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, naturalized in temperate parts of the Southern Hemisphere.

Description
Common knotgrass is an annual herb with a semi-erect stem that may grow from 10 to 40 cm 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. It is a common carrier of the parasitic pathogen powdery mildew, which can give the leaves a whitish appearance.

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
It is common on roadsides and arable ground in the British Isles.

Chemistry
Polygonum aviculare contains the flavonols avicularin, myricitrin and juglanin. The flavanoids astragalin and betmidin, and the lignan aviculin have also been found. The diterpene alkaloid panicudine is another known component.

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.