Astragalus alpinus is a loosely mat-forming herb in the pea family (Fabaceae) with pale purple flowers and pinnate leaves that grows in gravel bars, open soil, rocky sites, and meadows from the lowlands into the alpine zone in Denali. The stems grow from branching horizontal subterranean stems arising from a thick taproot, plants reach 5-25 cm tall. Leaves are 5-15 cm long, pinnately compound with 8-11 pairs of oblong-ellipticentire leaflets. Astragalus alpinus has a spreading raceme consisting of 5 to 30 pale blue to violet flowers, the flowering stems covered in black appressed hairs. Flowers are bilaterally symmetric (like all members of the pea family), petals 9-15 mm long. Seed pods (legumes) are reflexed and held on a stalk, 10-17 mm long with obvious black hairs. This species can be distinguished from other members of the genus by its prostrate habit (instead of upright), many-flowered inflorescence and stalked seedpods.
This species generally flowers in early to mid-summer (late May through July).
Astragalus alpinus is insect pollinated, typically by bumblebees (Kudo and Molau 1999). Plants are self-incompatible, not setting any seed if not visited by insects. Seeds are dispersed by wind and water after the legume splits open. Seeds are reported to stay afloat for 3-13 days and scarification may be important for germination (Danvind and Nilsson 1997). Plants can also spread vegetatively via rhizomes.
Astragalus alpinus is a widespread circumpolar species. This species is widespread in North America ranging from Alaska across all of Canada and south down the Rocky Mountains to New Mexico in the west and in the Great Lakes states and New England in the east. In Alaska, A. alpinus occurs occasionally in suitable habitat statewide, except is absent from the Aleutian Islands. In Denali, this species occurs in scattered locations across the mountains on both sides of the Alaska Range, and radiates into boreal areas along major river corridors.
Astragalus alpinus has been found in Denali National Park from 94 m to 1573 m with an average plot elevation of 876 m. This species is mostly frequently found in plots above 1100 m. It appears to prefer west and east facing slopes, but it shows no significant preference for south or north facing slopes. It has been found in plots ranging from flat to 39 degree slopes, with an average plot slope of 10.24 degrees. The highest frequency of occurrence for this species is seen in moderately steep plots (20-28 degrees).
This species, as many nitrogen-fixing legumes, is an early successional colonizer on gravel bars, lakeshores, moraines, scree slopes, alluvium and frost heaves in heaths. It prefers well-drained areas of gravel, sand, silt or rock. Reportedly, this species is common on calcareous and siliceous parent materials.