The species in this genus are generally plants of weedy, disturbed habitats, and there is much doubt as to their native range. All except A. retroflexus, A. tuberculatus, and possibly A. albus, have presumably spread into Michigan from farther south or west (or from cultivation) in historic time. Several species have long been cultivated as grain crops, as ornamentals, or as potherbs.
In most species, the fruit (a thin-walled, one-seeded structure called a utricle) is circumscissile, that is, it dehisces neatly at maturity along a circular belt (which is easily visible even on immature material), the top falling away like a lid. Sometimes younger specimens of monoecious species may falsely appear entirely staminate, or older ones appear falsely pistillate, and care must be taken not to assume that such a plant is really one of the truly dioecious species with characteristic slender elongate spikes.
Two additional species found in adjacent regions might be expected in the southern Lower Peninsula: A. spinosus L. is readily recognized by the presence of sharp rigid spines in the axils of the leaves; A. palmeri is dioecious, like A. tuberculatus and A. arenicola, but with long-acuminate outer tepals.
1. Flowers in small axillary clusters; plants monoecious; utricles circumscissile.
2. Tepals 3, much surpassed by the slender bracts; seeds ca. 1 mm or slightly less in diameter; stem ± erect or ascending, stiffly branched.
2. Tepals (4-) 5, scarcely if at all exceeded by the bracts; seeds ca. 1.5 mm in diameter; stem and branches ± prostrate.
1. Flowers in elongate simple or paniculate, chiefly terminal spikes; plants and utricles various (but dioecious with utricle not circumscissile in A. tuberculatus, of which some leafy-bracted pistillate plants might be said to have axillary clusters).
3. Stem densely pubescent around the inflorescence; leaves with petioles and often also midrib beneath pubescent.
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3. Stem glabrous or sparsely pubescent, at least toward the summit; leaves glabrous or nearly so.
4. Plants dioecious.
5. Utricle circumscissile; pistillate flowers with 5 conspicuous spatulate tepals; staminate tepals ca. 2-2.5 mm long, often ± broadly rounded and apiculate at apex and half or slightly more as wide as long.
5. Utricle indehiscent (irregularly breaking open); pistillate flowers with tepals absent or rudimentary; staminate tepals (at least the largest) ca. 2.3-3.5 mm long, less than half as wide, ± tapered or gradually rounded toward the apex, usually terminating in a short weak awn.
4. Plants monoecious.
6. Utricle indehiscent, with a very rough or warty surface; bracts not over 1 mm long, at most minutely pointed (not spine-tipped); inflorescence of slender branches mostly less than 6 mm thick.
6. Utricle circumscissile, with smoothish or rough surface; bracts longer and usually spine-topped; inflorescences often more robust.
7. Flower clusters not elongate (usually ± hemispherical or nearly spherical), present (though small) even in the lowermost leaf axils (those at top of plant more crowded into elongate spikes); tepals 3.
7. Flower clusters ± elongate and crowded in middle and upper axils, absent from lowest axils; tepals ordinarily 5.
8. Larger tepals of at least some pistillate flowers ca. (2.5-) 3-3.5 mm long.
9. Tepals of pistillate flowers ± truncate or broadly rounded (and spinulose or apiculate) at the tip; stamens 5; plants ± densely pubescent on stems (especially around the inflorescence), on the petioles, and on the base of the midrib of the blades beneath.
9. Tepals of pistillate flowers ± acute or tapered to the weakly spinulose tip; stamens 3 or 5; plants glabrous or sparsely pubescent around the inflorescence and on leaves.
10. Seed nearly circular in outline, ca. 1.2-1.3 mm broad; stamens 5.
10. Seed obovate in outline, ca. 1 (-1.2) mm broad; stamens mostly 3.
8. Larger tepals of pistillate flowers not over 2.2 (-2.4) mm long.
11. Tepals of pistillate flowers obovate or spatulate, conspicuously overlapping; terminal inflorescence thick and greatly elongate, drooping from near its base, much more strongly developed than lateral inflorescences.
11. Tepals of pistillate flowers narrowly elliptic to oblong, scarcely if at all overlapping; terminal inflorescence not disproportionately developed.
12. Tepals of pistillate flowers ± obtuse or rounded at apex; bracts acute, narrowly acuminate, or short-spined, shorter than tepals or up to ca. 1.5 times as long; inflorescences usually reddish or purplish.
12. Tepals (at least the outer ones) of pistillate flowers acute; bracts generally long-spined, often twice as long as the tepals; inflorescences usually green.
Citation:
MICHIGAN FLORA ONLINE. A. A. Reznicek, E. G. Voss, & B. S. Walters. February 2011. University of Michigan. Web. June 4, 2025
https://mifloradev.lsa.umich.edu/flora-demo/#/genus/Amaranthus