Bitterroot Milkvetch - Astragalus scaphoides
State Rank Reason (see State Rank above)
Bitterroot milkvetch occurs only in Lemhi County, Idaho and Beaverhead County, Montana. In Montana, the documented occurrences are confined to an area from the Grasshopper Creek drainage south to the Tendoy Mountains. The total number of individual plants has been estimated in the tens of thousands, but occupied habitat is likely less than 700 acres.
Bitterroot milkvetch is a stout, herbaceous perennial with several erect stems 2-6 dm in height arising from a branched rootcrown. The pinnately compound leaves (10-25 cm long) have 15-21 narrowly elliptic leaflets, and the foliage is glabrous to sparsely hairy. Crowded clusters of 15-30 spreading yellowing flowers are borne in the axils of upper leaves, becoming less dense as the plant matures. Corollas are about 20 mm long and have a reflexed upper petal and a blackish-hairy calyx (8-10 mm long). Glabrous, green to reddish, oblong pods (15-20 mm long by 4-6 mm wide) are 2-chambered in cross-section, and are borne on a stalk of approximately equal length that spreads out and then arches up, holding the fruit nearly erect.
Flowering occurs in late May-early June, with fruit maturing in July.
Astragalus atropubescens is similar, but has white flowers and smaller diameter (2-4 mm) fruits borne on sharply erect (immediately ascending) stalks. Astragalus canadensis also differs in having sharply erect fruit stalks. Astragalus terminalis has white flowers and unstalked fruits.
Beaverhead County, Montana, and Lemhi County, Idaho.
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)
Bitterroot milkvetch occurs in sagebrush grassland, generally with a dense cover of sagebrush on silty soils with a moderate to high content of coarse material derived from limestone, basalt or diabase (Lesica 1984). Populations are often found along drainages in the ecotonal area between rocky, steep upper slopes and nearly level benches along drainageways, and seem to be most frequent on warmer, south- and southwest-facing slopes. The habitat usually has a high percentage of bare ground, and shrubs other than sagebrush sometimes dominate. Frequent associates include Artemisia tridentata, Chrysothamnus nauseosus, Artemisia nova, Agropyron spicatum, Agropyron smithii, Stipa comata, Oryzopsis hymenoides, Opuntia polyacantha, Gutierrezia sarothrae, Phlox hoodii and Artemisia frigida.
Ecological Systems Associated with this Species
The number of above-ground stems is highly variable among years, making accurate population assessments difficult (Lesica 1995).
The tendency of this species to occur on lower slopes above drainages makes it particularly vulnerable to road construction (Lesica 1984), which has likely impacted some populations.
Herbivory by insects and mammals may limit the reproductive ability of this species (Lesica 1984, 1995). However, three sample populations remained stable or increased in size in spite of significant seed predation (Lesica 1995). Plants are most vulnerable to grazing as they mature, between May 15 and July 15. Limiting livestock grazing during this period minimizes soil compaction and damage to reproductive plants (Lesica 1984). Rest-rotation grazing regimes may allow enough recruitment to maintain stable populations, while chronic severe grazing could reduce adult vigor and lead to population decline (Lesica 1995).
- 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.
- Horn, G. 1980. Report on inventory of threatened or endangered, rare, or sensitive plants-Headwaters Resource Area, Butte District, Bureau of Land Management, Montana. Unpublished report. 54 pp.
- Lesica, P. 1995. Demography of Astragalus scaphoides and effects of herbivory on population growth. Great Basin Naturalist 55: 142-150.
- Lesica, P. 1984. Report on the Conservation Status of A. Scaphoides, a Candidate Threatened Species (Unpublished).
- Lesica, P. 1994. Demographic monitoring of Astragalus scaphoides at two sites in Montana and Idaho: final report. Unpublished report for the Bureau of Land Management, Butte District. Montana Natural Heritage Program, Helena, MT.
- Lesica, P., M. T. Lavin, and P. F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
- Vanderhorst, J.P. 1995. Sensitive plant survey in the Horse Prairie Creek drainage, Beaverhead County, Montana. Unpublished report to the Bureau of Land Management, Butte District. Montana Natural Heritage Program, Helena, Montana. 42 pp. plus appendices.
- Vanderhorst, J.P. and P. Lesica. 1994. Sensitive plant survey in the Tendoy Mountains, Beaverhead County, Montana. Unpublished report to the Bureau of Land Management, Butte District. Montana Natural Heritage Program, Helena, MT. 59 pp. plus appendices.