Polygonum


Polygonum is a genus of about 130 species of flowering plant in the buckwheat and knotweed family Polygonaceae. Common names include knotweed and knotgrass. In the Middle English glossary of herbs Alphita, 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 as Polygonum fagopyrum. Former genera such as Polygonella have been subsumed into Polygonum; other genera have been split off.
The genus primarily grows in northern temperate regions. The species are very diverse, ranging from prostrate herbaceous annual plants to erect herbaceous perennial plants.
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.

Description

When the genus is defined narrowly, Polygonum species are annual or perennial herbaceous plants, rarely shrubby, with much branched stems. The leaves are arranged alternately, usually less than long, with a length greater than the width. They have a membranous ochrea. The flowers are usually bisexual, rarely unisexual, and have five tepals, the outer being slightly different from the inner ones. There are usually four to six stamens and three styles. The fruit is three-sided.

Taxonomy

The genus Polygonum was first described by Carl Linnaeus in 1754. The genus name is usually said to be from the Greek πολυ- and γόνυ, in reference to the swollen jointed stem. However, long before Linnaeus, the name was used in Greek and Latin for a plant used medically. Discussing the plant he knew as polygonum in 1655, Matthias Martinius referred to Scribonius Largus and gave an alternative etymology, based on γόνος, the meaning of the name then being the Latin foecundus, i.e. 'fecund', 'with many offspring'. The Flora of North America says that a derivation meaning 'many seeds' is the "grammatically correct interpretation".
Many members of the family Polygonaceae that are now placed in separate genera were at one time or other placed in Polygonum, including species of Fagopyrum, Fallopia, Persicaria and Reynoutria, and older sources frequently use much wider definitions of the genus. Molecular phylogenetic studies, particularly in the 21st century, have led to major changes. Clarifying the circumscription of genera split from Polygonum was described in 2015 as "still ongoing".

Classification and phylogeny

Polygonum is placed in the tribe Polygoneae of the subfamily Polygonoideae. Within the tribe, it is most closely related to the genera Duma and Atraphaxis, forming the so-called "DAP clade".

Species

Between 65 and 300 species have been recognised at various times, depending on the circumscription of the genus. A number of species that had been included in Polygonum have been moved into several other genera, including Bistorta, Fagopyrum, Fallopia, Koenigia, Persicaria and Reynoutria. Other genera, such as Polygonella, have been subsumed into Polygonum.
, Plants of the World Online accepted 129 species.
Many species formerly placed in Polygonum have been moved to other genera in the subfamily Polygonoideae. Some synonyms are listed below.

''Polygonum'' species that have been reclassified as ''Bistorta''

Several species can be eaten cooked, for example during famines. The species Polygonum cognatum, known locally as "madimak", is regularly consumed in central parts of Turkey.
Care should be taken not to confuse Polygonum with Polygonatum – an entirely different genus of plants.