Photo © Robert K. Moseley
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Astragalus aquilonius (Barneby) Barneby

Lemhi milkvetch

Fabaceae (Pea; Legume family)

General Description: A taprooted, herbaceous, short-lived perennial with numerous, decumbent or trailing stems up to about 35 cm long and often purplish-tinged. The herbage is greenish-ashy in color due to short, fine hairs. The compound leaves consist of 9-23 oval, oblong, or oblanceolate, rounded to apically notched leaflets 5-16 mm long. The inflorescence is a lose raceme of 4-15 greenish-white flowers, each about 1 cm long. The keel petal is often purplish tipped, while the calyx has white and gray-brown, more or less straight, appressed hairs. The one-celled fruit pods are 2.5-4 cm long by 1.3-1.7 cm in diameter, sessile, inflated, ellipsoid, membranous, green and not mottled, and glabrous to minutely hairy.

Photo © Robert K. Moseley
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Illustration.

Field Identification Tips: Lemhi milkvetch is the only Astragalus species in east-central Idaho with a large, bladdery, unilocular fruit pod. Robust plants with numerous stems can have a low, rounded shape. The purplish-tinged stems and greenish-gray color of the herbage also help distinguish this species.

Phenology: Plants in flower from mid- to late May into July. Fruits may still be present into September.

Similar Species: The range of Lemhi milkvetch partly overlaps and is most likely to be confused with A. amblytropis (Challis milkvetch) because of its similar habit and greatly swollen fruit pod. Challis milkvetch differs, however, by having a two-chambered fruit.

Habitat: On dry, gentle to often steep and unstable slopes, talus, washes, alluvial debris, and flats. It occurs on various, but often southerly aspects having gravelly and sandy, to ashy and occasionally clayey soils. The surrounding shrub-steppe vegetation is dominated by Artemisia tridentata ssp. wyomingensis, Atriplex confertiflora, Pseudoroegneria spicata, Elymus elymoides, Poa secunda, and Leymus salinus ssp. salmonis. Along the Salmon River it is often associated with two other Challis region endemics, Astragalus amblytropis and Oxytropis besseyi var. salmonensis.

Global Distribution: Endemic to east-central Idaho, in Custer, Butte, and Lemhi counties.

Idaho Distribution: The main center of distribution for Lemhi milkvetch includes the lower slopes of the Salmon River valley from near Ellis to Clayton, and the East Fork Salmon River upstream to the vicinity of Herd Creek. Populations are also known from the southern end of the Lemhi Range, the Lemhi River valley around Lemhi, and scattered locations in the Pahsimeroi and Lost River valleys.

References:

Moseley, R. K. 1989. Field investigation of four astragali, all Region 4 sensitive species on the Salmon National Forest, with notes on two others. Idaho Department of Fish and Game, Conservation Data Center, Boise, ID. 23 pp. plus appendices.

Barneby, R. C. 1964. Atlas of North American Astragalus. Memoirs of the New York Botanical Garden Vol. 13. New York Botanical Garden, Bronx, NY. 1188 pp.