1. Amaranthus L. (pigweed, amaranth)

Plants annual, monoecious or dioecious (rarely a few perfect flowers present in monoecious species). Stems erect to spreading, glabrous or variously pubescent, green to yellowish white or reddish purple, often with pink to purple longitudinal lines or ridges. Leaves alternate, short- to long-petiolate. Leaf blades 0.8–20.0 cm long, variously shaped, usually pubescent with short, curved hairs when young, usually becoming nearly glabrous (except often along the margins) at maturity. Inflorescences axillary and/or terminal, dense spikes, spikelike racemes, panicles (often appearing as spicate branches arranged racemosely along a central spicate axis), or axillary clusters. Flowers imperfect. Sepals 1–5 or sometimes absent in pistillate flowers, free, more or less similar or differentiated into inner and outer series, often with an awnlike extension, this sometimes becoming somewhat hardened and spinelike at maturity, herbaceous to hardened or leathery, green, at least centrally, but often with thin, papery margins. Staminate flowers with the stamens 3–5, the filaments free. Pistillate flowers with the ovary ovoid. Ovule 1. Style absent or very short, the 2 or 3 stigmas tapered and slender, persistent. Fruits mostly with papery walls, ovoid or ellipsoid, with 2 or 3 small beaks, indehiscent or more commonly with irregular or circumscissile dehiscence, 1-seeded. Seeds often somewhat flattened, circular or nearly so in outline, rounded or angled along the rim, the surface smooth (sometimes somewhat roughened in A. blitum and A. viridis), shiny. Sixty to 70 species, nearly worldwide.

The genus Amaranthus has great economic importance. Several of the Missouri species (the weedy amaranths) are serious weeds in crop fields. All of the pigweeds produce copious quantities of potentially allergenic, wind-dispersed pollen that are major causes of hay fever during some times of the year. On the plus side, however, certain species have a long history of cultivation for food. Three species, A. caudatus, A. cruentus, and A. hypochondriacus, have histories of cultivation in Latin America as pseudo–grain crops that stretch back to long before European colonization of the Americas, and these cereal amaranths also subsequently became cultivated in parts of Europe and Asia (Sauer, 1967; Robertson, 1981; Costea et al., 2001 a, b). Red-pigmented forms of these species and others, principally A. hybridus, also have been cultivated as garden ornamentals, and in Latin America they are used to produce a red dye for religious ceremonies (Robertson, 1981; Heiser, 1985). Yet other species, such as A. tricolor, have been used widely for food, fresh as greens and boiled as potherbs, and also as a green supplement to some livestock feeds. That these uses persist today is evidenced by the fact that all of the species mentioned have been recorded outside of cultivation in Missouri. In fact, grain amaranths are available to consumers in many grocery and health food stores, the seeds dried and ground into flour, popped for use in baked goods and cereals, and powdered as a component of health food beverages (Robertson, 1981). They are quite nutritious, a good source of protein, and contain high levels of lysine. Wild food enthusiasts are cautioned, however, that amaranths growing in the wild occasionally can absorb and accumulate in their foliage toxic substances present in the soil.

The species of Amaranthus considered native in Missouri and the central portion of the United States occur in a variety of habitats with moist, bare soil. The natural limits of the distributional ranges of these species are not well understood, as they have been spread in recent times by agriculture and other human activities as well as by natural means. These plants are primary colonizers of both habitats prone to frequent natural disturbance, especially in the floodplains of major rivers and streams, and also anthropogenically altered sites, like crop fields and railroad embankments. Distributional limits of these disturbophiles had already expanded both westward across the Great Plains and eastward to the Atlantic seaboard as a result of European colonization by the time that botanists began paying attention to the biogeography of such species (Sauer, 1957, 1967, 1972).

The weedy amaranths frequently grow in mixed populations, and a number of hybrids have been reported. These hybrids are sterile, producing inflorescences with many bracts but with the flowers lacking or vestigial, or in some cases with reduced flowers having shrunken, misshapen, or absent fruits, so detecting such plants is not difficult. However, evaluating the presumed parentage of such sterile individuals with confidence is often not possible. Carl Sauer, a specialist on the genus, determined one or a few specimens as representing each of the following hybrid combinations in Missouri: A. hybridus × A. palmeri, A. hybridus × A. retroflexus, A. hybridus × A. rudis, A. hybridus × A. tuberculatus, and A. retroflexus × A. rudis (note, however that A. rudis is reduced to synonymy under A. tuberculatus in the present treatment). Other combinations are to be expected where two or more species grow together, but apparently they are rare, even in large mixed populations.

Identification of Amaranthus species is difficult, especially when only staminate flowers are present, not only because of the presence of occasional hybrids but also because determination depends on observation of details of the small, morphologically similar flowers and fruits. Within an inflorescence, the numerous flowers are at various stages of development, and fruits often continue to mature in the plant press during drying. Taxonomists traditionally have divided the genus into two main subgroups, the dioecious subgenus Acnida (L.) Aellen ex K.R. Robertson and the monoecious subgenus Amaranthus. Recently, a few authors (Costea et al., 2001a) have segregated subgenus Albersia (Kunth) Gren. & Godr. from subgenus Amaranthus, based on its mostly axillary clusters of flowers (vs. mostly terminal spikes or panicles), indehiscent (vs. circumscissile) fruits, and minute differences in leaf and seed anatomy, but most botanists still treat this group as section Blitopsis Dumort. of subgenus Amaranthus. In spite of the instinctive appeal of these groupings, the validity of maintaining such infrageneric taxa is drawn into question by the existence of various hybrids between them.