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Key to Amaranthus, Key B: Amaranthus, subgenera Albersia and Amaranthus

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1 Inflorescences axillary clusters of glomerules (sometimes leafy terminal spikes also present); [subgenus Albersia].
  2 Pistillate flowers usually with 3 tepals; utricles usually regularly dehiscent (indehiscent in A. blitum).
    3 Utricles indehiscent; leaf blades usually deeply notched at the tip.
      4 Plants prostrate; leaves (0.75-) 1-2 (-3.5) cm long; inflorescences axillary cymes (terminal inflorescences reduced or absent)
      4 Plants ascending; leaf blades 2-4 (-6) cm long; inflorescences primarily terminal, thin and flexuous
    3 Utricles dehiscent; leaf blades obtuse, acuminate, or very shallowly notched at the tip.
        5 Tepals of pistillate flowers acute to short-acuminate at the tip, not reflexed; seeds 0.6-1.0 mm in diameter
        5 Tepals of the pistillate flowers long-aristate at the tip, usually reflexed outward; seeds 1.0-1.4 mm in diameter
  2 Pistillate flowers usually with (4-) 5 tepals; utricles usually indehiscent or tardily dehiscent (regularly dehiscent in A. blitoides).
          6 Inflorescence axes thickened, becoming indurate at maturity
          6 Inflorescence axes not thickened, not indurate at maturity.
             7 Utricles with regular, circumscissile dehiscence
             7 Utricles indehiscent (or tardily and irregularly dehiscent).
               8 Leaves crisped-erose, conspicuously undulate (non planar)
               8 Leaves entire or erose, plane or slightly undulate.
                 9 Leaves ovate, obovate-rhombic, to narrowly ovate or lanceolate; plants not fleshy; [exotic of disturbed situations]
                 9 Leaves orbicular or obovate; plants fleshy; [native of sea-beaches]
1 Inflorescences terminal spikes or panicles, leafless or nearly so at least in the distal portions (axillary spikes or clusters usually also present).
                   10 Utricles indehiscent; tepals of pistillate flowers usually 2-3 (5 in A. spinosus); inflorescence bracts shorter than the tepals.
                     11 Stems with paired nodal spines; tepals of pistillate flowers 5; [subgenus Amaranthus]
                     11 Stems lacking spines; tepals of pistillate flowers 2-3; [subgenus Albersia].
                       12 Utricles distinctly rugose, equaling or slightly exceeding the tepals; terminal inflorescences usually thin and interrupted
                       12 Utricles smooth to faintly rugose (occasionally wrinkled or rugose in dried material), distinctly exceeding the tepals; terminal inflorescences usually thick and dense (or thin and interrupted in some forms of A. blitum).
                          13 Utricles subglobose to obovate, compressed; seeds filling the fruit almost completely; leaf blades usually deeply notched at the tip; annual.
                            14 Plants ascending; leaf blades 2-4 (-6) cm long; inflorescences primarily terminal, thin and flexuous
                            14 Plants prostrate; leaves (0.75-) 1-2 (-3.5) cm long; inflorescences axillary cymes (terminal inflorescences reduced or absent)
                          13 Utricles ellipsoid, slightly to distinctly inflated; seeds filling only the proximal portions of the fruit; leaf blades shallowly notched at the tip; short-lived perennials, or annuals
                   10 Utricles dehiscent; tepals of pistillate flowers usually 5 (3-5 in A. powellii); inflorescence bracts exceeding the tepals (shorter than the tepals in some cultivated forms); [subgenus Amaranthus].
                              15 Fully developed inflorescences large and robust, usually brightly colored (red, purple, occasionally white or yellow, rarely green); bracts usually not exceeding style branches at maturity (occasionally longer than the style branches in A. hypochondriacus); seeds white, ivory, red, brown, or black; [cultivated, only weakly naturalized].
                                16 Inflorescences stiff, erect
                                16 Inflorescences lax, erect to drooping.
                                  17 Tepals of pistillate flowers (at least the inner tepals of the pistillate flowers) obovate or spatulate, the tip obtuse to slightly notched; style branches spreading or reflexed
                                  17 Tepals of pistillate flowers oblong to lanceolate, the tip acute; style branches erect or slightly reflexed
                              15 Fully developed inflorescences moderately large, usually green (rarely with some whitish or reddish coloration); bracts exceeding the style branches and tepals; seeds brown or black; [wild and weedy].
                                    18 Tepals of pistillate flowers obtuse, rounded, or slightly notched at the tip; plants rather densely pubescent
                                    18 Tepals of pistillate flowers acute, acuminate, or aristate at the tip; plants slightly pubescent when young, becoming glabrous or nearly so.
                                       19 Bracts 2-4 mm long; inflorescences usually soft and lax, with spreading branches
                                       19 Bracts 4-7 mm long; inflorescences usually stiff, with erect branches