Asclepias syriaca

common milkweed

By Will Clausen, 2012 Chester B. Allen Horticulture Fellow

As we quickly transition into summer, attention in the garden moves from the woodland spring ephemerals to their taller meadow relatives. The peak time for blooming meadow plants is still a month or two away, but if you wander out into the sunlight now you’ll find a few plants already adding some splashes of color to the landscape. One such early plant is common milkweed (Asclepias syriaca). A botanical beauty in its own right, it is also an important habitat plant.

Long, ovate leaves are oppositely arranged and light green with distinctly red veins. In late spring to early summer, clusters of green buds appear in the upper leaf axils. These buds quickly turn pink and then open to reveal the unique flowers typical of all milkweeds. Each flower has five downward oriented petals with five pink-purple hoods positioned above the petals. The fragrant inflorescences bloom early in the summer with light green warty seed pods up to four inches long developing midsummer. The pods turn brown as they dry out and eventually split open to release the feathery seeds which are dispersed by the wind.

This medium sized herbaceous perennial can spread quickly in medium soils with full sun. Because of its aggressive rhizomatous growth it can become weedy, but in shadier spots or places with well-established plants, the spread is slower and it can be managed. Other milkweed species are less vigorous spreaders and perhaps better suited for the hands-off gardener.

Few plants have a more well-known faunal association than common milkweed. Where you find milkweed, you’re likely to find monarch butterflies and where you find monarchs, you’re likely to find milkweed. The lifecycle of the monarch depends upon this one plant. Despite the great diversity of plant life in meadows, monarchs will only lay their eggs on the underside of common milkweed leaves. The leaves provide food for the young caterpillars, and a place for chrysalises to be attached. Adult monarch butterflies find nectar waiting for them in the flowers that bloom throughout the summer. Monarch caterpillars are immune to the heart arresting compound called cardenolide which is found in the milky sap of milkweed. In a great example of the marvels of nature and a result of munching on milkweed leaves, monarchs become toxic to all but a select few of their feathery predators. So flows the life saving toxin.

The first blooms of common milkweed are among the harbingers of all things summer; always a welcome sight to northern eyes.