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

Amaranthaceae

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 the pistillate flowers long-aristate at the tip, usually reflexed outward; seeds 1.0-1.4 mm in diameter
        5 Tepals of pistillate flowers acute to short-acuminate at the tip, not reflexed; seeds 0.6-1.0 mm in diameter.
          6 Bracts ca. 2x as long as the tepals (of pistillate flowers), without apical spines; [widespread in e. US, with native and non-native populations]
          6 Bracts not conspicuously longer than the tepals (shorter to slightly longer), with apical spines; [waif, NJ]
  2 Pistillate flowers usually with (4-) 5 tepals; utricles usually indehiscent or tardily dehiscent (regularly dehiscent in A. blitoides, A. tamaulipensis, and A. scleropoides).
             7 Inflorescence axes thickened, becoming indurate at maturity
               8 Utricles with regular, circumscissile dehiscence; utricle surface smooth to tuberculate distally; [OK and TX south into Mexico]
             7 Inflorescence axes not thickened, not indurate at maturity.
                   10 Tepals narrowly ovate to broadly linear, leaf blades obovate to elliptic or spathulate; seed surface dull; [widespread]
                       12 Leaves ovate, obovate-rhombic, to narrowly ovate or lanceolate; plants not fleshy; [exotic of disturbed situations]
                       12 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).
                            14 Stems lacking spines; tepals of pistillate flowers 2-3; [subgenus Albersia].
                              15 Utricles distinctly rugose, equaling or slightly exceeding the tepals; terminal inflorescences usually thin and interrupted
                              15 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).
                                16 Utricles subglobose to obovate, compressed; seeds filling the fruit almost completely; leaf blades usually deeply notched at the tip; annual.
                                16 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
                          13 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].
                                    18 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].
                                         20 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
                                    18 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].