Narrowleaf Milkweed - Asclepias stenophylla
Species of Concern
* (see State Rank Reason below)
MNPS Threat Rank
State Rank Reason (see State Rank above)
In Montana, Asclepias stenophylla is known from only a few occurrences in two southeastern counties. So far, surveys in Montana have documented a total population that numbers only several hundred plants. Trends are unknown.
Narrowleaf Milkweed is a perennial herb with few, erect and usually unbranched stems that are decumbent or upright, 2-8 dm high and arise from a thick, woody root. The alternate or opposite leaves are 8-18 cm long, but only 1-5 mm wide. Foliage is moderately to sparsely hairy, and the sap is milky. Umbrella-shaped clusters of 10-25 flowers are nearly stemless in the leaf axils on the upper half of the stem. Each greenish-white to yellow flower is only 7-9 mm long and is borne on a hairy stalk that can be up to 1 cm long. Flowers have 5 reflexed corolla lobes which are ca. 5 mm long, and 5 erect hoods that are distinctly 3-lobed at the tip and which surround a central column. Erect spindle-shaped fruits are 9-12 cm long and less than 1 cm wide. Each fruit has numerous seeds, each with a white tail that is ca. 3 cm long.
Flowering in June-early July.
Asclepias stenophylla resembles narrow-leaved forms of A. viridiflora but has flowers with horns. Both A. pumila and A. verticillata also have narrow leaves, but often filiform and less than 1.5 mm wide. A hand lens reveals the three-lobed hoods of A. stenophylla that are diagnostic.
Number of Species Occurrences:
*Not all Species Occurrences are mapped
Midwest and Great Plains, from IL to MT, south from AR and TX to CO. Peripheral.
Summary of Observations Submitted for Montana
Number of Observations:
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Map Help and Descriptions
(Observations spanning multiple months or years are excluded from time charts)
Sandy soils of prairies and open pine woodland. Common associated species include Stipa comata, Carex filifolia, Carex inops, Psoralea spp., Tradescantia occidentalis, Pinus ponderosa, Schizachyrium scoparium, Heterotheca villosa and Calamovilfa longifolia. Several other milkweed species may also co-occur with A. stenophylla. Non-native bromes (B. japonicus and B. tectorum) may also be prevalent.
Ecological Systems Associated with this Species
- Commonly Associated with these Ecological Systems
- Additional ReferencesLegend: View Online Publication
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- Heidel, B.L. and K.H. Dueholm. 1995. Sensitive plant survey in the Sioux District, Custer National Forest, 1994, Carter County, Montana and Harding County, South Dakota. Unpublished report to the Custer National Forest. Montana Natural Heritage Program, Helena, Montana. 95 pp. plus appendices.
- Lesica, P., M. T. Lavin, and P. F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
- Vanderhorst, J.P., S.V. Cooper, and B.L. Heidel. 1998. Botanical and vegetation survey of Carter County, Montana. Unpublished report prepared for the Bureau of Land Management. Montana Natural Heritage Program, Helena. 116 pp. + app.