swamp milkweed

Swamp milkweed is an erect, perennial forb. One to several stems rise from a short, fibrous rhizome. It often grows in clumps. The leaves and stems contain a milky juice.

The stems are erect and usually repeatedly branched above the middle. They are usually hairless except for a vertical line of hairs on each side between the leaf nodes on the upper half.

The leaves are opposite, untoothed, 4″ to 7″ long, 1″ to 2″ wide, and lance-shaped or oblong. They gradually taper to a sharp point. They are attached to the stem on short leaf stalks. The lower surface is hairless.

The inflorescence is usually several large, dense, umbrella-shaped clusters (umbels) rising from the upper leaf axils and at the end of the stem. The clusters are 3″ to 4″ in diameter and erect on a hairy, 2″ long stalk. They typically have 20 to 30 flowers.

The structure of the typical milkweed flower is unique and instantly recognizable. There are 5 petals bent backward at the base and hanging downward. Subtending the petals are 5 much shorter, light green, lance-shaped sepals. There are 5 stamens. Formed from the filament of each stamen is a petal-like appendage. The appendage consists of a tubular hood surrounding an awl-shaped horn in the center of the hood. The stamens and the stigma are fused together into a crown-like structure (gynostegium). Each stigma has a long slit designed to catch the legs of a pollinating insect. A small, dark, sticky gland above this slit is attached to pollen sacs from adjacent anthers. These glands are designed to break off as an insect pulls its leg free of the slit, and remain attached to the insects leg. The flowers are pollinated by larger insects strong enough to lift off with the pollen sacs attached. Smaller insects are caught in a death trap or leave behind their detached legs.

The flowers of this plant are shaped like the typical milkweed flower and are fragrant. They are ⅜″ tall and ¼″ wide. The petals are pink to red. They bend backward at the base, hang downward, then curl upward near the tip. They are separated from the hoods by a distinct column. The horns are much taller than the hoods. They project from the hoods and are curved inwards.

The fruit is a narrow, spindle-shaped pod. It is 3″ to 5″ long and hairless. It is held erect on an erect stalk. It opens on one side exposing the seeds to spreading by the wind. The seeds have a tuft of silvery-white, silky hairs at the tip.

2′ to 5′

Pink or red

Common milkweed (Asclepias syriaca)stems are unbranched and hairy. The upper surface of the leaves are softly hairy. The umbels have 20 to 130 flowers. The horns are shorter than the hoods. The fruits are fat and are covered with warts.

Purple milkweed (Asclepias purpurascens) stems are unbranched and are covered with short, fine, soft hairs. The leaves are wider, elliptical or oblong, and downy on the underside. The umbels have up to 50 flowers. The horns are much shorter than the hoods.

Wet to moist. Open swamps, marshes, streambanks, ditches, wet prairies, wet fields, meadows. Full sun.

June to August

Milkweeds are the only plants that Monarchs lay their eggs on. The eggs are laid on the underside of healthy young leaves.

This and other milkweeds contain cardiac glycosides and may be poisonous to both humans and livestock.

Sources

2, 3, 4, 5, 7, 24, 28, 29, 30.

Native

Common

Plant

Plant

Plant

Buds

Inflorescence

Inflorescence

Flower

Flower

Gynostegium

Flower

Leaves

Leaves

Infructescence

Infructescence

Infructescence

Fruit

Fruit

Swamp Milkweed (Asclepias incarnata)
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Swamp Milkweed - Asclepias incarnata blooming at Ion Exchange
Ionxchange

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Monarch larva eating Swamp Milkweed
ii386

Location: Lake Shore, Cass County

Location: Joy Park

Location: Joy Park