Polygonum aviculare
Polygonum aviculare or common knotgrass is a plant related to buckwheat and dock. It is also called prostrate knotweed, birdweed, pigweed and lowgrass. It is an annual found in fields and wasteland, with white flowers from June to October. It is widespread across many countries in temperate regions, apparently native to Eurasia and North America, naturalized in temperate parts of the Southern Hemisphere.

Description 
Common knotgrass is an annual herb with a semi-erect stem that may grow from  high.  The leaves are hairless and short-stalked. They are longish-elliptical with short stalks and rounded bases; the upper ones are few and are linear and stalkless. The stipules are fused into a stem-enclosing, translucent sheath known as an ochrea that is membranous and silvery. The flowers are regular, green with white or pink margins. Each has five perianth segments, overlapping at the base, five to eight stamens and three fused carpels. The fruit is a dark brown, three-edged nut. The seeds need light to germinate which is why this plant appears in disturbed soil in locations where its seeds may have lain dormant for years.

Subspecies 
Polygonum aviculare has a wide distribution as an arable weed and plant of fields, shingle, sand, roadsides, yards and waste places. There is much morphological variation among different populations and several different sub-species are recognized:
 Polygonum aviculare subsp. aviculare – very widespread
Polygonum aviculare subsp. boreale (Lange) Karlsson – Greenland, Labrador, Newfoundland, Scandinavia
Polygonum aviculare subsp. buxiforme (Small) Costea & Tardif – North America
 Polygonum aviculare subsp. depressum (Meisn.) Arcang. – Europe, North America
Polygonum aviculare var. fusco-ochreatum (Kom.) A.J.Li – northeastern China, Russian Far East
Polygonum aviculare subsp. neglectum (Besser) Arcangeli – Europe, North America
 Polygonum aviculare subsp. rurivagum (Jord. ex Boreau) Berher  – Europe, North America

Distribution
Widespread and common in Great Britain, Ireland, and Scandinavia.

Habitat
Common on roadsides and arable ground in the British Isles.

Chemistry 
Polygonum aviculare contains the flavonols avicularin, myricitrin, juglanin, astragalin, betmidin and the lignan aviculin.

Fossil record
One fossil fruit of Polygonum aviculare has been extracted from borehole samples of the Middle Miocene fresh water deposits in Nowy Sacz Basin, West Carpathians, Poland.

Cuisine

It formed a traditional ingredient in porridge consumed by Germanic peoples of western Europe, and has been found in numerous autopsies of peat bodies, including the Tollund Man. 

In Vietnam, where it is called rau đắng, it is widely used to prepare soup and hot pot, particularly in the southern region.