narrow-leaved milkweed

(Asclepias stenophylla)

               
Conservation Status

IUCN Red List

not listed

not listed

LC - Least Concern

 

No image available

NatureServe

NNR - Unranked

S1 - Critically Imperiled

Minnesota

Endangered
Nativity

Native

Occurrence

 

Habitat

Dry. Prairies, wood openings.

Flowering

July to August

     
Flower Color

Pale greenish to yellow or slightly purplish

     
Height

8 to 40

     
 
Identification

This 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 2 to 6 long, less than 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 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 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.

 
Similar
Species

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.

 
Distribution Distribution Map  

Sources: 2, 3, 4, 5, 6.

The only records of this plant in Minnesota are in Houston County, 3½ to 4 miles west of Hokah, in and near Mound Prairie Scientific and Natural Area.

 
Comments

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

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

 
Taxonomy

Family:

Apocynaceae (dogbane)

 

Subfamily:

Asclepiadoideae

 

Tribe:

Asclepiadeae

 

Subtribe:

Asclepiadinae

 
Synonyms

Acerates angustifolia

Polyotus angustifolius

 
Common
Names

narrow-leaf milkweed

narrow-leaved green milkweed

narrowleaved milkweed

narrow-leaved milkweed

slimleaf milkweed

slim-leaf milkweed

Potential
Sites

Located within the "Mound Prairie SNA" at base of third, S face bluff N of Hwy.16. At base of bluff prairie; in dry prairie, less than 50% herbaceous cover, loose sand, 10-20% slope; with Muhlenbergia cuspidata, Bouteloua curtipendula, Schizachyrium scoparium.

Houston Co. Gap Hill #1; ca. 4 mi W of Hokah on N side of MN hwy 16. NW1/4 SW!/4 Sec.34 T104N R5W, SSW-facing bluff.

 

 

 

 

 

 

 

 

Glossary

Axil

The upper angle where the leaf stalk meets the stem.

 

Gynostegium

A crown-like structure of plants of the genus Asclepias formed by the fusion of the anthers with the stigmas.

 

Linear

Long, straight, and narrow, with more or less parallel sides, like a blade of grass.

 

Rhizome

A horizontal, usually underground stem. It serves as a reproductive structure, producing roots below and shoots above at the nodes.

 

Umbel

A flat-topped or convex, umbrella-shaped cluster of flowers or buds arising from more or less a single point.

       
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