Asclepias syriaca

Description

Asclepias syriaca is a tall, robust, rhizomatous perennial and the most abundant milkweed in eastern North America. Plants reach 60-180 cm in height, with thick, upright stems and large, opposite, broadly oblong leaves 15-30 cm long with prominent central veins and woolly undersides. All parts exude a sticky white latex when cut. The deep, spreading rhizome system allows the plant to form extensive colonies over time.

The flowers are among the most fragrant of any native wildflower: large, pendulous, rounded umbels 5-10 cm across, each bearing 30-100 individual pink to mauve florets. The scent is rich, sweet, and detectable from metres away. Seed pods (follicles) are large, 8-12 cm long, covered in soft, warty protrusions — a key distinguishing feature from other milkweeds. When ripe, they split open to release hundreds of seeds, each attached to a silky coma for wind dispersal.

Growing Conditions

Thrives in full sun and well-drained soils across an exceptionally broad range of conditions. Tolerates sandy, loamy, and clay soils, and is highly drought-tolerant once established. Its deep rhizomes allow it to persist in compacted, nutrient-poor, and disturbed soils where few other native perennials can compete. Often found along roadsides, railway embankments, old fields, and prairie remnants.

Hardy from Zone 3 through Zone 9. In Ontario, it is one of the most common and widely distributed native plants, occurring from the Carolinian zone to the boreal transition. The rhizomatous spread makes it aggressive in garden settings — best planted in large meadow or naturalized areas where it has room to roam.

Phenology

Emerges in late spring (May) from overwintering rhizome buds. Flowering extends from June through August, peaking in July. The fragrance is strongest in the evening, attracting nocturnal as well as diurnal pollinators. Seed pods develop through late summer and split open in September-October. The foliage turns yellow in fall before dying back to the ground. Winter dormancy is complete; above-ground stems are annual.

Ecology

Asclepias syriaca is the single most important Monarch butterfly host plant across the species' eastern North American range. Monarch caterpillars feed exclusively on Asclepias foliage, and Common Milkweed supports the largest share of the Monarch population due to its sheer abundance and distribution.

The flowers are a pollinator magnet, attracting an enormous diversity of bees (including Bombus), butterflies, moths, wasps, beetles, and flies. The copious nectar production makes it a keystone nectar source in mid-summer landscapes. Specialist milkweed herbivores — including the Large Milkweed Bug (Oncopeltus fasciatus), Milkweed Beetle (Tetraopes), and Milkweed Leaf Beetle (Labidomera clivicollis) — feed on the foliage, seeds, and latex, forming a distinctive insect community.

All parts of the plant contain cardiac glycosides, rendering it toxic to most vertebrate herbivores. Deer and rabbits avoid browsing it.

Propagation

Seeds require 30 days of cold-moist stratification. Sow in fall or stratify in a refrigerator for spring sowing. Plants grow rapidly from seed, often flowering in their second year. Rhizome divisions taken in early spring establish quickly and are the fastest way to increase plants. Given its aggressive rhizomatous nature, site selection should allow for spread, or install a root barrier.