Amaranthaceae, the Amaranth Family
Herbs, with alternate leaves, and inconspicuous greenish or reddish flowers without petals, which are axillary or in dense clusters, blooming in summer.
| 1a. Flower-clusters axillary — 2. | |
| 1b. Flower-clusters in terminal spikes or panicles, sometimes also axillary — 3. | |
| 2a. Plant prostrate or decumbent; seed about 1.5 mm. broad | Pigweed, Amaranthus blitoides. |
| 2b. Plant erect or ascending, widely branched; seeds about 1 mm. broad (3-10 dm. high) | Tumble Weed, Amaranthus graecizans. |
| 3a. Principal leaves with a pair of spines at their base | Thorny Amaranth, Amaranthus spinosus. |
| 3b. Spines none at the base of the leaves — 4. | |
| 4a. Weedy plants of cultivated or waste ground; flowers monoecious or polygamous; pistillate flowers with a calyx (Pigweed) — 5. | |
| 4b. Plants of swamps or stream-banks; flowers dioecious; pistillate flowers without calyx (Water Hemp) — 7. | |
| 5a. Spikes short, 1-8 cm. long, crowded in dense ovoid panicles; the terminal spike not conspicuously elongated beyond the appressed or ascending lower ones | Pigweed, Amaranthus retroflexus. |
| 5b. Spikes slender, 1-12 cm. long; the terminal spike greatly exceeding the short inconspicuous divergent lower ones — 6. | |
| 6a. Bracts subulate, sharply awned | Pigweed, Amaranthus hybridus. |
| 6b. Bracts merely acuminate | Pigweed, Amaranthus paniculatus. |
| 7a. Flowers in leafy spikes, or the lower in separate clusters | Water Hemp, Acnida tuberculata. |
| 7b. Flowers in separate distinct clusters | Water Hemp, Acnida tuberculata var. subnuda. |