Inflorescence paniculate or spike-like and either terminal with some branches in upper leaf axils, or axillary with the larger clusters in the lower axils. Bracts usually 3 (1 bract and 2 bracteoles) subtending each flower, others subtend inflorescence branches. Flowers mostly unisexual, usually mixed on the inflorescence. Perianth segments 3–5, mostly scarious with a coloured midrib. Stamens 3–5, not necessarily equal to the number of perianth segments. Ovary sessile, circumsciss or indehiscent, sometimes inflated and spongy; styles mostly not developed; stigmas 3, mostly sessile; ovule 1.
| | Key to the species | |
| 1 | Perianth segments 3 | 2 |
| Perianth segments 5 | 7 |
| 2 | Inflorescence axillary, larger clusters in lower axils | 3 |
| Inflorescence mostly terminal with smaller branches in upper axils only
Back to 1 | 4 |
| 3 | Bracts with long spines, longer than perianth | Amaranthus albus |
| Bracts with short spines, shorter than perianth
Back to 2 | Amaranthus macrocarpus |
| 4 | Fruit much longer (1.3–2 times) than perianth | 5 |
| Fruit equal to or slightly longer (less than 1.2 times) than perianth
Back to 2 | 6 |
| 5 | Stems and young leaves with scattered hairs | Amaranthus deflexus |
| Stems and leaves glabrous
Back to 4 | Amaranthus blitum |
| 6 | Bracts and perianth segments sharply pointed; bracts longer than perianth | Amaranthus powellii |
| Bracts and perianth segments not sharply pointed; bracts shorter than or equal to perianth
Back to 4 | Amaranthus viridis |
| 7 | Inflorescence axillary, larger clusters in lower axils | 8 |
| Inflorescence mainly terminal with smaller branches in upper axils only
Back to 1 | 9 |
| 8 | Perianth segments 2–3 mm long | 19 |
| Perianth segments 5–7 mm long, broad-spathulate
Back to 7 | Amaranthus grandiflorus |
| 9 | Inflorescence to 30 cm long, dark red to maroon | Amaranthus caudatus |
| Inflorescence not as above
Back to 7 | 10 |
| 10 | Two spinescent bracts, 10–15 mm long, present at the base of each short inflorescence branch | Amaranthus spinosus |
| Spinescent bracts absent
Back to 9 | 11 |
| 11 | Fruit indehiscent or rupturing irregularly | 12 |
| Fruit circumsciss
Back to 10 | 14 |
| 12 | Perianth segments oblong | Amaranthus muricatus |
| Perianth segments spathulate
Back to 11 | 13 |
| 13 | Perianth segments 2–3 mm long | Amaranthus mitchellii |
| Perianth segments 5–7 mm long
Back to 12 | Amaranthus grandiflorus |
| 14 | Stems and leaves hairy, especially when young | 15 |
| Stems and leaves glabrous
Back to 11 | 17 |
| 15 | Bracts 5–6 mm long | Amaranthus retroflexus |
| Bracts 2–5 mm long
Back to 14 | 16 |
| 16 | Bracts 3–4 mm long | Amaranthus hybridus |
| Bracts 2–3 mm long
Back to 15 | Amaranthus cruentus |
| 17 | Outer perianth segments recurved at the apices | Amaranthus quitensis |
| Apices of perianth segments straight
Back to 14 | 18 |
| 18 | Bracts 5–6 mm long; perianth segments 2.5–3 mm long | Amaranthus powellii |
| Bracts 3–4 mm long; perianth segments 1.5–2 mm long
Back to 17 | Amaranthus hybridus |
| 19 | Fruit wrinkled, with prominent wavy ribs; confined to river floodplains with grey, brown or black | Amaranthus mitchellii |
| Fruit wrinkled, without wavy ribs; grows on red sand and rocky areas
Back to 8 | Amaranthus cuspidifolius |
