12. Amaranthus spinosus L. (spiny pigweed, thorny amaranth, careless weed)
Pl. 198 c, d;
Map 821
Plants monoecious.
Stems 30–150 cm long, erect or ascending, glabrous or nearly so, with
conspicuous pairs of slender, straight, spreading spines at most nodes. Leaves
mostly long-petiolate. Leaf blades 2–10 cm long, narrowly ovate to ovate,
narrowed or tapered to a bluntly or sharply pointed tip, narrowed or tapered at
the base, glabrous or nearly so. Inflorescences dull or grayish green,
occasionally dull reddish-tinged; axillary and terminal; the axillary
inflorescences dense, small, globose clusters or elongate spikes; the terminal
inflorescence a spike or panicle with few to rarely numerous ascending
branches; the flowers often grouped into discontinuous clusters or regions
along the basal portions of the spikes; the tip straight or somewhat curved at
maturity; the main axis and branches glabrous or nearly so. Bracts 0.5–1.0 mm
long, shorter than the sepals, lanceolate to narrowly lanceolate or linear,
narrowed or tapered to a sharply pointed tip, with a somewhat thickened green
midrib and relatively narrow, thin, papery margins, the midrib not or only
slightly extending beyond the main body as a minute, sharp point. Staminate
flowers with 5 more or less similar sepals, these 1.0–1.6 mm long, somewhat
outward-curved, oblong-lanceolate, narrowed or tapered to a sharp point at the
tip. Stamens 5. Pistillate flowers with 5 more or less similar sepals, these
1.0–1.6 mm long, somewhat outward-curved, oblong to oblong-elliptic or
oblong-lanceolate, narrowed or tapered to a blunt or sharp point. Stigmas 3,
ascending to erect. Fruits 1.5–2.0 mm long, dehiscing irregularly or less
commonly indehiscent or with more or less circumscissile dehiscence, the
surface somewhat roughened or wrinkled above the midpoint when dry. Seeds 0.7–1.0
mm in diameter, rounded along the rim, the surface black. 2n=32, 34.
June–October.
Introduced,
scattered, mostly south of the Missouri River (probably originally a native of
New World tropics, now distributed nearly throughout tropical and
warm-temperate regions, in North America introduced north to California,
Nebraska, Minnesota, and Maine; also Canada). Bottomland forests, banks of
streams and rivers, and margins of sloughs; also fallow fields, crop fields,
barnyards, feedlots, pastures, roadsides, railroads, and open, disturbed areas.