lepidoptera that feed on Polygonum, a number are considered weedy, especially in moisture retentive soils in the USA.
At least one species has been eaten during times of famine.
The genus Polygonella has a number of morphological similarities with Polygonum and some authors have included Polygonella in Polygonum.
Selected species include: