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Montana Field Guides

Broad-keeled Milkvetch - Astragalus platytropis

Native Species

Global Rank: G5
State Rank: S3S4

Agency Status


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General Description
Broad-keeled milkvetch is a small perennial herb without prolonged stems; all leaves are clustered at the base of the plant on short branches of the rootcrown clothed with old leaf stalks. Pinnately compound leaves, 1-9 cm long, have 7-15 oblong to elliptic, often blunt-tipped leaflets. Foliage is sparsely covered with gray or silver hairs. Spreading flower stalks arise from amongst the leaves and bear 3-9 flowers near the tip. Whitish, pea-like flowers are 6-9 mm long with an upper petal that is barely reflexed and nearly the same length as the others. The calyx is 3-5 mm long and sparsely covered with white or black hairs. Inflated, egg-shaped fruits are papery, purple-mottled, and 15-33 mm (ca. 1 in) long. Fruits that resemble small Chinese lanterns are usually found on the ground near the base of the plant.

Flowering in April and May, fruiting in June

Diagnostic Characteristics
It resembles the widespread Astragalus miser var. hylophilus in vegetative condition, except that it has leaves with petioles that are usuallly much longer than the blade, and typically has a trailing growth form. The only other species of milkvetch in Montana with inflated pods that lie along the ground is Astragalus crassicarpus, but it differs in having a papery pod with a mottled pattern rather than a very fleshy green or red pod. papery, purple-mottled fruit are diagnostic.

Species Range

Range Comments
Montana and central Idaho, southward through Nevada to eastern California, and at the edge of its range in western Wyoming.

Observations in Montana Natural Heritage Program Database
Number of Observations: 49

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



(Observations spanning multiple months or years are excluded from time charts)

Limestone- or crystalline-derived soils in open grasslands and sagebrush steppe in the valleys and montane zone.

The following animal species have been reported as pollinators of this plant species or its genus where their geographic ranges overlap: Bombus vagans, Bombus appositus, Bombus auricomus, Bombus bifarius, Bombus borealis, Bombus centralis, Bombus fervidus, Bombus flavifrons, Bombus huntii, Bombus mixtus, Bombus nevadensis, Bombus rufocinctus, Bombus ternarius, Bombus terricola, Bombus occidentalis, Bombus pensylvanicus, Bombus griseocollis, and Bombus insularis (Macior 1974, Thorp et al. 1983, Mayer et al. 2000, Colla and Dumesh 2010, Wilson et al. 2010, Koch et al. 2012, Miller-Struttmann and Galen 2014, Williams et al. 2014).

  • Literature Cited AboveLegend:   View Online Publication
    • 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.
    • Macior, L.M. 1974. Pollination ecology of the Front Range of the Colorado Rocky Mountains. Melanderia 15: 1-59.
    • Mayer, D.F., E.R. Miliczky, B.F. Finnigan, and C.A. Johnson. 2000. The bee fauna (Hymenoptera: Apoidea) of southeastern Washington. Journal of the Entomological Society of British Columbia 97: 25-31.
    • Miller-Struttmann, N.E. and C. Galen. 2014. High-altitude multi-taskers: bumble bee food plant use broadens along an altitudinal productivity gradient. Oecologia 176:1033-1045.
    • 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.
    • Williams, P., R. Thorp, L. Richardson, and S. Colla. 2014. Bumble Bees of North America. Princeton, NJ: Princeton University Press. 208 p.
    • Wilson, J.S., L.E. Wilson, L.D. Loftis, and T. Griswold. 2010. The montane bee fauna of north central Washington, USA, with floral associations. Western North American Naturalist 70(2): 198-207.
  • Additional ReferencesLegend:   View Online Publication
    Do you know of a citation we're missing?
    • Heidel, B.L. and J. Vanderhorst. 1996. Sensitive plant species surveys in the Butte District, Beaverhead and Madison Counties. Unpublished report to the Bureau of Land Management. Montana Natural Heritage Program, Helena, Montana.
    • Lesica, P. 1992. Sensitive plant inventory of the Birch Creek Grazing Allotment, BeaverheadCounty. Uunpublished report to the Bureau of Land Management. Montana Natural Heritage Program, Helena. 3 pp.
    • Lesica, P. 1994. Sensitive plant survey of BLM lands along the Big Hole River and in the Whitehall Valley. Montana Natural Heritage Program, Helena, MT. 13 pp.
    • Lesica, P., M.T. Lavin, and P.F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
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Citation for data on this website:
Broad-keeled Milkvetch — Astragalus platytropis.  Montana Field Guide.  .  Retrieved on , from