Knotgrass

Polygonum aviculare is a taxonomically controversial polyploid complex of selfing annuals. Although members of the complex have been considered inbreeders, they possess some structures that make cross pollination possible. Cleistogamous and chasmogamous flowers, heterostyly, protandry, and the capacity to secrete nectar suggest an ancestral mixed-mating system. Isoenzyme studies showed that the complex has an allopolyploid origin (P. Meerts et al. 1998) and has evolved as a swarm of inbreeding lines (“Jordanons”) (J. Gasquez et al. 1978). The six subspecies included here have been treated variously (T. Karlsson 2000; M. Costea and F. J. Tardif 2003). Complex intergradation patterns among them make their recognition at the species level impractical. Multivariate analysis and isoenzyme studies show that populations with intermediate characteristics may occur (Meerts et al. 1990, 1998). Except for subsp. boreale, which occurs in Greenland and Labrador, all subspecies are partially sympatric and their distributions have been influenced greatly by humans.

A cosmopolitan weed, usually number of sagregates from this highly variable taxon have been described as independent species on the basis of habit, heterophyllous and isophyllous leaves and the length of perianth tube. However, if large number of specimens are studied, the characters often break down in our region. We have presently maintained three species P. aviculare, P. arenastrum and P. olivascens on basis of habit and characters of leaves and nuts. Sometimes some intermediate are also found between P. aviculare and P.arenastrum. Experimental studies are needed to solve the problem.

Plants green or bluish green, green after drying, sometimes whitish from powdery mildew, homophyl-lous or heterophyllous. Stems prostrate to erect, branched, flex-uous, 5-200 cm. Leaves: ocrea 3-15 mm, proximal part cylindric or ± funnelform, distal part silvery, hyaline, soon disintegrating into persistent fibers or nearly completely deciduous; petiole 0.3-9 mm; blade green to gray-green, narrowly elliptic, lanceolate, elliptic, obovate, or spatulate, 6-50(-60) × 0.5-22 mm, margins flat, apex acute, obtuse, or rounded; stem leaves 1-4 times as long as adjacent branch leaves; distal leaves overtopping flowers. Inflorescences axillary; cymes uniformly distributed or aggregated at tips of stems and branches, 1-6(-8)-flowered. Pedicels enclosed in or exserted from ocreae, 1.5-5 mm. Flowers closed or semi-open; perianth 1.8-5.5 mm; tube 20-57% of perianth length; tepals overlapping or not, green or reddish brown with white, pink, or red margins, petaloid, not keeled, oblong to obovate, often cucullate in fruit; midveins branched or unbranched, thickened or not; stamens 5-8. Achenes enclosed in or exserted from perianth, light to dark brown, ovate, (2-)3-gonous, 1.2-4.2 mm, faces subequal or unequal, apex not beaked, edges slightly concave, dull, usually coarsely striate-tubercled, sometimes obscurely tubercled; late-season achenes common or not, 2-5 mm.

Herbs annual. Stems prostrate, ascending, or erect, 10-40 cm tall, much branched from base. Petiole short or nearly absent, articulate at base; leaf blade lanceolate or narrowly elliptic, 1-4 cm × 3-12 mm, both surfaces glabrous, midvein and lateral veins conspicuous, base cuneate, margin entire, apex acute or nearly obtuse; ocrea: lower part brown, upper part white or throughout brown, membranous, veined, apex lacerate. Flowers 1-5; axillary; bracts thinly membranous. Pedicel slender, articulate at apex. Perianth green, margin white or pinkish, 5-cleft to 2/3-3/4; tepals elliptic, 2-2.5 mm. Stamens 8; filaments dilated at base. Styles 3, free, short; stigmas capitate. Achenes included or slightly exceeding persistent perianth, black-brown, opaque, ovoid, trigonous, 2.5-3 mm, minutely granular striate. Fl. May-Jul, fr. Jul-Aug. 2n = 40, 60.

Suberect-erect, ascending or decumbent, glabrous annual herb, branched mostly from base. Leaves heterophyllous, lower larger on main branches, smaller on the lateral and upper branches, 0.8-2.5 x 0.25-1.0 cm, elliptic - lanceolate or ovate, acute, entire, dotted. Ochrea 0.75-1.25 cm long, bifid, silvery, membranous lacerate. Inflorescence solitary, axillary or 3-5 clusters. Flower 0.5-0.75 mm across, pedicel 0.5-0.75 (-1.0) mm. Ochrealae minute. Tepals 5, 1.5-2.0 x 0.5-1 mm, elliptic-lanceolate or ovate, obtuse-acute, entire. Stamens 5 (-4), filaments short, equal; anthers dorsifixed. Ovary 0.25-0.5 mm, ovate - circular, trigonous with 3 very short styles and capitate stigmas. Nuts 2-2.5 x 1.0-1.5 mm, ovate, trigonous, black, shining, striate.

Widely distributed in temperate and subtropical regions.

Anhui, Fujian, Gansu, Guangdong, Guangxi, Guizhou, Hainan, Hebei, Heilongjiang, Henan, Hubei, Hunan, Jiangsu, Jiangxi, Jilin, Liaoning, Nei Mongol, Ningxia, Qinghai, Shaanxi, Shandong, Shanxi, Sichuan, Taiwan, Xinjiang, Xizang, Yunnan, Zhejiang [widely distributed in N temperate zone; widely naturalized in S temperate zone].

Distribution: Widely distributed in temperate and subtropical regions of both the hemispheres.

2200-3800 m

Fl. Per.: March-September.

Near fields, roadsides, waste places; sea level to 4200 m.

Grows from plains to 3500 m, as a weed in area of cultivation, on waste ground, moist and shady areas.

Polygonum centinodum Lamk., Fl. Francies 3: 237. 1778; P. erectum Roth Beitr. Bot. 2: 131. 1783; P.heterophyllum Lindm. Svensk. Bot. Tidskr. 6: 960. 1912; P. aviculare var. heterophyllum (Lindm.) Munshi & Javeid, l.c. 55.