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Montana Field Guide

Montana Field Guides

Spiny Milkvetch - Astragalus kentrophyta

Native Species

Global Rank: G5
State Rank: S4

Agency Status


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General Description
PLANTS: A mat-forming perennial plant that grows from an intricately branched caudex. Hairs on plant are strigillose (stiff, straight, sharp, and appressed) to villous (long, soft, crooked, and not tangled). Stems prostrate with upper ends ascending 1–3 cm. Source: Lesica et al. 2012.

LEAVES: Leaves alternately arranged on the stem and pinnately divided into 3 to 7 leaflets. Leaflets are narrowly lanceolate in shape, 3–9 mm long, and with mucronate tips. Terminal leaflet is confluent with the rachis of the leaf. Stipules are 2–5 mm long, sessile (basally fused). Source: Lesica et al. 2012.

INFLORESCENCE: Raceme of 1 to 3 flowers growing in the axils of a leaf and shorter than the leaves. Papilionaceous flowers: Petals are white to purple; Banner is reflexed, 4–8 mm long; and Keel is 3–7 mm long. Sepals are green with white and/or black strigillose hairs, 1-3 mm long. Legume is ovoid, compressed, strigose, and 3–5 mm long. Source: Lesica et al. 2012.

In Montana we have two varieties: kentrophyta and tegetarius (Lesica et al. 2012).

Astragalus is derived from the Greek word for ankle bone, astragalos which may refer to the shape of the leaves or fruits (legumes) (Giblin et al. [eds.] 2018).

Diagnostic Characteristics
At the species-level Astragalus kentrophyta is unique in having a combination of these characteristics:
*Height: Grows low along the soil; mat-forming.
*Leaflets: 3-9mm long and tipped with an abrupt, short, sharp, and terminal tip (mucronate). Terminal leaflet is confluent (continuous) with the rachis.
*Inflorescence: 1-3 flowers

Variety kentrophyta

Variety tegetarius

Species Range

Range Comments
Alberta and Saskatchewan, Canada south to California, Arizona, New Mexico, and Nebraska, United States (Lesica et al. 2012).

NOTE: The Montana maps below only reflect observations reported at the species-level and not variety-level.

Observations in Montana Natural Heritage Program Database
Number of Observations: 30

(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)

Stony or gravelly, sparsely vegetated soil of grasslands, steppe, and fellfields in the plains, valleys, and alpine zones of Montana (Lesica et al. 2012).

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).

Reproductive Characteristics
Plants reproduce by seed. Seeds have the potential for long term viability in natural seedbanks (Tara Luna personal communication). In eastern Glacier County it has been observed that plants do not flower and reproduce during drought conditions (Tara Luna personal communication).

*Species belonging to the Pea Subfamily (Papilionoideae) have papilionaceous flowers (Elpel 1998). This applies to almost all species in the northern latitudes (Elpel 1998).
*It refers to the petals of the flowers appearing like a butterfly.
*The 5 petals (collectively called the corolla) have a bilateral symmetry: large upper petal (banner), two lateral petals (wings), and usually two fused lower petals (keeled). The keel petals resembling a boat.

In north-central Montana, livestock have been observed to avoid plants.

  • 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.
    • Elpel, Thomas. 2000. Botany In A Day, Elpel's Herbal Field Guide to Plant Families. 4th Edition, January. Hollowtop Outdoor Primitive School Press, Pony, Montana.
    • Hitchcock, C.L. and A. Cronquist. 2018. Flora of the Pacific Northwest: An Illustrated Manual. Second Edition. Giblin, D.E., B.S. Legler, P.F. Zika, and R.G. Olmstead (eds). Seattle, WA: University of Washington Press in Association with Burke Museum of Natural History and Culture. 882 p.
    • Koch, J., J. Strange, and P. Williams. 2012. Bumble bees of the western United States. Washington, DC: USDA Forest Service, Pollinator Partnership. 143 p.
    • Lesica, P., M.T. Lavin, and P.F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 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?
    • Martin, S.A. 1985. Ecology of the Rock Creek bighorn sheep herd, Beartooth Mountains, Montana. M.Sc. Thesis. Bozeman, MT: Montana State University. 152 p.
    • Seipel, T.F. 2006. Plant species diversity in the sagebrush steppe of Montana. M.Sc. Thesis. Bozeman, MT: Montana State University. 87 p.
    • Williams, K.L. 2012. Classification of the grasslands, shrublands, woodlands, forests and alpine vegetation associations of the Custer National Forest portion of the Beartooth Mountains in southcentral Montana. M.Sc. Thesis. Bozeman, MT: Montana State University. 376 p.
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Citation for data on this website:
Spiny Milkvetch — Astragalus kentrophyta.  Montana Field Guide.  .  Retrieved on , from