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Astragalus gilviflorus E. Sheldon

Plains milkvetch; Plains orophaca

Fabaceae (Pea; Legume family)

Synonyms: Astragalus triphyllus Pursh; Orophaca triphylla (Eaton & Wright) Britton

General Description: A stemless, tufted perennial forming small mats up to about 15 cm in diameter, and foliage with silvery, lustrous, straight, appressed hairs. The compound leaves are comprised of 3, sessile, palmately arranged leaflets 5-30 mm long, and having a reverse lance- or reverse egg-shape narrowing at the point of attachment. The short inflorescence has 1-3 (mostly 2) erect, yellowish to whitish flowers tucked in the leaf axils. Banner petals are 16-28 mm long. The calyx is narrowly cylindric and 9-20 mm long. Fruit pods are erect and commonly concealed among the persistent sepals and stipules. They are ovoid-elliptic in shape, hairy, one-celled, 6-10 mm long by 2.5-5 mm broad, and have thin fleshy valves that become leathery in age.

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Illustration.

Field Identification Tips: The stemless, densely tufted habit, palmately tri-foliate leaves of steely gray-blue color, and relatively large yellowish to whitish flowers tucked within or barely exceeding the leaves readily identifies plains milkvetch in the field.

Phenology: Flowering in early May to about early June.

Similar Species: Although a number of other low-growing Astragalus species occur in east-central Idaho, none should be confused with plains milkvetch. Astragalus calycosus (Torrey's milkvetch) may look superficially similar when flowers or fruits are not present, but it does not have palmately 3-foliate leaflets.

Habitat: Open, more or less sparsely vegetated, rocky, gentle to steeper limestone slopes with little soil development. It occurs on all aspects. Associated species include Petrophytum caespitosum, Artemisia nova, A. frigida, Cercocarpus ledifolius, Tanacetum nuttallii, Penstemon spp., Hymenopappus filifolius var. idahoensis, and Arenaria kingii.

Photo © Robert K. Moseley
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Global Distribution: Widespread on the high plains from southern Alberta and Manitoba, south to Oklahoma, west to the Rocky Mountain foothills, and in the Intermountain Region in east-central Idaho and northeastern Utah.

Idaho Distribution: West slope of the Beaverhead Range in and near the Lemhi River and Birch Creek valleys in Lemhi and Clark counties. Plains milkvetch has also been collected from near Henrys Lake in Fremont County.

References:

Barneby, R. C. 1989. Intermountain Flora. Vascular plants of the Intermountain West, U.S.A. Vol. 3, Part B, Fabales. The New York Botanical Garden, Bronx, NY. 279 pp.

Cholewa, A. F., and D. M. Henderson. 1984. A survey and assessment of the rare vascular plants of the Idaho National Engineering Laboratory. DOE/ID-12100. U.S. Department of Energy, Idaho Operations Office, Radiological and Environmental Sciences Laboratory, Idaho Falls, ID. 45 pp.

Spahr, R., L. Armstrong, D. Atwood, and M. Rath. 1991. Threatened, endangered, and sensitive species of the Intermountain Region. U.S. Forest Service, Intermountain Region, Ogden, UT.