Polygonum

Polygonum is a genus in the Polygonaceae family. Common names 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. For example, Buckwheat 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 aquatic, growing as floating plants in ponds. The smooth-edged leaves range from 1–30 cm long, and vary in shape between species from narrow lanceolate to oval, broad triangular, heart-shaped, or arrowhead forms. The stems are often reddish or red-speckled. The small flowers are, pink, white, or greenish, forming in summer in dense clusters from the leaf joints or stem apices.

The genus name is from the Greek poly, "many" and gonu, "knee" in reference to the swollen jointed stem.

Polygonum species are occasionally eaten by humans, and are used as food plants by the larvae of some Lepidoptera species - see list. Most species are considered weedy, especially in moist soils in the USA.