Astragalus osterhoutii
Author: M.E. Jones

Kremmling Osterhout milkvetch

Fabaceae (Pea Family)

Close up of Astragalus osterhoutii by Alan Carpenter
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Close up of Astragalus osterhoutii by Denise Culver
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Close up of Astragalus osterhoutii in fruit by Denise Culver
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Ranks and Status

Global rank: G1
State rank: S1
Federal protection status: USFWS Endangered
State protection status: None

[+] Description and Phenology

Astragalus osterhoutii by Dorothy DePaulo
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General description: Relatively tall (up to 100 cm) herbaceous, perennial plants with numerous bright green stems and white to cream colored flowers. Flowers are about 2 cm long and clustered toward the end of branches. Leaflets are 15-30 mm long, subsessile, and linear-oblong or lanceolate. Pods are 40 mm in length, stipitate, and strongly laterally compressed. Pods are bright green and turn red/maroon as they mature (Spackman et al. 1997).

Look Alikes: Astragalus osterhoutii can be distinguished from look alikes by its height, flower color, and pod length. A. duchesnensis is 15-40 cm in height, has pink-purple flowers with white wing tips, and pods 20-35 mm in length, and is not known to occur in Grand county. A. convallarius is 20-50 cm in height, has greenish white flowers which are often tinged with purple, and pods 10-12 cm in length. Also similar to A. scopulorum which has oblong to elliptic leaflets. A. bisulcatus occurs in the same habitat but has bicolored flowers.

Phenology: Flowers June to August (Colorado Natural Heritage Program 2012).

[+] Habitat

Habitat of Astragalus osterhoutii by Jill Handwerk
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Habitat of Astragalus osterhoutii by Alan Carpenter
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Highly seleniferous, grayish-brown clay soils derived from shales of the Niobrara, Pierre, and Troublesome formations. On moderate slopes, sometimes found growing up through sagebrush (Spackman et al. 1997). Associated species include: Artemesia tridentata, Chrysothamnus viscidiflorus, Gutierrezia sarothrae, Eurotia lanata, Phlox hoodii, Eriogonum brevicaule, and Agropogon smithii.

Elevation Range: 7,365 - 8,012 feet (2,245 - 2,442 meters)

[+] Distribution

Distribution of Astragalus osterhoutii in Colorado
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Colorado endemic: Yes
Global range: Endemic to Grand County, Colorado. Estimated range is 120 square kilometers, calculated in GIS in 2008 by the Colorado Natural Heritage Program by drawing a minimum convex polygon around the known occurrences. Imprecisely reported occurrences are not included.

[+] Threats and Management Issues

The major threat is from habitat destruction from all terrain vehicles going off road. OHVs target the barren hillsides that are Astragalus osterhoutii's habitat. The Colorado Natural Areas Program has worked with the State Land Board and the local OHV community on the Idiot's Hill population to mitigate OHV impacts, by keeping folks on trail and away from A. osterhoutii, and in 2011 put up an interpretative sign at the site. Other threats include road maintenance (including weed control and road widening), mineral extraction/mining, and expansion of Wolford Mountain Reservoir. Some occurrences were impacted by habitat loss due to the creation of Wolford Mountain Reservoir. The threat still exists because of future plans to enlarge the reservoir. Mineral extraction is becoming a more imminent threat because of increased demands from mining interests. The primary pollinator is a ground bumble bee which could also be impacted by surface disturbance and inundation.

[+] References

    • Ackerfield, J. 2012. The Flora of Colorado. Colorado State University Herbarium. 433 pp.
    • Barneby, R. C. 1964. Atlas of North American Astragalus. Memoirs of New York Botanical Garden, vol. 13. New York Botanical Garden, Bronx, NY.
    • Baskin, C.C., J.M. Baskin and E. Quarterman. 1972. Observations on the ecology of Astragalus tennesseensis. Amer. Midl. Natur. 88(1): 167-182.
    • Colorado Native Plant Society. 1989. Rare plants of Colorado. Rocky Mountain Nature Association, Colorado Native Plant Society, Estes Park, Colorado. 73 pp.
    • Dawson, C. 2009. Personal communication with Colorado Natural Heritage Program staff regarding BLM rare plant monitoring in Colorado.
    • Dawson, C.A. 1999. The autecology of Astragalus osterhoutii Jones. Dissertation. University of Denver, Denver, CO.
    • Elliott, B. A., S. Spackman Panjabi, B. Kurzel, B. Neely, R. Rondeau, M. Ewing. 2009. Recommended Best Management Practices for Plants of Concern. Practices developed to reduce the impacts of oil and gas development activities to plants of concern. Unpublished report prepared by the Rare Plant Conservation Initiative for the National Fish and Wildlife Foundation.
    • Green, T.W. and G.E. Bohart. 1975. The pollination ecology of Astragalus cibarius and Astragalus utahensis (Leguminosae). Amer. J. Bot. 62(4): 370-386.
    • Kartesz, J.T. 1994. A synonymized checklist of the vascular flora of the United States, Canada, and Greenland. 2nd edition. 2 vols. Timber Press, Portland, OR.
    • NatureServe. Central Databases. Arlington, Virginia. U.S.A. Online. Available:
    • Neely, B., S. Panjabi, E. Lane, P. Lewis, C. Dawson, A. Kratz, B. Kurzel, T. Hogan, J. Handwerk, S. Krishnan, J. Neale, and N. Ripley. 2009. Colorado Rare Plant Conservation Strategy, Developed by the Colorado Rare Plant conservation Initiative. The Nature Conservancy, Boulder, Colorado, 117 pp.
    • O'Kane, S. L. 1988. Colorado's Rare Flora. Great Basin Naturalist. 48(4):434-484.
    • Panjabi, S. and B. Neely. 2011. Middle Park Conservation Action Plan, 2011 Update. Prepared by The Nature Conservancy and the Colorado Natural Heritage Program. Unpublished report prepared for the National Fish and Wildlife Foundation.
    • Rocky Mountain Society of Botanical Artists. 2009. RARE Imperiled Plants of Colorado, a traveling art exhibition. Exhibition catalogue developed by the Denver Botanic Gardens and Steamboat Art Museum.
    • Spackman, S., B. Jennings, J. Coles, C. Dawson, M. Minton, A. Kratz, and C. Spurrier. 1997. Colorado rare plant field guide. Prepared for Bureau of Land Management, U.S. Forest Service and U.S. Fish and Wildlife Service by Colorado Natural Heritage Program.
    • Taliga, Christine E., 2011. Plant Guide for Kremmling milkvetch (Astragalus osterhoutii). USDA-Natural Resources Conservation Service, Colorado State Office. Denver, CO 80225-0426.
    • U. S. Fish and Wildlife Service. 1988. Proposal to determine Astragalus osterhoutii and Penstemon penlandii to be endangered species. Federal Register 53(128): 25181-25185.
    • USDA, NRCS. 2013. The PLANTS Database. National Plant Data Team, Greensboro, NC 27401-4901 USA.
    • Weber, W. A. and R. C. Wittmann. 2012. Colorado Flora, Western Slope, A Field Guide to the Vascular Plants, Fourth Edition. Boulder, Colorado. 532 pp.
    • Western Resource Development. 1990. Osterhout milkvetch inventory: Kremmling Resource Area, Grand County, Colorado.

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