Narrow-leaved or Mexican or Mexican whorled milkweed

Herbs. Stems few–numerous, erect, sparsely to moderately branched, 50–150 cm, glabrous, not glaucous, rhizomes absent. Leaves 3–5-whorled, sessile or petiolate, with 1–3 stipular colleters on each side of petiole on a ciliate interpetiolar ridge; petiole 0–4 mm, margins puber­ulent with curved trichomes; blade linear to linear-lanceolate or narrowly elliptic, often somewhat condu­plicate, 4.5–13 × 0.2–1.8 cm, membranous, base cune­ate, margins entire, apex acute or attenuate to obtuse, mucronate, venation obscure to faintly eucamptodro­mous, surfaces glabrous, margins eciliate, laminar col­leters absent. Inflorescences terminal and extra-axillary at upper nodes, sometimes branched, pedunculate, 10–37-flowered; peduncle 0.4–5.5 cm, puberulent with curved trichomes in a line to glabrate, with 1 caducous bract at the base of each pedicel. Pedicels 9–14 mm, puberulent with curved trichomes to pilosulous. Flowers erect; calyx lobes lanceolate, 1.5–2 mm, apex acute, puberulent with curved trichomes to pilosulous; corolla pale to dark pink, rarely pale green with a pink tinge, lobes reflexed with spreading tips, oval, 3–5 mm, apex acute, glabrous abaxially, minutely papillose at base adaxially; gynostegial column 1–1.5 mm; fused anthers green, cylindric, 1.5–2 mm, wings narrowly right-triangular, closed, apical appendages deltoid; corona segments cream, often tinged or striped pink, stipitate, cupulate, dorsally somewhat flattened, 1.5–2 mm, exceeded by style apex, apex obtuse, glabrous, internal appendage acicular, exserted, arching towards style apex, glabrous; style apex shallowly depressed, cream. Follicles erect on straight pedicels, fusiform, 6–9 × 0.5–1 cm, apex attenuate, smooth, glabrous. Seeds ovate, 6–7 × 4–5 mm, margin winged, faces minutely rugulose; coma 2.5–3 cm. 2n = 22.

Flowering Apr–Oct; fruiting (Jun–)Jul–Nov. Valleys, slopes, hills, streamsides, ditches, seeps, hot springs, wet depressions, arroyos, vernal pools, basalt, granite, limestone, clay, sandy, and silty soils, native, non-native, and shrubby grasslands, oak, pine-oak, juniper, pinyon-juniper, and riparian woodlands, chaparral, coastal sage scrub, pine and mixed-conifer forests, sometimes following fires; 0–2300 m; Calif., Idaho, Nev., Oreg., Wash.; Mexico (Baja California).

Asclepias mexicana Decaisne was misapplied to A. fascicularis in the past; this is the legitimate name of a related species endemic to southern and eastern Mexico. The common name Mexican (whorled) milkweed stems from this past confusion. Compared to its close relatives A. angustifolia, A. linearis, A. pumila, A. subverticillata, and A. verticillata, the leaves of A. fascicularis are not particularly narrow (despite the implication of another common name). However, very narrow leaves are found in A. fascicularis when it is growing at relatively dry sites, especially at the eastern limit of its range in southeastern Idaho. Such specimens (for example, Mumford 272 [MO], Atwood 28495 [NY]) have been attributed in the past to A. subverticillata in error. Asclepias fascicularis is completely allopatric with its Incarnatae clade relatives (species 7–16). Like these species, it is easily cultivated, and its seeds are widely available. In Washington, the range of A. fascicularis is restricted largely to the valleys of the Columbia and Spokane rivers and in Idaho to the Snake and Weiser rivers.