Polygonum arenastrum Boreau, Equal-leaved Knotgrass
Account Summary
Introduction, archaeophyte, locally frequent. Eurasian wide-temperate, but also widely naturalised.
1937; Praeger, R.Ll.; Co Fermanagh.
June to December.
Growth form, identification and preferred habitats
This small stress-tolerant ruderal summer annual is similar to P. aviculare s.s. (Knotgrass), but it is always prostrate or procumbent, rather than erect. It has a strong, deep taproot sometimes reaching down to around 40 cm, and its wiry stem is often much branched, allowing it to form a dense surface-spreading mat (Salisbury 1964). P. arenastrum has evenly-sized leaves, whereas those of P. aviculare s.s. noticeably decrease in size towards the tip of the shoot. Both knotweed species have a long flowering period stretching from July to November and both are mobile, well-adapted, pioneer colonists of cultivated and disturbed open habitats, occupying a wide range of moderate soils of acid to neutral reaction (pH usually above 5.0 according to Grime et al. 1988). They both show preference for moderately fertile, damp to dry, often winter-wet sites and full sun situations. Unlike many other annuals, both of them have the ability to recover after early season damage, including grazing pressure and trampling.
P. arenastrum is typically found in dry or well-drained, often sandy, gravelly or stony soils in wayside and waste ground habitats which are open and disturbed. It can survive, and indeed even thrive, in very compacted, heavily trampled surfaces, eg on paths, around the sides of yards, in gateways, between paving stones and in cracks in concrete or asphalt, often in droughted, dwarfing conditions which P. aviculare s.s. simply could not tolerate (Lousley & Kent 1981; Sell & Murrell 2018).
Both Knotgrass species also commonly occur as successful weeds of arable agriculture and horticulture, preferring fertile, damp, loose, recently disturbed soils, newly sown grassland, or disturbed or overgrazed meadows and pastures where patches of bare earth are present. They may also be present together in much more ruderal, artificial, man-made habitats, eg in waste ground, rubbish tips and along the kerbsides of busy roads, tolerating the spray and slipstream buffeting provided by passing road traffic. Presence of these two species in coastal situations and along roadside kerbs immediately adjacent to metalled surfaces is strong evidence of their salt-tolerance (Grime et al. 1988).
Reproduction
Polygonum species are typically inbreeding, although in Scandinavian countries the flowers are often wide open, and at least the larger flowered species are occasionally visited by flies and other small insects so could from time to time be cross-pollinated (Jonsell et al. 2000). Normally, however, seed production in all species of the genus is assured by habitual self-fertilization, and a robust plant can produce up to 1,000 relatively large seed. The seeds (often referred to as nuts or achenes), are dispersed by both man and animals, eg in mud, with harvested crops, as an impurity of agricultural seed, or they may be ingested and transported internally by both domestic stock and wild birds (Ridley 1930).
The flowers of P. arenastrum are borne in 3-7 flowered cymes on the upper parts of stems and branches from July to September. The perianth is 2-4 mm, the five tepals laterally overlapping, but forced apart as the fruit grows. The tepals are green with wide white margins and they usually contain the nut fully, although the tip may be visible. The three-sided or biconvex nuts, 2.5-3.0 mm, are blackish brown, the faces almost smooth or coarsely papillose.
Variation
The high degree of inbreeding found in this genus leads to the formation of numerous infraspecific races and ecotypes within it. In taxonomic terms, the number of useful morphological identification characters is limited, and when this is combined with polyploidy and great plasticity, the taxonomic limits between forms are often obscured (Jonsell et al. 2000).
Fermanagh occurrence
In Fermanagh, P. arenastrum is widely but thinly scattered throughout lowland areas, having been recorded in 81 tetrads, 15.3% of those in the VC.
Irish occurrence
The New Atlas hectad map shows P. arenastrum is widespread throughout most of Ireland, particularly in the N and in the region around Dublin plus in the extreme S, but more patchily dispersed elsewhere.
Status in Britain and Ireland
The editors of the New Atlas regard P. arenastrum as an ancient introduction (an archaeophyte), while they take the ecologically very similar P. aviculare s.s. to be indigenous. It is not obvious why they make a status distinction between the two. The fossil diagram and associated text for P. aviculare s.l. in Godwin (1975), indicates that a gap occurs in the fossil record in the current interglacial period (called the Flandrian in Britain and the Littletonian in Ireland), until a sudden expansion of fossils (both fruits and pollen) occurs in close association with archaeological situations in the Bronze Age. In a paper subsequent to publication of the New Atlas, Preston et al. (2004) remark that the authors of most accounts of the fossil record do not distinguish between the segregates of P. aviculare (ie palaeoecologists like Godwin treat it as sensu lato). Nevertheless, Preston et al. (2004) single out P. arenastrum and label it an ancient introduction. Both these species, and P. aviculare s.s. in particular, have a long history as common cornfield and broadleaf crop weeds (Styles 1962). The current author (Ralph Forbes) considers them as equally probable ancient archaeophytes which accidentally and repeatedly have been transported to these islands amongst crop seed by farmers from the Neolithic period or the Bronze Age, onwards.
For comparison, in the critical account of these taxa given in Flora Nordica, Jonsell et al. (2000) treat these two segregates of P. aviculare s.l. as subspecies. Subsp. aviculare is regarded as indigenous on the coasts of Denmark, S Norway and W & S Sweden, while inland it is closely associated with man and is considered a probable archaeophyte, except in the north. The status of P. arenastrum (as subsp. microspermum (Jord. ex Boreau) Berher) is given as, "anthropochorus in Norden" (ie associated with man throughout the whole survey area), and "probably archaeophytic, except in the north." (Jonsell et al. 2000).
European and world occurrence
The distribution is imperfectly known, but P. arenastrum is regarded as probably common throughout continental Europe, extending well into northern regions of Scandinavia where it is an archaeophyte (Jalas & Suominen 1979, but not mapped; Jonsell et al. 2000). It is fairly common as an established alien in N America. Hultén & Fries (1986) map the Polygonum aviculare complex of around 50 taxa as circumpolar and very widely introduced worldwide (Map 645).
Names
The genus name 'Polygonum' is a combination of two Greek words, 'polys', many or much, and either 'gonos', meaning offspring or seed, an allusion to the numerous seeds characteristic of the plants, or 'gony', knee-joint, an allusion to the swollen joints of the stems. The Latin specific epithet 'arenastrum', means 'resembling Arenaria', from 'arena', 'sand', an allusion to sandy places where many of these species prefer to grow (Stearn 1992).
Threats
None.