Wind River Milkvetch - Astragalus oreganus
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Wind River milkvetch is a regional endemic known in Montana only from southern Carbon County. Although populations are relatively large, there are few known occurrences in the state and negative impacts or potential impacts to the species from livestock grazing, ORV use and extractive industries have been noted.
Wind River Milkvetch is an herbaceous perennial with lax stems 6-20 cm long and arise singly from deep, cord-like rhizomes. Spreading, alternate, pinnately compound leaves are 5-15 cm long with 9-15 broadly elliptical leaflets. Foliage is thinly to densely covered with long, ashen hairs. Nearly cylindrical inflorescences, 3-7 cm long, are densely 20-35-flowered and arise from the axils of the upper leaves. Off-white or yellowish, pea-like flowers have a reflexed upper petal that is notched at the tip and a calyx that is 6-10 mm long and thinly covered with light-colored hairs. The fleshy, green, oblong pods are 10-15 mm long, round in cross section, and held nearly erect.
Flowering and fruiting occur in June and July.
This is distinguished from other species of Astragalus in our area by the combination of the broadly elliptic to nearly round leaflets, the dolabriform foliage hairs, and the papery sheaths at the base of the petioles that completely surround the stems (Dorn 1984, 1992). The similar A. canadensis has more oblong leaflets and usually occurs in moister habitats.
W. WY and south-central MT. Regional endemic.
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)
Wind River milkvetch is a regional endemic of south-central Montana and central Wyoming. In Montana, it is only known from the Pryor Mountain desert area, while it is widespread over western Wyoming. In Montana this species forms large colonies in sandy soil below 5,000 ft and is commonly associated with Artemisia tridentata and Stipa comata. It most often occurs on the Chugwater Formation (Lesica and Achuff 1992).
Wind River milkvetch always occurs in habitats with sparse vegetation, suggesting that it is a poor competitor for light, water, and/or nutrients.
Areas where this plant occurs are grazed by livestock, but grazing is not likely to adversely affect this plant because it is probably unpalatable and would benefit from reduced competition. In one area, it grows close to a gravel road on the border of the Big Horn NRA, and may be vulnerable to vehicular disturbance or competition from the weed Halogeton glomeratus, which is present in low numbers along the road.
- Additional ReferencesLegend: View Online Publication
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- Barneby, R.C. 1964. Atlas of North American Astragalus. 2 Vols. New York Botanical Garden, Bronx, New York. 1188 pp.
- Dorn, R. D. 1984. Vascular Plants of Montana. Cheyenne, WY: Mountain West Publishing. 276 pp.
- Dorn, R. D. 1992. Vascular plants of Wyoming. Second edition. Mountain West Publishing, Cheyenne, WY. 340 pp.
- Lesica, P. and P.L. Achuff. 1992. Distribution of vascular plant species of special concern and limited distribution in the Pryor Mountain desert, Carbon County, Montana. Unpublished report to the Bureau of Land Management. Montana Natural Heritage Program, Helena, MT. 105 pp.
- Lesica, P., M. T. Lavin, and P. F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.