Common knotgrass

Appearance

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.

Plants green, heterophyllous (often lacking larger proximal stem leaves at maturity). Stems 1-3, ascending or erect, sometimes decumbent, basal branches divar-icate, (10-)25-75(-100) cm. Leaves: ocrea (5.5-)6-10(-14) mm, proximal part cylindric, distal part silvery, hyaline, soon lacerate and disintegrating into fibers; petiole 1-6(-8) mm; blade green, lateral veins visible but not raised abaxially, elliptic to oblanceolate, 18-50(-60) × (6-)10-20 mm, 2-4.5 times as long as wide, apex acute or obtuse; stem leaves 1.4-4 times as long as branch leaves. Cymes aggregated at tips of stems and branches, 3-8-flowered. Pedicels mostly exserted from ocreae, 2-5 mm. Flowers: perianth (2.3-)2.8-4.7(-5) mm, 1.8-2.8 times as long as wide; tube (15-)20-37% of perianth length; tepals overlapping, green with pink, red, or white margins, oblong, flat (cucullate in fruit), outer tepals not pouched at base; veins branched, thickened; stamens 7-8. Achenes enclosed in or barely exserted from perianth, brown to dark brown, ovate, 3-gonous, (2.1-)2.7-3.7 mm, faces subequal, concave, apex straight, striate-tubercled; late-season achenes uncommon, 3.5-5 mm. 2n = 40, 60.

Naming

Polygonum aviculare var. eximium (Lindman) Ascherson & Graebner
Polygonum aviculare subsp. heterophyllum (Lindman)Ascherson & Graebner
Polygonum aviculare subsp. monspeliense (Thiébaut-de-Berneaud ex Persoon) Arcangeli
Polygonum aviculare var. vegetum Ledebour
Polygonum heterophyllum Lindman
Polygonum monspeliense Thiébaut-de-Berneaud ex Persoon
Polygonum aequale Lindm.
Polygonum arenastrum Jord. ex Boreau
Polygonum aviculare var. arenastrum (Jord. ex Boreau) Ruoy
Polygonum aviculare var. depressum Meisn.
Polygonum microspermum Jord. ex Boreau

''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'' Karlsson – Greenland, Labrador, Newfoundland, Scandinavia
''Polygonum aviculare'' subsp. ''buxiforme'' Costea & Tardif – North America
''Polygonum aviculare'' subsp. ''depressum'' Arcang. – Europe, North America
''Polygonum aviculare'' var. ''fusco-ochreatum'' A.J.Li – northeastern China, Russian Far East
''Polygonum aviculare'' subsp. ''neglectum'' Arcangeli – Europe, North America
''Polygonum aviculare ''subsp''. rurivagum'' Berher – Europe, North America

Distribution

Most subspecies are native to Europe, but has been extensively introduced elsewhere. It is now a widespread weed in nearly all the temperate regions of the world and, more locally, in warmer climates. Its range may still be extending. Its extension towards equatorial regions could be restrained by chilling requirements.

Polygonum aviculare subsp. aviculare is introduced into North America and recorded from St. Pierre and Miquelon; Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon; Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., La., Maine, Md., Mass., Mich., Minn., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., Wyo.

Habitat

Fields, uncultivated areas, waste places, roadsides, cracks in sidewalk.

Reproduction

Selfing annual

Evolution

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.