Montana Field Guide

Raceme Milkvetch is a coarse, herbaceous perennial with few to many erect stems, 15-70 cm high, and arising from a woody taproot and branched rootcrown. The pinnately compound leaves are 4-15 cm long with 11-31 narrowly to broadly elliptic leaflets. Foliage is thinly to densely covered with short hairs. Inflorescences with 15-70 densely clustered flowers arise from the axils of the upper leaves. The nodding, whitish flowers are 16-21 mm long with a partially reflexed upper petal and purplish tinged lower petals. The calyx is 8-11 mm long and glabrous or with scattered hairs. The pendant, oblong-elliptic pods have a basal, 3-7 mm long stalk, are glabrous, triangular in cross-section, with 3 sides that are nearly flat and equal-width, and 15-30 mm long.

Our plants are variety racemosus.

Flowering occurs in late June-early July; fruits persisting.

The combination of coarse, erect stems, short pubescence, and pendant pods, triangular in cross-section, is diagnostic of this species in the n. Great Plains. The more common Astragalus drummondii is similar but has pods dorsiventrally compressed with 2 parallel grooves on the bottom, and has foliage with long spreading hairs.

Raceme milkvetch typically grows in grasslands on heavy soil derived from shale. It is an indicator of - and confined to - selenium soils (Great Plains Flora Assoc. 1986), typically occurring on "gumbo" in "low sites with an abundance of alkali" (Barr 1983). Soils with a high selenium content are found locally in Montana on substrates derived from marine shales (Vanderhorst et al. 1998). Vegetation at these sites varies from grasslands to sagebrush steppe and greasewood communities.

The largest and most robust population in Montana occurs on the gentle slopes of a low relief shale ridge. This site supports a high quality Elymus lanceolatus/Stipa viridula grassland with eroded areas dominated by Eriogonum pauciflorum, and has among the highest diversity of forbs observed in Carter County (Vanderhorst et al. 1998). Another large population was documented in Stipa comata/Carex filifolia grassland at the contact zone between shale and sandstone, with Stipa comata/Carex filifolia dominant on the sandstone and wheatgrasses (Elymus lanceolatus and/or Pascopyrum smithii) on the shale.

In the breaklands of southeastern Carter County, it occurs sparsely among sandstone and shale outcrops dominated by a mix of sagebrush steppe and Stipa comata/Carex filifolia grasslands. It has also been occasionally documented in shrub dominated communities, including Artemisia tridentata ssp. wyomingensis/Elymus lanceolatus and Sarcobatus vermiculatus/Elymus lanceolatus. Another selenium indicator, Stanleya pinnata (prince's plume), grows in the same habitat at two sites in Carter County (Vanderhorst et al. 1998).

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).

Raceme milkvetch accumulates selenium and may cause poisoning of livestock. However, it is apparently rarely eaten by livestock (Great Plains Flora Association, 1988). No browsed plants have been observed during surveys in Montana, as populations documented were in rangelands without evidence of recent livestock use (Vanderhorst et al. 1998). The site of the largest population has an unusually high forb diversity and cover of Nasella viridula (green needlegrass) indicating excellent condition for that range site. It is possible that livestock are excluded from areas known to have selenium soils or that grazing in those areas is timed seasonally to avoid poisonous plants (Vanderhorst et al. 1998). The range literature does not indicate whether raceme milkvetch is an increaser or decreaser under grazing. The fact that its largest known numbers occur in areas of high quality range suggests that it warrants some attention from range managers and livestock operators.

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