1. Chenopodium album L. (pigweed, lamb’s quarters)

Pl. 353 f–h; Map 1521

Plants annual, without an odor. Stems 10–150 cm long, erect or ascending, usually few- to several-branched above the base and below the inflorescence, glabrous or more commonly sparsely to moderately white-mealy, sometimes reddish-tinged or reddish purple–striped, but usually lacking a pronounced reddish purple area at the base of each leaf. Leaves mostly long-petiolate. Leaf blades 1–6(–12) cm long, mostly 1–3 times as long as wide (1–4 cm wide), often more than 1.5 times in the largest (lowermost) leaves, rhombic to ovate-rhombic, ovate-triangular, or lanceolate, the uppermost usually linear to narrowly lanceolate, angled or tapered to a bluntly or sharply pointed tip, the middle lobe not appearing unusually elongate, angled at the base, green or reddish-tinged, thin and herbaceous to thickened, somewhat leathery, and slightly succulent in texture, the margins entire to wavy or irregularly several-toothed (the basal pair of teeth usually larger than the others, sometimes appearing shallowly lobed), the upper surface glabrous or sparsely to moderately mealy at maturity, not shiny, the undersurface moderately to more commonly densely white-mealy. Venation noticeably branched, with 1 or 3 main veins. Inflorescences axillary and terminal, consisting of short spikes with small clusters of flowers, the terminal ones usually grouped into small to relatively large panicles. Flowers not all maturing at the same time. Calyx 5-lobed nearly to the base, usually covering the entire fruit except sometimes for a minute area surrounding the style, occasionally somewhat spreading in a few flowers of a given inflorescence, the lobes 0.7–1.2 mm long, ovate to triangular-ovate, bluntly pointed at the tip, usually with a relatively pronounced broad keel or raised area along the midvein dorsally, moderately to densely white-mealy. Stamens 5. Stigmas 2. Fruits 1.2–1.5 mm wide, depressed-ovoid, the seeds positioned horizontally, the wall thin, membranous, and somewhat translucent, smooth or finely roughened, not appearing honeycombed, usually difficult to separate from the seed. Seeds black, shiny, smooth or nearly so, rounded to very bluntly angled along the rim. 2n=54. May–October.

Introduced, common nearly throughout the state (nearly worldwide, probably of Eurasian origin). Banks of streams, rivers, and spring branches; also crop fields, fallow fields, gardens, roadsides, railroads, and open, disturbed areas.

Chenopodium album has been interpreted taxonomically in a variety of ways. On the one hand, it has been split into numerous subspecies, varieties, and forms (see Wahl [1952–1953] for synonymy), some of which have been recognized as separate species (Mohlenbrock, 2001). At the other extreme, some authors have circumscribed C. album as a highly polymorphic cosmopolitan species, reducing to synonymy or to infraspecific rank taxa treated here as C. berlandieri, C. bushianum, C. missouriense, and C. opulifolium, along with some others not yet found growing in Missouri. Although the temptation in any group of plants in which taxa are difficult to identify is to lump them together into a single species, this probably does not reflect the actual taxonomy of the C. album complex and it certainly is not the approach taken by those monographers who have had experience with the group on a worldwide basis. Further biosystematic and molecular studies may help to resolve classification in these pigweeds.

Steyermark (1963) separated var. lanceolatum (Muhl. ex Willd.) Coss. & Germ. from var. album based upon its more spreading branches, somewhat narrower and less-toothed leaves, and more discontinuous flower clusters in the inflorescence. Mohlenbrock (2001) and some other authors have treated this as a separate species, C. lanceolatum Muhl. ex Willd., and have noted other morphological distinctions, including less mealy calyces with whitened (vs. yellowish) lobe margins and supposedly slightly smaller seeds. The status of this relatively uncommon taxon is unclear, especially as its fruits are sometimes faintly reticulate on the surface. A number of specimens exist that are intermediate for one or more of the characters. Perhaps var. lanceolatum is merely a component of the morphological variation in C. album, but it also may represent the effects of past hybridization between this species and C. berlandieri var. boscianum. The specimens in question appear somewhat different in leaf and fruit morphology from the rare putative hybrids between C. album and C. berlandieri var. zschackei, which are discussed under the treatment of the latter species. Further studies are needed.