Montana Field Guide

Broad-keeled milkvetch is a small perennial herb without prolonged stems; all leaves are clustered at the base of the plant on short branches of the rootcrown clothed with old leaf stalks. Pinnately compound leaves, 1-9 cm long, have 7-15 oblong to elliptic, often blunt-tipped leaflets. Foliage is sparsely covered with gray or silver hairs. Spreading flower stalks arise from amongst the leaves and bear 3-9 flowers near the tip. Whitish, pea-like flowers are 6-9 mm long with an upper petal that is barely reflexed and nearly the same length as the others. The calyx is 3-5 mm long and sparsely covered with white or black hairs. Inflated, egg-shaped fruits are papery, purple-mottled, and 15-33 mm (ca. 1 in) long. Fruits that resemble small Chinese lanterns are usually found on the ground near the base of the plant.

Flowering in April and May, fruiting in June

It resembles the widespread Astragalus miser var. hylophilus in vegetative condition, except that it has leaves with petioles that are usuallly much longer than the blade, and typically has a trailing growth form. The only other species of milkvetch in Montana with inflated pods that lie along the ground is Astragalus crassicarpus, but it differs in having a papery pod with a mottled pattern rather than a very fleshy green or red pod. papery, purple-mottled fruit are diagnostic.

Limestone- or crystalline-derived soils in open grasslands and sagebrush steppe in the valleys and montane zone.

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).