Asclepias angustifolia

The Arizona milkweed is a cold-hardy herbaceous perennial that forms a taproot, growing to 60–90 cm (24–35 in) in height. The stems arise from a single crown, and can be many.

Leaves - opposite, linear with short petioles, glabrous, 4–12 cm (1.6–4.7 in) in length, 2–11 mm (0.079–0.433 in) in width; Inflorescence - 2–4 cm (0.79–1.57 in), erect umbel, single peduncle per node, with one or more peduncles per stem, typically borne towards the top of the stem; Flowers - 3 mm (0.12 in) in diameter and in length, pedicels 5 mm (0.20 in), horns extend beyond the hoods, corollas reflexed, flower color ranges from whitish to pink; Pods - slender, upright, 3.5–9 cm (1.4–3.5 in) long, containing few to many seeds; Seeds - ca. 4mm in length, attached to white silky coma c. 4 mm (0.16 in) in length.[4][5]

Found at higher elevations (around 1,050 to 2,100 metres [3,440 to 6,890 ft]), it is known only from Pima, Santa Cruz, and Cochise counties in Arizona, United States. Native substrate mostly consists of dry rocky soils.[5] Habit includes riparian woodlands, floodplain meadows, cienega edges, canyons, and arroyo bottoms. It is considered rare in Arizona, and restricted to the borderlands. This species is a known host plant to the Monarch Butterfly.[1] It has been observed to be pollinated by small species of Carpenter Bees (Ceratina sp.).[6]

The Arizona milkweed is commercially available by both seed and propagated plants. Seed readily germinates, and mature flowering plants can be grown in as little as three months. Plants can be successfully grown in containers as small as a quart. This species is tolerant of excess watering and lack of watering, in which substrate has not been seen to be an important factor in growth. Substrates utilized have been both organic-rich and replacement to sandy loam profiles.[6] Cold hardiness has been noted to −12 °C (10 °F).[5]