Swamp Milkweed

Asclepias incarnata

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Photo of swamp milkweed, three plants with flower clusters.
Swamp milkweed’s flower clusters form at the tops of the stems.
Rob Routledge, Sault College, Bugwood.org
Family

Apocynaceae (dogbanes); formerly Asclepiadaceae (milkweeds)

Description

Swamp milkweed is a perennial herb. Stems smooth, tall, flexible, branching. Sap milky latex. Flowers mostly terminal (positioned at the tops of the stems), in many loose umbels (each rounded cluster arises from the same point); pink or rarely white, with a delicate fragrance. Blooms June–September. Leaves mostly opposite, narrowly lance-shaped, to 6 inches long and about 1 inch wide. Fruits slender pods, to 4 inches long, in pairs, smooth but usually slightly hairy, bearing seeds with silky parachutes.

Similar species: Swamp milkweed can be distinguished from similar milkweeds by its pink-flowered umbels (flower clusters) positioned at the tops of the plant stalks (not arising from the sides of the stems), relatively narrow leaves, and moist bottomland habitat.

Size

Height: to 6½ feet.

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Photo of swamp milkweed colony in bloom.
Swamp Milkweed Colony
Swamp milkweed occurs in moist, open bottomlands, especially in the Missouri and Mississippi floodplains.

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Photo of swamp milkweed flower cluster.
Swamp Milkweed Flowers
Swamp milkweed’s flowers have the same unique flower structure of most other milkweeds.

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Photo of swamp milkweed, top of plant with flower cluster.
Swamp Milkweed
Swamp milkweed has its pink flowers at the tops of the stalks, rather narrow leaves, and a moist bottomland habitat.

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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 swamp milkweed, dry fruit opening to show seeds.
Swamp Milkweed Seeds
Swamp milkweed pods dry and split open, releasing seeds with silky parachutes.

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Photo of several swamp milkweed flower clusters.
Swamp Milkweed
Swamp milkweed is popular with native plant gardeners who wish to help monarch butterflies.

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Photo of swamp milkweed and prairie cordgrass at Ripgut Prairie NA
Swamp Milkweed at Ripgut Prairie Natural Area
Swamp milkweed is one of the notable plants to be seen in the wet bottomland prairie habitat at Ripgut Prairie NA in Bates County.
Habitat and conservation

Occurs in moist, open bottomlands, especially in the Missouri and Mississippi floodplains, where it is often the dominant plant. Look for it in swamps, sloughs, marshes, margins of ponds and lakes, banks of streams and rivers, bottomland prairies, and occasionally bottomland forests; also ditches and railroads.

Distribution in Missouri

Scattered nearly throughout the state.

Status

The entire former milkweed family (Asclepiadaceae) has recently been rolled into the dogbane family (Apocynaceae). For many years, botanists have known the two families were closely related. The milkweed group, with its distinct floral structures, is still considered a unique subfamily or tribe of the dogbane family. As you consult various sources, you can expect to see milkweeds grouped in either family.

Human connections

Because they are a required larval food plant of monarchs, milkweeds are increasingly popular with native plant gardeners concerned about the decline of those butterflies, and swamp milkweed is a favorite for this. Young shoots and leaves of swamp milkweed, however, are browsed by rabbits and deer.

Ecosystem connections

The roots are eaten by muskrats and other wetland mammals. Monarch butterflies use milkweeds as larval food plants, collecting the sap’s toxic cardiac gycosides in their bodies and becoming unpalatable to predators. Many insects visit the flowers for nectar.