Montana Field Guide

Railhead milkvetch is a tufted, taprooted perennial herb with several erect stems that reach 5-30 cm in height. The pinnately compound leaves are 5-20 cm long and have 13-21 oblong leaflets with blunt tips. Foliage is sparsely covered with forked gray hairs that branch at the base and spread in opposite directions and are appressed to the surface of leaves or stem. Dense clusters of 10-30 flowers are borne in the axils of upper leaves and become less crowded as the plant matures. The white, pea-like corollas are 12-16 mm long, with the upper petal reflexed and the lower petal purple-spotted. The calyx reaches 4-5 mm and is covered with white or black hairs. The cigar-shaped fruits are glabrous, unstalked, 3-sided in cross-section, and 15-20 mm long. The alpine ecotype is much smaller than plants from the valleys.

Flowering takes place from June to mid-July.

Astragalus atropubescens, A. canadensis and A. scaphoides all have stalked fruits, and the latter two species usually have yellowish flowers.

For the Intermountain Region, Barneby (1989) describes this species' habitat as "open, stony hillsides and benches along rivers, commonly associated with low sagebrush and calcareous bedrock." In Montana, its habitat varies widely from valley grasslands (Idaho fescue/bluebunch wheatgrass) and open eroding slopes, to ridgetops, barren clay buttes and dry subalpine meadow. Thus it is difficult to predict where this species will occur.

Southeast of Bannack, it is found on sagebrush and grassland slopes between more gentle slopes. At another site in Beaverhead County, it grows in grasslands dominated by bluebunch wheatgrass and needle-and-thread grass, sometimes with sagebrush (Artemisia tridentata and A. tripartita). In the Centennial Valley, it occurs in an Artemisia tridentata-Festuca idahoensis community. In the Upper Madison Valley, it grows in fine-textured soils at the base of eroding, sparsely vegetated slopes. This species appears to favor relatively barren sites that are often alkaline (Heidel and Vanderhorst 1996), and frequently occurs with other "barren site indicators," such as Astragalus vexilliflexus and Haplopappus acaulis (Lesica and Vanderhorst 1995).

Astragalus terminalis may have the capability for prolonged dormancy, since it is morphologically similar and closely related to A. scaphoides which has been shown to remain dormant in some years (Lesica 1995). This can result in the number of above-ground stems varying significantly among years, making accurate population counts difficult (Lesica and Steele 1994).

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).

Observations suggest that railhead milkvetch is palatable and may decrease under some livestock grazing regimes. In one exclosure in the Upper Madison Valley, plants were much denser inside than outside, and many of the plants in a nearby population had inflorescences removed, probably by game. No plants were found across the cattle guard in a grazed pasture (Heidel and Vanderhorst 1996).

Railhead milkvetch faces competition from invasive weeds, especially spotted knapweed, yellow sweet clover and cheatgrass. Centaurea maculosa (spotted knapweed) is invading this species' habitat from roadsides in the Upper Madison Valley (Heidel and Vanderhorst 1996), and has the potential to establish and compete aggressively there and in other areas. Melilotus officinalis (yellow sweet clover) and Bromus tectorum (cheatgrass) were also noted invading a site in Beaverhead County.

STATE THREAT SCORE REASON
Reported threats to Montana’s populations of Beck Water-marigold are due to presence of non-native American Water-lily (Nymphaea odorata) at some sites. American Water-lily has large, floating leaves, and is a strong competitor for light on the surface of standing water. Partially submerged Beck Water-marigold plants are relatively vulnerable to direct competition for light, and populations are unlikely to establish where habitat quality is damaged by non-natives. Reported threats to Montana’s populations of Railhead 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.