Montana Field Guide

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.

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.

The number of above-ground stems is highly variable among years, making accurate population assessments difficult (Lesica 1995).


POLLINATORS
The following animal species have been reported as pollinators of this plant species or its genus where their geographic ranges overlap: Bombus vagans, Bombus appositus, Bombus auricomus, Bombus bifarius, Bombus borealis, Bombus centralis, Bombus fervidus, Bombus flavifrons, Bombus huntii, Bombus mixtus, Bombus nevadensis, Bombus rufocinctus, Bombus ternarius, Bombus terricola, Bombus occidentalis, Bombus pensylvanicus, Bombus griseocollis, and Bombus insularis (Macior 1974, Thorp et al. 1983, Mayer et al. 2000, Colla and Dumesh 2010, Wilson et al. 2010, Koch et al. 2012, Miller-Struttmann and Galen 2014, Williams et al. 2014).

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).

STATE THREAT SCORE REASON
Reported threats to Montana’s populations of Bitterroot Milkvetch are currently assigned as unknown (MTNHP Threat Assessment 2021). Spotted Knapweed (Centaurea stoebe) is found in the vicinity of some populations, but reports indicate that severity of impacts is unknown. Information about the severity of impacts to populations due to noxious weeds or other threats is needed.