Non-technical Description: Precocious milkvetch is a stemless perennial forb forming low cushions 2-3 dm in diameter. The herbage is densely silvery-whitish throughout with ascending, twisted or spreading hairs 1.5-3 mm long. Leaves are densely clustered, 1-3.5 mm long, and have three narrowly oblanceolate to elliptic leaflets 5-9 mm long. Stipules are fused at the back, whitish-membranous, 7-12 mm long, and glabrous except for ciliate margins. The yellow or whitish pea-like flowers (often tinged with lavender or pink, especially on the throat) are up to 17 mm long, sessile, erect, and borne in pairs among the basal leaves. The calyx is 8-10.5 mm long with a cylindric tube 6-6.5 mm long. The banner petal is distinctly constricted in the middle (giving the whole banner a fiddle-like shape) and is glabrous on the back. Fruit pods are sessile, 7-10 mm long, narrowly elliptic to ovoid, slightly flattened from the sides, densely fine-hairy, and have 11-14 ovules. Flowering May-mid June (Barneby 1964; Dorn and Dorn 1980; Fertig et al. 1994; Isley 1998).
Similar Species: Astragalus gilviflorus has larger flowers and spoon-shaped banner petals 16-28 mm long. A. hyalinus has fiddle-shaped banners that are hairy on the back and smooth pubescence on the stems, calyces, and foliage.
Habitat: Astragalus proimanthus occurs primarily on sparsely vegetated rims and gullied upper slopes of benches, bluffs, and mesa-like ridges at the elevations of 6400-7200 feet (1950-2195 m). Populations are typically found in cushion plant/bunchgrass communities dominated by Phlox hoodii, Haplopappus nuttallii, Cryptantha sericea, and Elymus spicatus in openings within Artemisia tridentata or Juniperus osteosperma grasslands. Occasionally, colonies may also occur on open toe slopes of ridges within a matrix of Artemisia nova, Sarcobatus vermiculatus, Juniperus osteosperma, and Grayia spinosa. Vegetation cover is typically less than 5-10%. Soils are whitish, fine-textured, dry, shallow, calcareous shale clays covered with a dense layer of coarse cobbles, whitish shaley flakes, and dark volcanic rock. Populations occur on the Eocene-age Bridger Formation and Laney member of the Eocene Green River Formation. Colonies may occur on any aspects, but are most common on flats or south to west-facing slopes of 1-30% (Fertig & Welp 2001).
References: Barneby, R. C. 1964. Atlas of North American Astragalus. Memoirs of the New York Botanical Garden 13(II):1-1188. Dorn, R.D. and J.L. Dorn. 1980. Illustrated Guide to Special Interest Vascular Plants of Wyoming. Published by US Fish and Wildlife Service and Bureau of Land Management. Fertig, W. and L. Welp. 2001. Status of Precocious milkvetch (Astragalus proimanthus) in southwest Wyoming. Prepared for the Bureau of Land Management Wyoming State Office by the Wyoming Natural Diversity Database, Laramie, WY. Fertig, W., C. Refsdal, and J. Whipple. 1994. Wyoming Rare Plant Field Guide. Wyoming Rare Plant Technical Committee, Cheyenne Wyoming. Isely, D. 1998. Native and Naturalized Leguminosae (Fabaceae) of the United States (exclusive of Alaska and Hawaii). Monte L. Bean Life Science Museum, Brigham Young Univ., Provo, UT.
Author: Walter Fertig, Moenave Botanical Consulting, Kanab, UT. April 2017
Citation: The vPlants Project. vPlants: A Virtual Herbarium of the Chicago Region. http://www.vplants.org
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