1	Dioecious	2
1	Monoecious	9
2	"Plant in hand pistillate"	3
2	"Plant in hand staminate"	6
3	"Perianth parts 1--2, 1 narrowly lanceolate, 1 rudimentary or 0"	"Amaranthus tuberculatus var. rudis"
3	"Perianth parts 5, well developed, fan- or spoon-shaped"	4
4	"Bracts <= perianth parts"	"Amaranthus arenicola"
4	"Bracts > than perianth parts"	5
5	"Plant +- glabrous; outer perianth parts +- acuminate"	"Amaranthus palmeri"
5	"Plant glandular-hairy especially on bracts; outer perianth parts obtuse to +- notched, mucronate"	"Amaranthus watsonii"
6	"Bracts <= 2 mm"	"Amaranthus tuberculatus var. rudis"
6	"Bracts (1.4)2--6 mm"	7
7	"Outer perianth parts acuminate, spine-tipped; bracts >= perianth parts"	"Amaranthus palmeri"
7	"Outer perianth parts mucronate; bracts <= perianth parts"	8
8	"Inflorescence axis and bracts glabrous"	"Amaranthus arenicola"
8	"Inflorescence axis and bracts generally sparsely glandular-hairy"	"Amaranthus watsonii"
9	"Nodes with 2 spines"	"Amaranthus spinosus"
9	"Nodes without spines"	10
10	"Perianth parts 1--3, only 1 well developed"	"Amaranthus californicus"
10	"Perianth parts (2)3--5, all well developed."	11
11	"Pistillate perianth parts fan-shaped, margins fringed or finely dentate"	"Amaranthus fimbriatus"
11	"Pistillate perianth parts not fan-shaped, margins entire"	12
12	"Inflorescence of axillary clusters only, or axillary and terminal"	13
12	"Inflorescence terminal only"	18
13	"Perianth parts (3)4--5"	14
13	"Perianth parts (2)3"	15
14	"Perianth parts narrow-ovate to oblong; seeds 1.3--1.7 mm"	"Amaranthus blitoides"
14	"Perianth parts narrow-spoon-shaped; seeds 0.9--1.1 mm"	"Amaranthus torreyi"
15	"Fruit dehiscent; bracts 1.5--4 mm, tip +- spined"	"Amaranthus albus"
15	"Fruit indehiscent; bracts <= 1 mm, tip not spined"	16
16	"Stem hairy distally; fruit 2--3 mm, smooth"	"Amaranthus deflexus"
16	"Stem +- glabrous; fruit 1.2--1.5 mm, wrinkled"	17
17	"Leaf tip entire or +- notched; seeds dull, dotted"	"Amaranthus viridis"
17	"Leaf tip deeply notched to 2-lobed; seeds shiny, only margins dotted"	"Amaranthus blitum subsp. emarginatus"
18	"Inflorescence green; bracts > stigma; seeds brown to black; often weedy"	19
18	"Inflorescence brightly colored, not green; bracts generally <= stigma; seeds white to +- red-brown or black; cultivated, rarely escaped"	21
19	"Perianth parts spoon-shaped to obovate, overlapping, reflexed in age, tip rounded to +- notched"	"Amaranthus retroflexus"
19	"Perianth parts linear to lance-oblong, not overlapping, erect, tip acute to awned"	20
20	"Leaf blades rhombic-ovate or widely lanceolate; inflorescence of many thin, flexible branches; bracts 2--4 mm"	"Amaranthus hybridus"
20	"Leaf blades elliptic, rhombic to widely lanceolate; inflorescence of few rigid, erect branches; bracts 4.5--6(8) mm"	"Amaranthus powellii"
21	"Inflorescence main axis generally drooping or nodding; perianth parts widely spoon-shaped, margins overlapping, tip obtuse to rounded"	"Amaranthus caudatus"
21	"Inflorescence not drooping or nodding; perianth parts linear to elliptic (obovate), margins not overlapping, acute to acuminate (rounded)"	22
22	"Inflorescence erect to flexible, lateral axes thin, flexible, 3--8(12) cm, 0.5--1 cm wide; bracts 1.8--2.5 mm; fruit tip abruptly narrowed to a beak"	"Amaranthus cruentus"
22	"Inflorescence erect, lateral axes thick, stiff, 9--20(30) cm, 1.2--2 cm wide; bracts 3--5(6) mm; fruit tip gradually narrowed"	"Amaranthus hypochondriacus"