Asclepiadaceae, the Milkweed Family
Herbs, with simple entire leaves and regular flowers; juice usually milky; except in the first species, which is a twining vine. The flowers have an unusual structure: calyx of 5 sepals; petals 5, united with each other, and spreading or reflexed so that they conceal the calyx; stamens 5, united with each other and with the stigma to form a complex organ in the center of the flower; ovaries 2; on the back of each stamen is a colored projecting hood, which is frequently the most conspicuous part of the flower, and may be mistaken for the corolla.
| 1a. Twining vine, with dark purple flowers (summer) | Black Swallow-wort, Cynanchum nigrum. |
| 1b. Stems not twining — 2. | |
| 2a. Leaves whorled (3-6 dm. high; summer) (Milkweed) — 3. | |
| 2b. Leaves opposite or alternate — 4. | |
| 3a. Leaves in whorls of 4, lanceolate (flowers pink) | Milkweed, Asclepias quadrifolia. |
| 3b. Leaves in whorls of 4-7, linear (flowers greenish-white) | Milkweed, Asclepias verticillata. |
| 4a. Umbel sessile (4-8 dm. high; flowers green, summer) | Green Milkweed, Acerates viridiflora. |
| 4b. Umbel peduncled — 5. | |
| 5a. Leaves linear or narrowly linear-lanceolate (4-8 dm. high; flowers greenish-white, summer) | Green Milkweed, Acerates floridana. |
| 5b. Leaves lanceolate or broader (flowers in summer) — 6. | |
| 6a. Leaves pubescent beneath — 7. | |
| 6b. Leaves glabrous or nearly so — 10. | |
| 7a. Flowers brilliant orange (3-6 dm. high) | Butterfly Weed, Asclepias tuberosa. |
| 7b. Flowers red or purple — 8. | |
| 8a. Reflexed lobes of corolla merely purple-tinged (1-2 m. high) | Milkweed, Asclepias syriaca. |
| 8b. Reflexed lobes of corolla bright-red or purple — 9. | |
| 9a. The erect hoods of each flower about 5 mm. long (7-12 dm. high) | Milkweed, Asclepias purpurascens. |
| 9b. The erect hoods of each flower about 3 mm. long (6-10 dm. high) | Swamp Milkweed, Asclepias incarnata var. pulchra. |
| 10a. Leaves broadly rounded and almost sessile at base (flowers purplish) — 11. | |
| 10b. Leaves narrowed at the base, distinctly petioled (8-15 dm. high) — 12. | |
| 11a. Umbel solitary, terminal and erect on a long peduncle (4-8 dm. high) | Milkweed, Asclepias amplexicaulis. |
| 11b. Umbels terminal or lateral, bent toward one side (7-12 dm. high) | Milkweed, Asclepias sullivantii. |
| 12a. Corolla (not hoods) red (1-2 m. high) | Swamp Milkweed, Asclepias incarnata. |
| 12b. Corolla (not hoods) greenish (8-15 dm. high) | Milkweed, Asclepias exaltata. |