Plants glabrous or sparsely pubescent in the distal younger parts of stems and branches. Stems erect or sometimes ascending proximally, much-branched and bushy, rarely nearly simple, 0.3-1(-2) m; each node with paired, divergent spines (modified bracts) to 1.5(-2.5) cm. Leaves: petiole ± equaling or longer than blade; blade rhombic-ovate, ovate, or ovate-lanceolate, 3-10(-15) × 1.5-6 cm, base broadly cuneate, margins entire, plane or slightly undulate, apex acute or subobtuse to indistinctly emarginate, mucronulate. Inflorescences simple or compound terminal staminate spikes and axillary subglobose mostly pistillate clusters, erect or with reflexed or nodding tips, usually green to silvery green. Bracts of pistillate flowers lanceolate to ovate-lanceolate, shorter than tepals, apex attenuate. Pistillate flowers: tepals 5, obovate-lanceolate or spatulate-lanceolate, equal or subequal, 1.2-2 mm, apex mucronate or short-aristate; styles erect or spreading; stigmas 3. Staminate flowers: often terminal or in proximal glomerules; tepals 5, equal or subequal, 1.7-2.5 mm; stamens 5. Utricles ovoid to subglobose, 1.5-2.5 mm, membranaceous proximally, wrinkled and spongy or inflated distally, irregularly dehiscent or indehiscent. Seeds black, lenticular or subglobose-lenticular, 0.7-1 mm diam., smooth, shiny. Flowering summer-fall. Waste places, fields, roadsides, railroads, barnyards, overgrazed pastures, other disturbed habitats; 0-700 m; introduced; Man., Ont.; Ala., Ark., Calif., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Minn., Miss., Mo., Nebr., N.J., N.Y., N.C., Ohio, Okla., Pa., R.I., S.C., Tenn., Tex., Vt., Va., W.Va., Wis.; Mexico; West Indies; Central America; South America; introduced nearly worldwide. Amaranthus spinosus is native to lowlands in tropical America; at present it is a pantropical weed that also occurs in some warm-temperate regions.
From Flora of Indiana (1940) by Charles C. Deam
This is a very objectionable weed on account of its many spines. It is restricted mostly to our southern counties in barnyards and lanes where it is often very abundant. I do not understand why farmers do not try to exterminate it when first they discover it on their premises but I have never met one who was making the attempt. All who had a common name for it called it careless, a name sometimes applied to species of the pigweed family. I never could learn the origin or significance of this name and it seems to me to be very inappropriate.
Monoecious; stem erect, branched, to 1 m, bearing at most nodes a pair of divergent spines 5-10 mm; lvs lance-ovate to ovate, 3-6 cm, narrowed to an obtuse, mucronate tip, broadly cuneate to the long petiole; spikes numerous, 5-15 cm, 6-10 mm thick, the terminal often chiefly or wholly staminate, the basal part and the axillary clusters mostly pistillate; sep of the pistillate fls 5, oblong, 1-1.5 mm; fr 1.5-2 mm, indehiscent or bursting irregularly, the terminal part spongy and roughened; seed suborbicular, 0.7-1 mm; 2n=32, 34. A pantropical weed, probably originally from the New World, extending n. in our range to N.Y., Pa., Ind., and Mo., seldom farther n.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.