Montana Field Guide

Geyer's Milkvetch is a small annual herb with simple or branched stems arising 5-20 cm high from a slender taproot. The pinnately compound leaves are 2-10 cm long with 5-13 linear to narrowly elliptic leaflets. Foliage is sparsely hairy. Ascending flower stalks arise from the leaf axils and are 6-25 mm long with 2-8 flowers. Each nodding pea-like flower is 3-15 mm long with a somewhat recurved upper petal that is 5-8 mm long, and a calyx that is 2-4 mm long and sparsely covered with light-colored hairs. Greenish, bladdery, pendant fruits are 15-24 mm long and shaped like a half moon.

Plants bloom in late May and June, then become dry and many break off by the middle of the growing season (Heidel 1994).

Geyer's milkvetch is unusual among our Astragalus species, being a taprooted annual lacking multiple stems. The only species similar in this regard is A. lotiflorus, which has hairy pods that are flattened top to bottom. Astragalus agrestis and A. leptaleus also have unbranched stems but they arise from a rhizome rather than a taproot and are usually found in moist habitats. Only var. geyeri occurs in Montana.

This milkvetch occupies loose, sandy soils with little or no organic matter and soil development; most Montana records are from sandy alluvial plains and terraces. In the Pryor Mountains, Geyer's milkvetch is restricted to sandy alluvial plains and terraces. Associates there include Artemisia tridentata, Stipa comata and Bouteloua gracilis. In the Hell Creek Recreation Area (Garfield County), it grows in very early, unstable successional habitat dominated by Calamovilfa longifolia and Oryzopsis hymenoides, near but not intermingled with Psoralea lanceolata, Astragalus kentrophyta and Lupinus pusillus.

In the CM Russell National Wildlife Refuge of Garfield County, Geyer's milkvetch grows on outcrops of Fox Hill sandstone along the shore of Fort Peck Reservoir. Sandstone outcrops are widespread in the Fort Union Formation along the Mussellshell valley and above the Fox Hill sandstone, but they are not as easily eroded as Fox Hill sandstone, and numerous searches there have failed to locate this species.

Geyer's milkvetch is an annual, and is adapted to Great Basin desert conditions characterized by rainfall early in the growing season, followed by moisture stress. The droughty nature of the soils is exacerbated by the hot and dry microclimate of its habitat (Heidel 1994). Some populations have numbered over 1000 plants, however since this species is an annual, population sizes may be expected to fluctuate widely (Lesica and Achuff 1992). Occupied habitat in Montana has been estimated at less than 20 acres due to the highly localized nature of its habitat (Heidel 1994). The rounded, inflated seed pods can disperse by rolling downhill on slopes (Heidel 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).

Geyer's milkvetch occurs in sparsely vegetated habitats, so it can probably tolerate and may even require moderate levels of disturbance. However, over the long-term it may be affected by the encroachment of exotic annuals such as Halogeton glomeratus, Salsola kali and Melilotus officinalis.

STATE THREAT SCORE REASON
Threat impact not assigned because threats are not known (MTNHP Threat Assessment 2021).