narrow-leaved milkweed

not listed

NNR - Unranked

S1 - Critically Imperiled

Endangered

Narrow-leaved milkweed is an erect, perennial herb that rises on 1 or occasionally 2 stems from a carrot-like vertical root. The leaves and stems contain a white milky juice.

The stems are erect, mostly unbranched, green or purple, and have numerous leaves. The upper stem is covered with fine, short hairs that point downwards. The lower stem is mostly hairless.

The leaves are alternate but the alternating leaves may be close together on the stem and appear almost opposite or almost whorled. They are linear and 2⅓″ to 6″ long, less than ⅛″ to ¼″ wide, and have pointed tips. On the lower part of the stem the leaves are on short leaf stalks, on the upper part they are stalkless or nearly stalkless. The margins are untoothed and are often rolled backward toward the underside.

The inflorescence is few to several umbrella-shaped clusters (umbels) rising from the upper leaf axils. The clusters are 2″ to 3″ in diameter and are stalkless or nearly stalkless. They have 10 to 25 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 ⅓″ tall. The petals are whitish green to yellow or slightly purplish. They bend backward at the base, hang downward, then curl upward near the tip. They are attached directly below the hoods without a separating column. The hoods are whitish green. The horn is fused to the hood for most of its length, forming a rib. A short tip is free and projects between the terminal loges of the horn. Together the appearance is of a hood with a 3-lobed tip and no horn.

The fruit is a narrow, spindle-shaped pod. It is 3½″ to 4¾″ long and ¼″ to ⅓″ wide. It is held erect on a downward-curving stalk. It opens on one side exposing the seeds to spreading by the wind. The seeds have a tuft of tan hairs at the tip.

8″ to 40″

Pale greenish to yellow or slightly purplish

Whorled milkweed (Asclepias verticillata) is a shorter plant, reaching no more than 20″ at maturity. The leaves are in whorls of 3 to 6 and are shorter, ¾″ to 3″ long. The clusters are on a ½″ to 1½″ long stalk. The petals are separated from the hoods by a distinct column. The horns are destinctly separated from the hoods. The fruit is on an erect stalk.

Dry. Prairies, wood openings.

July 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, 6, 28, 29, 30.

Native

Rare