Monographs Details: Astragalus purshii Douglas ex Hook. var. purshii
Authors:Rupert C. Barneby
Authority: Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(2): 597-1188.
Synonyms:Tragacantha purshii (Douglas) Kuntze, Phaca purshii (Douglas) Piper, Xylophacos purshii (Douglas ex Hook.) Rydb., Astragalus purshii var. typicus Barneby, Phaca mollissima Nutt., Astragalus purshii var. interior M.E.Jones, Xylophacos incurvus Rydb., Astragalus purshii var. incurvus (Rydb.) Jeps.
Description:Variety Description - Stems 0-10 cm. long; leaves (1.5) 3-10 (15) cm. long, with (5) 7-15 (17) nearly always elliptic, elliptic-oblanceolate. or rhombic-elliptic and acute, rarely obovate and obtuse leaflets (2) 4-14 (20) mm. long; racemes (1) 2-5 (6)-flowered; calyx (12) 13-16 (19) mm. long, the tube 9.2-12 (13.6) mm. long, 2.9-5.1 mm. in diameter, the teeth 2.2-6 (7) mm. long; banner 19-25 (26) mm. long, 7.6-12 (13) mm. wide; wings 17.3-22.5 mm. long, the claws 10-13.5(14.2) mm., the blades (7.5) 8-12.6 mm. long, 2.7-3.6 mm. wide; keel (15) 17-21.2 mm. long, the claws (9.9) 11-13.8 mm., the blades (6.3) 6.6-8.8 (9.4) mm. long, 2.7-3.6 mm. wide; pod obliquely ovoid or broadly lance-ellipsoid, 1.3-2.3 (2.7) cm. long, 5-9.5 (13) mm. in diameter, not or very shallowly sulcate ventrally, the surface of the valves nearly always concealed by the dense shaggy vesture of hairs up to (2) 2.5-4 (5) mm. long; ovules (20) 22-34; seeds 1.9-2.7 (3.4) mm. long.

Distribution and Ecology - Dry hills and plains, characteristic of the sagebrush deserts of the northern and western Great Basin, but also on barren, gravelly slopes in piñon-juniper woodland, on sandy flats and banks in yellow pine forest, on river terraces, and gullied bluffs or knolls, without apparent rock preference, ranging from 1250 feet along the Columbia River up to 7600 feet in northern Colorado, widespread and locally abundant from southcentral Nevada through transmontane Oregon to eastern Washington, east through the Rocky Mountains to the plains of southern Alberta and southwestern Saskatchewan, southwestern North Dakota, and western South Dakota, south to the head of the North Platte, Grand, and Yampa Rivers in northwestern Colorado, and to the Uinta Basin in northeastern Utah.—Map No. 86.—April to June, rarely July, flowering in earliest spring.


The typical form of the Pursh milk-vetch is distinguished simply by its whitish or cream-colored flowers, differing from var. tinctus by no other consistent character. Over an immense tract of prairie country, extending from the Missouri Valley in North Dakota west into the Rocky Mountains of Montana, Wyoming, and Colorado, and again over a considerable area in northeastern Nevada, var. Purshii is the only representative of its species. In western Nevada it passes insensibly into var. tinctus, and here are plants, sometimes singly and sometimes in colonies, in which the ochroleucous banner and wings are variably tinged or margined with lilac or dingy purple. In the Columbia Basin var. Purshii is relatively uncommon, at least much less prevalent than var. glareosus, which differs, like var. tinctus, in its bright purple flower, but also in its ventrally sulcate fruit. The var. Purshii is polymorphic, but such variable features as the length of the pod's vesture, color of the calyx-hairs, and degree of caulescence are not geographically correlated. Two of the more remarkable forms deserve mention as minor variants:

M. v. 1. Stems well developed, forming loosely woven mats rather than the usual cespitose tufts, the plants superficially resembling A. inflexus. Occasional in the Great Basin, especially in low ground. The typus of A. Purshii, as described by Hooker, was partly of this form, which has since been collected on several occasions in Spokane County where, however, it does not replace the typical form. Cf. Piper 2288 (GH, NY, POM, WS, WTU).

M. v. 2. Stipules, bracts, and calyx-teeth all elongate; pod strongly incurved. A local race, of yellow pine forest on the east slope of the Sierra Nevada in Lassen, Plumas, Sierra, and Nevada Counties, California. The differential characters are not consistently present in this area and occur occasionally, either separately or together, well outside it (Xylophacos incurvus). Cf. Ripley & Barneby 4528 (CAS, GH, RSA).

Distribution:Colorado United States of America North America| Nevada United States of America North America| Oregon United States of America North America| Washington United States of America North America| Utah United States of America North America| California United States of America North America| Idaho United States of America North America| Montana United States of America North America| Wyoming United States of America North America| North Dakota United States of America North America| South Dakota United States of America North America| Alberta Canada North America| Saskatchewan Canada North America|