Milkweeds

Subfamily Asclepiadoideae

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Photo of whorled milkweed flowers.
This cluster of whorled milkweed blossoms exemplifes the unique look of milkweed flowers, recognizable at a glance.
Family

Apocynaceae (dogbanes); formerly Asclepiadaceae (milkweeds)

Description

In Missouri, milkweeds are perennial herbs or twining vines. Most have white latex (milky sap), but some have clear sap. Leaves are simple, most commonly opposite; blades variously shaped but often somewhat wavy. The unique, 5-parted flowers 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.

Size

Varies with species. Some are stout plants 3 feet high, some are vines with long twining stems, etc.

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Photo of prairie milkweed, or tall green milkweed, fruits.
Prairie Milkweed (Tall Green Milkweed) (Fruits)
Prairie milkweed, like most other milkweeds, bears pods holding numerous seeds, each with a parachute of silky hairs.

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Photo of common milkweed flower cluster
Common Milkweed

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Photo of butterfly weed flowers
Butterfly Weed (Flowers)

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Photo of fourleaf milkweed plant with flower clusters
Fourleaf Milkweed (Whorled Milkweed)
One of our earliest blooming milkweeds, fourleaf milkweed bears round clusters of pink or cream-colored flowers.

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Photo of swamp milkweed, three plants with flower clusters.
Swamp Milkweed
Swamp milkweed’s flower clusters form at the tops of the stems.

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Photo of swamp milkweed, green fruit pods on plant.
Swamp Milkweed Pods
Swamp milkweed bears paired, 4-inch, slender pods that are smooth but usually slightly hairy.

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Photo of sand vine, leaves with flower cluster.
Sand Vine (Climbing Milkweed; Blue Vine)
Beloved by bees, butterflies, and other insects for its nectar, sand vine is also a problem weed that can be difficult to eradicate.

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Photo of sand vine flower cluster.
Sand Vine (Climbing Milkweed)
Sand vine is a native milkweed vine that provides needed nectar for monarch butterflies as they migrate southward in late summer.

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Photo of climbing milkweed flowers and leaves.
Climbing Milkweed
The brown, starlike, spreading flowers of climbing milkweed differ from those of other milkweeds.

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Mead's Milkweed
Mead's Milkweed

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Great Spangled Fritillary, Wings Spread
Great Spangled Fritillary, Wings Spread
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.

image of Milkweeds distribution map
Distribution in Missouri

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).

Status

The entire former milkweed family (Asclepiadaceae) has 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.

Life cycle

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.

Human connections

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.

Ecosystem connections

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.