Milkweed is the common name of many plant species in the genus Asclepias, a genus that contains more than two hundred species that are distributed across North America, South America, and Africa. The common name comes from the plants' abundant, milky sap that oozes from nearly any broken tissue, from the flower to the root. This sap contains latex as well as a toxin group known as cardiac glycocides. And it's this toxin that Monarch butterflies depend on to make themselves toxic and unpalatable to predators. More on Monarchs later...
Not all milkweeds are built the same. Some are obedient garden bloomers that produce clusters of vibrant, sweet smelling flowers from late spring through summer. Other species employ multiple strategies to spread, notably through fast-growing rhizomes below the soil and by silk-bearing, dandelion-like seeds that spread by wind.
But the whole genus is unified by the unusually complex structure of their individual flowers. More in-depth botanical descriptions of the flowers exist on the Internet, but two features stand out. First, the flowers are particularly generous with nectar, which makes them excellent food plants for a wide range of insects. Secondly, the structure of the flower includes a "stigmatic slit," which actually traps the leg of a visiting insect. In its struggle to free itself, the insect is dabbed with a pollen-laden "pollinia" in an effort to fertilize the next milkweed flower it visits.
In all species, the flowers appear in clusters that open all at once, so they are showy and ornamental garden accents. Once pollinated (and they will always get pollinated), the flowers droop and then form a single conspicuous seed pod per each flower cluster. As summer passes, the seed pod begins to split along its length revealing overlapping rows of relatively large brown seeds. To each is attached a group of white, silky hairs known as the coma, or pappus. The seed pod can be easily removed to prevent self-sowing, but each stem bearing a seed pod can also be cut and dried for long lasting dried flower arrangements.
Monarch Butterflies
All butterflies (including moths), as well as a wide range of bees, flies, beetles, and even hummingbirds, will feed on the nectar-heavy flowers of all milkweed varieties. The Monarch, however, seeks milkweed out on which to lay her eggs. Monarch caterpillars require milkweed to feed on prior to pupating, and they tend not to thrive when presented with alternative food sources. Planting milkweed is thought to be the number one step North American gardeners can take to help the endangered Monarch.
Even in gardens that are outside of the Monarch's main migration routes, milkweed will attract and support bumble bees, carpenter bees, yellow-faced bees, plasterer bees, sweat bees, and leaf-cutting bees. The White-Lined Sphinx moth hovers as it feeds from milkweed flowers. Like the Monarch, the Milkweed Tussock moth and Queen butterfly will also lay their eggs on the plants so that their larvae can eat the leaves.
Other butterflies that are avid fans of milkweed include the Easter Tiger Swallowtail, the Pipevine Swallowtail, the Red Admiral, the Great Spangled Fritillary, the American Copper, Edward's Hairstreak and the Delaware Skipper. The beneficial predatory Syrphid fly (aka hover-fly) is attracted to milkweed flowers, as are long-horned beetles.
Milkweed Varieties
While Common Milkweed (A. syriaca) is still considered a noxious weed in some regions, several other species make excellent garden plants.
When presented with several species of milkweed side by side, the Monarch female appears to prefer Swamp Milkweed (A. incarnata) on which to lay her eggs. This perennial can reach 120cm (48") in height, and bears large clusters of bright pink flowers. It's deer-resistant and hardy to Zone 3.
Showy Milkweed (A. speciosa) is native from BC to the Maritimes and found across the northern USA. It is also a perennial, but a bit shorter at 90cm (36") tall, and produces clusters of pale pink flowers that have a distinctive starry shape. This plant produces rhizomatous roots that spread with gusto - simply grow it in a container to control any unwanted spread.
The bright orange colour of Butterfly Bush Milkweed (A. tuberosa) attract a wide range of butterflies, but it's also irresistible to bees. It's a bit shorter still and rarely reaches 90cm (36") tall. Also perennial, it's hardy to Zone 4 and blooms from late spring right through summer. Grow this one in full sun, in well drained soil.
All milkweed species can be started indoors in late winter (February and March on the coast) and transplanted out in mid-spring. Otherwise direct sow the seeds in late spring or early autumn. More on How to Grow Milkweed.