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Herbs. Stems 1 (rarely more, but forming dense colonies), erect, unbranched (rarely branched), 50–200 cm, tomen­tose to puberulent with curved trichomes, not glaucous, rhizo­matous. Leaves opposite, petio­late, with 1–5 stipular colleters on each side of petiole; petiole 5–15 mm, tomentose to puberulent with curved trichomes; blade oval or ovate to oblong or elliptic, 6–30 × 2.5–11 cm, chartaceous, base cuneate to rounded or truncate, margins entire, apex obtuse to rounded or acute, often mucronate, venation brochidodromous, sur­faces tomentose to pilosulous abaxially, tomentose to gla­brate adaxially, margins ciliate, 4–20 laminar colleters. Inflorescences extra-axillary, pedunculate, 24–113-flowered; peduncle 2–12 cm, tomentulose to pilose or puberulent with curved trichomes, with 1 caducous bract at the base of each pedicel. Pedicels 17–40 mm, densely pilose to puberulent with curved trichomes. Flowers erect to pendent; calyx lobes elliptic, 3–6 mm, apex acute, tomentulose; corolla dark to pale pink or green and pink-tinged, lobes reflexed with spreading tips, oblong to oval, 6–9 mm, apex acute, pilose abaxially, minutely hirtellous at base adaxially; gynostegial column 1–1.5 mm; fused anthers green, cylindric, 2–2.5 mm, wings narrowly right-triangular, slightly open, apical appendages ovate; corona segments reddish purple to cream, sessile, tubular, slightly flattened dorsally, 4–5 mm, exceeding style apex, apex obtuse, somewhat to strongly flared, glabrous, internal appendage falcate, exserted, sharply inflexed over style apex, glabrous; style apex shallowly depressed, green or pale to dark pink. Follicles erect on upcurved pedicels, lance-ovoid to ovoid, 7–12 × 2–4 cm, apex acuminate, smooth or muricate, tomentose. Seeds narrowly ovate, 7–8 × 4–5 mm, margin winged, faces rugulose; coma 3–4 cm. 2n = 22.

Phenology: Flowering May–Sep(–Oct); fruiting Jun–Oct.
Habitat: Flats, slopes, ridges, valleys, fields, meadows, pastures, ditches, pond and lake edges, marshes, bogs, fens, parks, urban lots, streamsides, swales, bluffs, sandhills, limestone, clay, silty, sandy, and rocky soils, prairies, forest openings and edges, riparian woods.
Elevation: 0–1300 m.

Distribution

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Man., N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Que., Sask., Ala., Ark., Conn., Del., D.C., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Pa., R.I., S.C., S.Dak., Tenn., Vt., Va., W.Va., Wis, introduced in Europe, sw Asia.

Discussion

Asclepias syriaca is surely the most familiar milkweed in North America, and one that evokes ambivalence. It has been considered an undesirable species because of its prolific rhizomatous spread and ability to invade and thrive in cultivated land. However, it has been used as a food plant by indigenous and colonizing peoples, and its pleasantly fragrant and nectariferous flowers are avidly sought by diverse insects, highlighting the ecolog­ical importance of A. syriaca. Moreover, it has come to be appreciated because of its importance as one of the most commonly utilized host plants of the monarch but­terfly, Danaus plexippus. There is some evi­dence that its population in agricultural lands in the upper mid­western United States has dramatically declined in the last several decades. However, its range and abundance prior to European settlement are not well understood and may have been much lower than in historical times, particularly in deforested areas of the eastern United States. It is considered to have been introduced to Newfoundland, Alabama, Georgia, Louisiana, Mississippi, and South Carolina and has been reported, but without documentation, from Texas. It has been documented from Salem, Oregon, but appears to not be established there (R. Halse, pers. comm.). It is considered to be of conservation concern at the northwesternmost edge of its range in Saskatchewan, where a single population is known in Estevan Municipality. Asclepias syriaca is the most promiscuous of milkweeds, as it is known to hybridize with at least seven other species (A. amplexicaulis, A. exaltata, A. ovalifolia, A. purpurascens, A. speciosa, A. sullivantii, A. tuberosa). Hybrids with A. speciosa are most frequent, as discussed under that species. Hybrids with A. exaltata are not infrequently encountered in the Appalachian Mountains and elsewhere. Other hybrids are highly localized. In all cases, putative hybrids are inferred from intermediate floral and vegetatative traits.

Selected References

None.

Lower Taxa

None.