Habitat. Bare, damp to moderately dry soil; favoured by a good nutrient supply but apparently not strongly nitrophilous (e.g. only rarely seen at dung heaps). - Gardens, fields, roadsides, plantations, tips and filling soil; outside settlements ditch banks and gravelly lake-shores.
Europe except for the extreme north, rare in the Mediterranean; Turkey, the Caucasus, W Siberia; also North America (not native).
Distribution. Nem-BNem(-SBor). - An archaeophytic weed in the south; in modern times brought in e.g. with grain, ballast and shipments to troops. - D fairly common to scattered on the islands and in 0Jy and eastern SJy, elsewhere rare. N frequent and well established in the southeastern lowlands north to southern He and Op and south to VA; rare and casual in coastal provinces from Ro Stavanger to SF Lærdal and Jølster, and in STSkaun 1930-37 (grain mill). S on the whole common, especially in farmland areas, north to BhG, Vg, southern Vsm and Gst, but fairly rare in parts of the southern uplands; rare north to southern Vrm, southeastern Dir and His; further north rare and ± casual near the coast; Jmt Brunflo 1930, Östersund 1935, PL Arjeplog 1941. F archaeophytic in the south; fairly common at least in V, U, southern St and southern EH\ further north mostly casual (tips, docks, or brought with wartime transports), known from EP Kaskinen, Kristiinankaupunki, Vaasa, PS Kuopio, Pieksämäki rural municipality, PK, KP ( ± established in Haapavesi and Raahe), Kn Hyrynsalmi, Kajaani, Paltamo, OP Oulu, PeP Kemi, Simo and KiL Muonio.
Therophyte (summer-annual). (2-)10-80(-100) cm, glabrous or very slightly farinose on young parts. Stem ± angular, green to red, hard, erect or sometimes procumbent, sparsely branched; lower branches (sub-) opposite, often long; secondary branches few. Leaves often tinged with brown or red; blades thin, glabrous or sparsely farinose below when young; margin entire. Lower leaves with 1-2.5 cm long petiole; blade broadly ovate to ovate, 1-6(-9) cm; apex apiculate or acute to obtuse or rarely emarginate. Middle leaves with narrower, more elliptic and more obtuse blade. Bracts elliptic, lanceolate or obovate. Fig. 7.
Chenopodium polyspermum (EH), Parts of inflorescences X 0.7. ILL. MARJA KOISTINEN Inflorescence leafy and bracteate throughout, composed of axillary dichasia which may be lax, wide and up to 10 cm long, or ± condensed and spike-like, with all flowers pedicellate or partly in small glomerules. Flowers bisexual. Tepals 5, entirely free, not contiguous, spreading in fruit, keeled at apex, with membranous margin and obtuse apex. Stamens 1-3 or rarely 5. Stigmas 2, 0.1 (-0.2) mm. Nut falling without the perianth; pericarp not adherent to the seed. Seed horizontal, orbicular in outline, 0.9-1.2 mm; edge rounded; seed-coat brown to blackish, glossy, obscurely pitted and with radial sinuous lines. - Early summer to early autumn. 2n=18 (F U, S Sk). - [2n=18]
Variation. Under long-day conditions plants are usually green to reddish and the leaf-blades are ovate with acute apex, whereas under short-day conditions (in the autumn) plants are green and the blades are elliptic with obtuse apex. Forms which show the same characters independently of day-length may exist, but they cannot be distinguished among this photoperiodically determined variation, and no taxonomic recognition can be adopted. - Pollen, seed and inflorescence characters indicate that C. polyspermum has no close relatives.