Arizona or narrow-leaved milkweed

Asclepias linifolia Kunth

Subshrubs or herbs, cespitose. Stems few–numerous, erect, sparsely to moderately branched, 20–60 cm, puberulent with curved trichomes in a line, not glaucous, rhizomes absent. Leaves persistent or gradually caducous from the base, opposite, petiolate, with 1 stipular colleter on each side of petiole on a ciliate interpetiolar ridge; petiole 2–4 mm, surfaces puberulent with curved trichomes to gla­brate; blade linear, 3.5–10 × 0.2–0.8 cm, chartaceous, base cuneate, margins entire, apex acute, venation faintly brochidodromous to obscure, puberulent with curved trichomes on midvein, margins ciliate, laminar colleters absent. Inflorescences extra-axillary at upper nodes, pedunculate, 3–15-flowered; peduncle 0.7–5 cm, puberulent with curved trichomes in a line, with 1 caducous bract at the base of each pedicel. Pedicels 8–18 mm, puberulent with curved trichomes in a line. Flowers erect; calyx lobes narrowly lanceolate, 2–3 mm, apex acute, puberulent with curved trichomes; corolla cream, sometimes pink-striped or -tinged, to pink, lobes reflexed with spreading tips, elliptic, 4–5 mm, apex acute, glabrous abaxially, minutely papillose at base adaxially; gynostegial column 0.8–1 mm; fused anthers brown, columnar, 1.5–2 mm, wings narrowly right-triangular, closed, apical appendages deltoid; corona segments cream, sometimes pink at base, stipitate, tubular, dorsally rounded, 3–3.5 mm, exceeding style apex, apex obtuse, glabrous, internal appendage acicular, exserted, arching over style apex, glabrous; style apex shallowly depressed, cream. Follicles erect on straight pedicels, narrowly fusiform, 4–10 × 0.4–0.7 cm, apex long-acuminate to attenuate, smooth, glabrous. Seeds ovate, 4.5–6 × 3–4 mm, margin narrowly winged, faces smooth; coma 2–3 cm.

Flowering Apr–Oct; fruiting Jul–Oct. Canyon bot­toms, streamsides, arroyos, sandy or rocky soils, ripar­ian forest, pine-oak-juniper forest; 1000–1800 m; Ariz.; Mexico.

The common name Arizona milkweed is misleading, as Asclepias angustifolia is narrowly distributed and uncommon in Arizona but widely distributed and locally common across mountainous regions of Mexico. Like A. linaria, this widespread species is probably relictual in its canyon habitat in southern Arizona. Asclepias angustifolia has been documented from the Atascosa, Dragoon, Huachuca, Pajarito, Rincon, Santa Catalina, and Santa Rita mountains (Cochise, Pima, and Santa Cruz counties). As with many milkweeds, the floral displays of well-developed plants are magnets for large numbers of Lepidoptera and Hymenoptera. Like A. curassavica and other members of the Incarnatae clade (species 7–16), A. angustifolia develops rapidly from seed and transplants well, making it a desirable species for the garden. Although its frost hardiness is limited, it can be used as a horticultural annual.