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Montana Field Guides

Narrowleaf Milkweed - Asclepias stenophylla

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Species of Concern

Global Rank: G4G5
State Rank: S2
* (see State Rank Reason below)

Agency Status
USFWS:
USFS:
BLM: SENSITIVE
MNPS Threat Rank:

External Links






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.
  • Details on Status Ranking and Review
    Narrowleaf Milkweed (Asclepias stenophylla) Conservation Status Review
    Review Date =
 
General Description
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.

Phenology
Flowering in June-early July.

Diagnostic Characteristics
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.

General Distribution
Mapped Observations

 


Distribution Comments
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: 11
Number of Occurrences: 7

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Relative Density

Recency

 

(Records associated with a range of dates are excluded from time charts)



Habitat
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

References
  • Additional ReferencesLegend:   View WorldCat Record   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. 2012. Manual of Montana Vascular Plants. BRIT Press. Fort Worth, TX.
    • Vanderhorst, J., S. V. Cooper, and B. 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.
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Citation for data on this website:
Narrowleaf Milkweed — Asclepias stenophylla.  Montana Field Guide.  Montana Natural Heritage Program.  Retrieved on August 20, 2014, from http://FieldGuide.mt.gov/detail_PDASC021U0.aspx
 
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