Swamp Milkweed

Photo of swamp milkweed, three plants with flower clusters.
Scientific Name
Asclepias incarnata
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.

Where To Find

Scattered nearly throughout the state.

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.

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.

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.

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.

Media Gallery
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
1/12
Swamp milkweed flower clusters in bloom at Twin Lakes, Columbia, Missouri
Swamp milkweed is a native wildflower visited by many kinds of pollinating insects.
Julianna Schroeder
2/12
Swamp milkweed stalk showing leaves
The leaves of swamp milkweed are opposite, lance-shaped, and to 6 inches long. The foliage feeds monarch butterfly caterpillars and several other insects that specialize in milkweeds.
Julianna Schroeder
3/12
Photo of swamp milkweed colony in bloom.
Swamp milkweed occurs in moist, open bottomlands, especially in the Missouri and Mississippi floodplains.
Jim Rathert
4/12
Photo of swamp milkweed flower cluster.
Swamp milkweed’s flowers have the same unique flower structure of most other milkweeds.
Rob Routledge, Sault College, Bugwood.org
5/12
Photo of swamp milkweed, top of plant with flower cluster.
Swamp milkweed has its pink flowers at the tops of the stalks, rather narrow leaves, and a moist bottomland habitat.
Rob Routledge, Sault College, Bugwood.org
6/12
Photo of swamp milkweed, green fruit pods on plant.
Swamp milkweed bears paired, 4-inch, slender pods that are smooth but usually slightly hairy.
Catherine Herms, Ohio State University, Bugwood.org
7/12
Photo of swamp milkweed, dry fruit opening to show seeds.
Swamp milkweed pods dry and split open, releasing seeds with silky parachutes.
Catherine Herms, Ohio State University, Bugwood.org
8/12
butterfly on a pink flower
Noppadol Paothong
9/12
bright flowers outside a house
Noppadol Paothong
10/12
Photo of several swamp milkweed flower clusters.
Swamp milkweed is popular with native plant gardeners who wish to help monarch butterflies.
Jim Rathert
11/12
Photo of swamp milkweed and prairie cordgrass at Ripgut Prairie NA
Swamp milkweed is one of the notable plants to be seen in the wet bottomland prairie habitat at Ripgut Prairie NA in Bates County.
Jim Rathert
12/12
Photo of swamp milkweed, three plants with flower clusters.
Swamp milkweed flower clusters in bloom at Twin Lakes, Columbia, Missouri
Swamp milkweed stalk showing leaves
Photo of swamp milkweed colony in bloom.
Photo of swamp milkweed flower cluster.
Photo of swamp milkweed, top of plant with flower cluster.
Photo of swamp milkweed, green fruit pods on plant.
Photo of swamp milkweed, dry fruit opening to show seeds.
butterfly on a pink flower
bright flowers outside a house
Photo of several swamp milkweed flower clusters.
Photo of swamp milkweed and prairie cordgrass at Ripgut Prairie NA
Similar Species
About Wildflowers, Grasses and Other Nonwoody Plants in Missouri
A very simple way of thinking about the green world is to divide the vascular plants into two groups: woody and nonwoody (or herbaceous). But this is an artificial division; many plant families include some species that are woody and some that are not. The diversity of nonwoody vascular plants is staggering! Think of all the ferns, grasses, sedges, lilies, peas, sunflowers, nightshades, milkweeds, mustards, mints, and mallows — weeds and wildflowers — and many more!