In Missouri, milkweeds are perennial herbs or twining vines. Most have white latex (milky sap), but some have clear sap. The leaves are simple, most commonly opposite; the blades variously shaped but often somewhat wavy.
There are 22 species in 4 genera of milkweeds in Missouri: Asclepias (milkweeds; 17 species), Cynanchum (2 species; sand vine is the most common), Gonolobus (angle-pod; 1 species), and Matelea (climbing milkweeds; 2 species).
The unique, 5-parted flowers of milkweeds grow in rounded clusters (umbels). The 5 calyx lobes are spreading or reflexed. The 5 corolla lobes are spreading, reflexed, or erect. In the center of the flower, the stigmas and anthers are fused into a 5-lobed or angled headlike structure called a gynostegium. Surrounding the gynostegium is a 5-parted corona (crown) of variously shaped petal-like structures that can resemble hoods or horns. Pollen is contained in pairs of tiny sacs called pollinia; these entire sacs are used in pollination. There are 2 separate pistils per flower, below the gynostegium. Each pistil has a single chamber.
The fruits (follicles) are podlike, sometimes growing in pairs. Each fruit contains numerous seeds, which are usually flattened and have a tuft of long, silky hairs at the tip.
Varies with species. Some are stout plants 3 feet high, some are vines with long twining stems, and so on.
Statewide. Different species have different habitat preferences and different distributions within the state.
Habitat and Conservation
Many of Missouri’s milkweeds are weedy and grow in open, disturbed soils, such as roadsides, pastures, and agricultural areas. Others are associated with high-quality native habitats such as prairies.
The increasing use of herbicides has drastically reduced agricultural and roadside weeds, including milkweeds. This is a problem for the monarch butterfly, which requires milkweeds as larval host plants. Monarch numbers have been plummeting in large part due to the dramatic reduction of milkweeds. In response, many people are cultivating milkweeds to help the monarchs survive.
The entire former milkweed family (Asclepiadaceae) has fairly recently been rolled into the dogbane family (Apocynaceae). Botanists have long known the two families were closely related. In about 2000, they announced that DNA evidence showed they were so close genetically that calling them separate families was not warranted. The milkweed group, with its distinctive floral structures, is yet considered a unique subfamily or tribe of the dogbane family.
A milkweed plant has many flowers, but relatively few fruits develop. Their unusual pollination biology, which promotes outcrossing, causes this. To be pollinated, a milkweed flower must be visited, first, by one insect pollinator, whose leg enters a slot in the flower and removes a pollen-bearing structure. Then it flies away with it stuck on its leg, perhaps to pollinate another milkweed. Meanwhile, the original milkweed flower waits for another insect to bring pollen from another milkweed.
Milkweeds have a long list of historical medicinal uses, and the milky latex was once explored as a potential source of rubber. The silky floss of the seedpods has been used for stuffing pillows and life preservers. Today, people are planting milkweeds to help monarch butterflies survive.
Many bees, butterflies, and skippers drink nectar from the flowers, and crab spiders often hide in the clusters, hunting them. Monarch butterflies use milkweeds as larval food plants, collecting the sap’s toxic cardiac gycosides in their bodies and becoming unpalatable to predators.