Monographs Details: Astragalus flavus Nutt. var. flavus
Authors:Rupert C. Barneby
Authority: Barneby, Rupert C. 1964. Atlas of North American Astragalus. Mem. New York Bot. Gard. 13(1): 1-596.
Family:Fabaceae
Synonyms:Tragacantha flaviflora Kuntze, Astragalus flaviflorus (Kuntze) E.Sheld., , Cnemidophacos flavus (Nutt.) Rydb., Astragalus confertiflorus var. flaviflorus (Kuntze) M.E.Jones
Description:Variety Description - Habit of the species, the individual plants long persisting, at length forming wide tufts 2-5 dm. in diameter, the caudex-branches often impacted in clay, aged plants thus acquiring a low-mounded outline; calyx either pilosulous or strigulose, sometimes thinly so, the hairs white, black, or of mixed colors; ovules (10) 13-17.

Distribution and Ecology - Adobe plains, barren clay flats, gullied knolls, and bluffs, in heavy alkaline soils, commonly but not exclusively on sandstone, 5300—7400 feet, locally abundant throughout the eastern half of the Colorado Basin, from the Green River in southwestern Wyoming south through the valleys of the Yampa, White, Grand, and Dolores Rivers in western Colorado and immediately adjoining Utah to the San Juan River in northwestern New Mexico and extreme northeastern Arizona, extending feebly northeast and southeast across the Continental Divide, in Wyoming to the upper forks of the North Platte, and in New Mexico to the west affluents of the Rio Grande.—Map No. 43.—Late April to July.

Discussion:The yellow milk-vetch, A. flavus, is common in all the valleys leading westward out of the Colorado Rocky Mountains and in places so abundant that it beautifies with its prolific blossoms extensive tracts of alkaline bottomland and playa otherwise given over to the gray monotony of halophytic shrubs. Around the north and east periphery of its range the epithet flavus is truly appropriate and descriptive of the flowers, although the yellow color varies somewhat in intensity and tends to turn deeper in drying. It is not possible to endorse Jones s restriction of genuine A. flavus (1923, p. 241, as var. flaviflorus) to sou era yoming, r there is no differential character which goes hand in hand with a distribution of this sort. It may be worth mentioning that the calyx-hairs tend to be longer and looser in Wyoming than southward, but even this inconsequential difference is inconstant. The var. flavus as understood here is easily distinguished by the key-characters given above from typical var. candicans as this occurs in the Sevier and Virgin Valleys in Utah and Nevada and in adjoining Arizona. In eastern Utah and northeastern Arizona, at relatively low elevations, the species is represented by many intermediate forms, some combining pure white petals with broad calyx-tube, others lemon-yellow petals with a narrow calyx; and the average flower varies in size around the crucial point of separation (as expressed in keel-length) brought out in the varietal key. The pod of the two varieties is identical in form, but tends to be a trifle larger in var. flavus, in which the ovules also are ordinarily more numerous, although this is not consistently so. It seems probable that if the typus of A. flavus had originated from near the center of the species-range, rather than at its northeast edge, the variations discovered subsequently would have been accepted as deviations in two directions from an original standard, and it would be reasonable even today, in the light of a complete series of intergradations between the extreme forms of var. flavus and var. candicans, to unite them under one name. However it seems preferable for the present to maintain the traditional categories which certainly correspond with real racial modifications within the species.
Distribution:Wyoming United States of America North America| Colorado United States of America North America| Utah United States of America North America| New Mexico United States of America North America| Arizona United States of America North America|