Photo © Robert K. Moseley CLICK PHOTO FOR A LARGER IMAGE

Astragalus oniciformis Barneby

Picabo milkvetch

Fabaceae (Pea; Legume family)

General Description: A wiry, prostrate perennial herb with numerous stems emanating at ground level from a loosely branched caudex attached to a slender taproot. The foliage is covered with a short, white pubescence that gives plants a greenish-ashy appearance. Stems are 10-25 cm long, sparsely leafy, and have flowering stalks emanating from all but the lowest leaf axils. Leaves are compound, with 17-25, distant, elliptic to oval, small leaflets 1-7 mm long. The inflorescence is a loose raceme of 4-12 cream-yellow flowers. Banner petals are 5-7 mm long. The calyx is 3-4 mm long with whitish hairs. Fruit pods are small, 7-12 mm long, pendulous, short stipitate, light green, papery, and have a more or less elliptic profile.

Photo © Robert K. Moseley CLICK PHOTO FOR A LARGER IMAGE

Illustration.

Field Identification Tips: Picabo milkvetch is recognized by its wiry, prostrate, multi-stemmed habit, small leaflets, small cream-yellow flowers, and small, hanging, greenish fruit pods.

Phenology: Flowering begins during mid-May most years.

Similar Species: Astragalus purshii (Pursh's milkvetch) and A. lentigenosus (freckled milkvetch) can occur with Picabo milkvetch. Both are readily distinguished from Picabo milkvetch by their larger leaflets, flowers and fruits. Astragalus atratus var. inseptus (mourning milkvetch) is an Idaho endemic with a range partly overlapping that of Picabo milkvetch. It has small leaflets like Picabo milkvetch, but differs in having larger, whitish flowers and larger, purplish or mottled pods.

Photo © Robert K. Moseley CLICK PHOTO FOR A LARGER IMAGE

Habitat: Picabo milkvetch occurs in sandy sites; in basins, bowls, and flats within rolling basalt topography having deep, stable, well-drained, sandy or sandy-loam soils. It occurs almost exclusively within the Artemisia tridentata ssp. wyomingensis/Hesperostipa comata habitat type. Other associated species include Artemisia tridentata ssp. tridentata, A. tripartata, Chrysothamnus spp., Achnatherum hymenoides, Poa secunda, Pseudoroegneria spicata, Gymnosteris nudicaulis, Phacelia heterophylla, Phlox spp., and Chaenactis douglasii. Populations are known from between approximately 3,700 and 5,200 feet elevation. Picabo milkvetch does not occupy unstable sand dune sites.

Global Distribution: Endemic to south-central Idaho.

Idaho Distribution: Endemic to the north-central portion of the eastern Snake River Plain in Lincoln, Minidoka, and southern Blaine counties.

References:

Barneby, R. C. 1989. Intermountain Flora. Vascular plants of the Intermountain West, U.S.A. Vol. 3, Part B, Fabales. The New York Botanical Garden, Bronx, NY. 279 pp.

McClain, P. 1998. An illustrated guide to the special plants of the Shoshone Resource Area (Upper Snake River District). Technical Bulletin 98-7. Bureau of Land Management, Boise, Idaho.

Moseley, R. K., and S. J. Popovich. 1995. The conservation status of Picabo milkvetch (Astragalus oniciformis Barneby). Idaho Bureau of Land Management Technical Bulletin No. 95-9. Bureau of Land Management, Idaho State Office, Boise, ID. 21 pp. plus appendices.