whorled milkweed

not listed

N5 - Secure

SNR - Unranked

not listed

FACU - Facultative upland

FACU - Facultative upland

UPL - Obligate upland

Whorled milkweed is a 8″ to 20″ tall, erect, perennial herb that rises from a rhizome and fleshy roots. It often forms colonies. The leaves and stems contain a milky juice.

The stems are erect, unbranched below the inflorescence, and ridged. They have short, soft hairs in lines on the ridges.

The leaves are numerous in whorls of 3 to 6. They are linear and are attached to the stem without a leaf stalk. They may be erect, semi-erect, or spreading. The blades are ¾″ to 3″ long, less than ⅛″ wide, with pointed tips. The margins are untoothed and rolled backward toward the underside.

The inflorescence is several loose, small, umbrella-shaped clusters (umbels) rising from the upper leaf axils. The clusters are 2″ to 3″ in diameter and are on a ½″ to 1½″ long stalk. They typically have 10 to 20 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. They are about ¼″ tall, ⅓″ wide, and are attached on ¼″ to ½″ long thread-like stalks. The petals are greenish white and are tinged with purple near the tip. 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 hoods are white. The horns are white and are longer than the hoods. They project from the hoods and bend inwards. There is little or no fragrance.

The fruit is a narrow, spindle-shaped pod. It is 3″ to 4″ long and ¼″ to ⅓″ wide. It is held erect or ascending on an erect stalk. It opens on one side exposing the seeds to spreading by the wind. The seeds have a tuft of hairs at the tip that are whitish and about 1⅛″ long.

8″ to 20″

White horns, greenish-white petals tinged with purple

Green milkweed (Asclepias viridiflora) leaves are much wider, lance-shaped to oblong, mostly opposite, with wavy margins. It usually has just 1 to 3 flower clusters. The clusters have 20 to 80 flowers. The flowers are not tinged with purple. The petals are attached directly to the hoods, not separated by a column.

Prairie milkweed (Asclepias hirtella) is a much taller plant, 16″ to 40″ tall at maturity. The leaves are longer and narrower, lance-shaped to linear, and alternate. It has 2 to 10 flower clusters. The clusters have 30 to 100 flowers.

Dry. Prairies, fields, open upland woods, roadsides.

July to September

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, 30.

Native

Whorled Milkweed

Plant

Inflorescence

Flowers

Leaves

Infructescence

Location: Kellogg Weaver Dunes SNA, Weaver Dunes Unit