Plants clump-forming, robust, 25–60 cm, densely strigulose; from shallow, subterranean cau­dex. Stems erect and ascending; densely strigulose. Leaves: dis­talmost unifoliolate, sometimes also proximalmost, 3–8 cm; stipules usually distinct throughout, rarely shortly connate-sheathing at proximal nodes, 1.5–9.5 mm, firm, early becoming papery; leaflets (1 or)3–7, blades elliptic to narrowly oblanceolate, 10–47 × 2–9(–13) mm, apex obtuse to retuse, surfaces strigose; terminal leaflet decurrent, not jointed to rachis. Peduncles erect and incurved-ascending, 2.5–15.5 cm. Racemes 7–30-flowered, flow­ers spreading-declined; axis 2–11 cm in fruit; bracts 1–2.5 mm; bracteoles 0–2. Pedicels 1.2–3 mm. Flowers (16–)20–24 mm; calyx light brown, cylindric-gibbous, (7–)8.2–11(–12) mm, strigose, tube (5.5–)6.5–9.2 mm, lobes subulate, 1.7–2.6(–4) mm; corolla ochroleucous, concolorous; banner recurved through 45°; keel 13.7–16.6 mm, apex bluntly or sharply deltate. Legumes pendulous, brown, straight, ellipsoid, dorsiventrally compressed, sometimes becoming subterete, 25–35 × 4–7.5 mm, papery, strigose; stipe 8–12 mm. Seeds 16–22.

Phenology: Flowering May–Jun.
Habitat: Tertiary Duchesne River and Wasatch, and less commonly on Cretaceous Mowry Shale, and Dakota, or other formations, pinyon-juniper and desert shrub communities.
Elevation: 1600–1900 m.

Distribution

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Colo., Utah.

Discussion

Astragalus hamiltonii is a specialized inhabitant of peculiar geological strata north and west of Vernal, Utah, especially the Tertiary Duchesne River and Cretaceous Mowry Shale formations. It also occurs in Dinosaur National Monument, Moffat County, Colorado.

Selected References

None.

Lower Taxa

None.