Narrowleaf Milkweed - Asclepias stenophylla
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.
Midwest and Great Plains, from IL to MT, south from AR and TX to CO. Peripheral.
Observations in Montana Natural Heritage Program Database
Number of Observations:
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(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
The following animal species have been reported as pollinators of this plant species or its genus where their geographic ranges overlap: Bombus vagans
, Bombus huntii
, Bombus rufocinctus
, Bombus ternarius
, Bombus terricola
, Bombus pensylvanicus
, Bombus bimaculatus
, Bombus griseocollis
, Bombus impatiens
, Bombus insularis
, and Bombus flavidus
(Plath 1934, Heinrich 1976, Thorp et al. 1983, Colla and Dumesh 2010, Colla et al. 2011, Koch et al. 2012, Williams et al. 2014, Tripoldi and Szalanski 2015).
Threats or Limiting Factors
STATE THREAT SCORE REASON
Threat impact not assigned because threats are not known (MTNHP Threat Assessment 2021).
- Literature Cited AboveLegend: View Online Publication
- Colla, S., L. Richardson, and P. Williams. 2011. Bumble bees of the eastern United States. Washington, DC: USDA Forest Service, Pollinator Partnership. 103 p.
- Colla, S.R. and S. Dumesh. 2010. The bumble bees of southern Ontario: notes on natural history and distribution. Journal of the Entomological Society of Ontario 141:39-68.
- Koch, J., J. Strange, and P. Williams. 2012. Bumble bees of the western United States. Washington, DC: USDA Forest Service, Pollinator Partnership. 143 p.
- Plath, O.E. 1934. Bumblebees and their ways. New York, NY: Macmillan Company. 201 p.
- Thorp, R.W., D.S. Horning, and L.L. Dunning. 1983. Bumble bees and cuckoo bumble bees of California (Hymenoptera: Apidae). Bulletin of the California Insect Survey 23:1-79.
- Tripoldi, A.D. and A.L. Szalanski. 2015. The bumble bees (Hymenoptera: Apidae: Bombus) of Arkansas, fifty years later. Journal of Melittology 50: doi: http://dx.doi.org/10.17161/jom.v0i50.4834
- Williams, P., R. Thorp, L. Richardson, and S. Colla. 2014. Bumble Bees of North America. Princeton, NJ: Princeton University Press. 208 p.
- 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.