Our milkweeds have distinctive and easily recognized flowers borne in hemispherical to spherical umbels. The corona consists of prominent erect cup- or hood-like structures, in most species surrounding a little horn or prolonged tooth. Measurements of flower length always include mature reflexed corolla lobes.
1. Leaves very narrowly linear, less than 2.5 (–4) mm broad, mostly whorled or nearly so.
1. Leaves all or mostly over 4 mm broad (usually much over), alternate or opposite.
2. Leaves all or mostly alternate.
3. Flowers greenish, purple tinged, ca. 7–9 mm long, the corona hoods without a horn; umbels axillary; sap milky.
3. Flowers (yellow-) orange, ca. 11–15.5 mm long, the corona hoods enclosing an elongate awn-like horn; umbels mostly terminal; sap clear.
2. Leaves all or mostly opposite.
4. Blades of leaves ± cordate or subcordate, sessile (or on petioles shorter than the basal lobes of blade).
5. Horns clearly exserted beyond the hoods; umbels solitary, terminal, with pedicels all or mostly puberulent at least toward base.
5. Horns shorter than the hoods (thus largely concealed); umbels usually 2 or more, with glabrous pedicels.
4. Blades of leaves ± rounded or tapered to a short or long petiole.
6. Horns conspicuously surpassing hoods; leaves glabrous or with thin puberulence beneath.
7. Flowers white or greenish, (13–) 14–17 (–18) mm long, on pedicels 2.5–4.5 cm long at maturity; follicles ca. 9.5–13 (–15) cm long.
7. Flowers rich rose-purple (rarely albino), 7–10.5 mm long, on pedicels 0.8–1.6 (–2.4) cm long; follicles ca. (5.5–) 6.5–9 cm long.
6. Horns shorter than hoods (or none); leaves with ± dense and uniformly distributed short pubescence beneath (sometimes glabrate in A. viridiflora).
8. Flowers 8–11 (–12) mm long; corolla lobes whitish, cream, or greenish (at most the lobes tinged with purplish beneath).
9. Hoods distinctly exceeding anthers, with a horn; pedicels much longer than flowers; plants small and slender, the stem less than 3 (–4) mm thick, with 4–6 (–7) pairs of leaves.
9. Hoods slightly shorter than winged tips of anthers, without a horn; pedicels mostly about equaling or shorter than the flowers; plants stouter, the stems usually 3–5 mm thick toward base, with 5–12 pairs of leaves.
8. Flowers 10.5–25 mm long; corolla lobes rose to purple on both surfaces (plants with small and/or pale flowers may be separated from the previous couplet by their stature too large for A. ovalifolia and the pedicels and hoods too long for A. viridiflora).
10. Hoods ca. 10–13 mm long, including a prolonged tongue-like apex; pedicels densely white-tomentose.
10. Hoods ca. (3–) 3.5–7 mm long, without a prolongation; pedicels glabrate to tomentose.
11. Corolla lobes glabrous; plant with a terminal umbel and sometimes 1 or 2 additional umbels in upper axils; hoods ca. 5–7 mm long; flowers (9–) 13–17 (–18) mm long.
11. Corolla lobes pubescent outside; plant usually with umbels in 3 or more axils; hoods (3–) 3.5–5 mm long; flowers 10.5–15 mm long.