Astragalus osterhoutii
M.E. Jones
Osterhout's Milkvetch
G1Critically ImperiledGlobal Rank
Very highThreat Impact
U.S. Fish & Wildlife Service, https://www.usa.gov/government-works
Identity
Unique IDELEMENT_GLOBAL.2.155315
Element CodePDFAB0F6E0
Record TypeSPECIES
ClassificationSpecies
Classification StatusStandard
Name CategoryVascular Plant
Endemicendemic to a single state or province
KingdomPlantae
PhylumAnthophyta
ClassDicotyledoneae
OrderFabales
FamilyFabaceae
GenusAstragalus
Other Common Names
Kremmling Milkvetch (EN) Kremmling milkvetch (EN)
Concept Reference
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.
Conservation Status
Rank Method Rank calculation - Biotics v2
Review Date2025-08-10
Change Date2025-08-10
Edition Date2025-06-06
Edition AuthorsS. Spackman, rev. Maybury/Spackman (1996), rev. S. Spackman and D. Anderson (2000), rev. K. Neuhaus, J. Handwerk, and S. Panjabi (2006), rev. J. Handwerk (2009, 2012, 2022)
Threat ImpactVery high
Range Extent100-250 square km (about 40-100 square miles)
Number of Occurrences1 - 20
Rank Reasons
Astragalus osterhoutii is endemic to small area of the western United States in Grand County, Colorado. It is known from seven occurrences within five localized areas. Threats to A. osterhoutii include include disturbance and habitat loss from off-highway vehicles, road maintenance (including snow plowing, and weed control), competition from non-native weedy species, black blister beetle herbivory, transmission line maintenance, and herbivory and trampling by cattle and wildlife. It is considered extremely vulnerable to climate change.
Range Extent Comments
Astragalus osterhoutii is endemic to the western United States, where it is found in Grand County, Colorado. The estimated range extent of 111 square kilometers was calculated in GeoCAT using occurrence data provided by the Colorado Natural Heritage Program (2022).
Occurrences Comments
As of 2022, there are seven known occurrences documented in the Colorado Natural Heritage Program database. All seven of the occurrences have been observed within the last 20 years. In addition, there is one extirpated occurrence that was last observed in 1981.
Threat Impact Comments
Threats to Astragalus osterhoutii include disturbance and habitat loss from off highway vehicles (OHVs) traveling off designated routes, road maintenance (including snow plowing and weed control), competition from non-native weedy species, black blister beetle (Epicauta puncticollis or E. pensylvanica) herbivory, transmission line maintenance, and herbivory and trampling by cattle and wildlife (Dawson 1999, Panjabi and Neely 2011, Taliga 2012, CNHP 2022). Dam maintenance at Wolford Mountain Reservoir is also of concern. This species is considered to have low adaptive capacity and is expected to be extremely vulnerable to climate change under both RCP 4.5 and 8.5 scenarios, as assessed for the 2025 Colorado State Wildlife Action Plan (CNHP 2025).
Ecology & Habitat
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).
Habitat
Astragalus osterhoutii grows on highly seleniferous, grayish-brown clay soils derived from shales of the Niobrara, Pierre, and Troublesome formations (Spackman et al. 1997). It occurs on moderate slopes, sometimes found growing up through sagebrush. Associated species include: Artemisia tridentata, Chrysothamnus viscidiflorus, Gutierrezia sarothrae, Eurotia lanata, Phlox hoodii, Eriogonum brevicaule, and Agropyron smithii.
Reproduction
Flowers bloom from July through August, followed by fruit (FNA 2023).
Terrestrial Habitats
Shrubland/chaparral
Other Nations (1)
United StatesN1
| Province | Rank | Native |
|---|---|---|
| Colorado | S1 | Yes |
Threat Assessments
| Threat | Scope | Severity | Timing |
|---|---|---|---|
| 2 - Agriculture & aquaculture | Large (31-70%) | Moderate - slight | High (continuing) |
| 2.3 - Livestock farming & ranching | Large (31-70%) | Moderate - slight | High (continuing) |
| 4 - Transportation & service corridors | Restricted (11-30%) | Moderate - slight | High (continuing) |
| 4.1 - Roads & railroads | Restricted (11-30%) | Slight or 1-10% pop. decline | High (continuing) |
| 4.2 - Utility & service lines | Restricted - small | Moderate - slight | High (continuing) |
| 6 - Human intrusions & disturbance | Restricted - small | Moderate - slight | High (continuing) |
| 6.1 - Recreational activities | Restricted - small | Moderate - slight | High (continuing) |
| 7 - Natural system modifications | Small (1-10%) | Extreme or 71-100% pop. decline | Low (long-term) |
| 7.2 - Dams & water management/use | Small (1-10%) | Extreme or 71-100% pop. decline | Low (long-term) |
| 8 - Invasive & other problematic species, genes & diseases | Pervasive - large | Moderate - slight | High (continuing) |
| 8.1 - Invasive non-native/alien species/diseases | Pervasive - large | Slight or 1-10% pop. decline | High (continuing) |
| 8.2 - Problematic native species/diseases | Large - restricted | Moderate - slight | High (continuing) |
| 11 - Climate change & severe weather | Pervasive (71-100%) | Extreme or 71-100% pop. decline | High (continuing) |
Plant Characteristics
DurationPERENNIAL
Economic Value (Genus)Yes
Roadless Areas (3)
Colorado (2)
| Area | Forest | Acres |
|---|---|---|
| Dome Peak | Routt NF | 35,716 |
| Troublesome South | Routt NF | 47,359 |
South Dakota (1)
| Area | Forest | Acres |
|---|---|---|
| Indian Creek | Buffalo Gap National Grassland | 24,666 |
References (15)
- Colorado Native Plant Society. 1989. Rare plants of Colorado. Rocky Mountain Nature Association, Colorado Native Plant Society, Estes Park, Colorado. 73 pp.
- Colorado Natural Heritage Program (CNHP). 2022. Biodiversity Tracking and Conservation System (Biotics 5). Colorado Natural Heritage Program, Colorado State University, Fort Collins.
- Colorado Natural Heritage Program (CNHP). 2025. Climate Change Vulnerability Assessment of Colorado Tier 1 and Tier 2 Plant Species of Greatest Conservation Need (SCGN) for the 2025 Colorado State Wildlife Action Plan. Colorado Natural Heritage Program, Colorado State University, Fort Collins.
- 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, Colorado.
- Handwerk, J., L. Grunau, and S. Spackman-Panjabi. 2015. Colorado Wildlife Action Plan: 2015 Rare Plant Addendum. Colorado Natural Heritage Program, Colorado State University, Fort Collins, Colorado, USA.
- 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.
- Krening, P. 2021. Osterhout’s milkvetch (<i>Astragalus osterhoutii</i>) population trend monitoring summary-2021. Prepared for the Colorado State Office - Bureau of Land Management, U.S. Department of the Interior.
- Krening, P. 2022. Osterhout’s milkvetch (<i>Astragalus osterhoutii</i>) Population Trend Monitoring Summary-2022. Prepared for the Colorado State Office, Bureau of Land Management, U.S. Department of the Interior.
- Krening, P. 2023. Osterhout’s milkvetch (<i>Astragalus osterhoutii</i>) Demographic Tend Monitoring Summary and Status Report - 2023. Prepared for the Colorado State Office, Bureau of Land Management, U.S. Department of the Interior.
- NatureServe. Central Databases. Arlington, Virginia. U.S.A. Online. Available: http://www.natureserve.org/explorer/
- 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.
- 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, C.E. 2012. Plant Guide for Kremmling milkvetch (<i>Astragalus osterhoutii)</i>. USDA-Natural Resources Conservation Service, Colorado State Office. Denver, Colorado.
- U.S. Fish and Wildlife Service (USFWS). 1988. Proposal to determine <i>Astragalus osterhoutii</i> and <i>Penstemon penlandii</i> to be endangered species. Federal Register 53(128): 25181-25185.