Southwest Colorado Wildflowers, Astragalus flavus and pattersonii

The Astragalus genus is large and complex. In Colorado Flora, Western Slope William Weber lists over five dozen species with many sub-species. The new Flora of the Four Corners lists fifty-eight species and several dozen varieties of Astragalus in the Four Corners drainage of the San Juan River. In Intermountain Flora Arthur Cronquist lists 156 species and 122 varieties. World-wide there are about 1600 species.

Astragalus species are difficult to identify and it is the seed pod, not the flower, that is often crucial in the identification process.

The common name, "Locoweed", is applied not to one plant but to many members of the Astragalus genus, for many of these plants absorb toxic soil substances, especially selenium, which cause grazing animals a variety of serious ailments. Further complicating the common name: some people use the name "Locoweed" not only for Astragalus but also for another Pea genus, Oxytropis. And, making common names even more confusing, many Astragalus also carry the common name of "Milk Vetch" (easily confused with other Peas known as "Vetch"). These common names are so confusing that they really should not be used (except in whispers to close friends).

The genus was named by Linnaeus in 1753 and the word "Astragalus" means "ankle bone" in Greek. It is an ancient Greek plant name perhaps given because of the seed shape in some members of the Astragalus genus or, the authors of Intermountain Flora conjecture, because the Greeks used rattling bones for dice and the sound made is similar to the rattling of dry Astragalus seeds in the pod.

Astragalus flavus
Fabaceae (Pea Family)

Semi-desert. Shrublands, openings. Spring.
McElmo Canyon, Canyons of the Ancients National Monument, April 18, 2007.

The light green, long leaves of Astragalus flavus grab your attention before the flowers emerge. Individual small leaflets are narrow and widely spaced compared to those of most other Astragalus. Flowers are numerous and on long, leafless, upright stems (although stems can be bent or arched, especially as they begin to grow). Flowers are usually yellow, but in the Four Corners area, they are most often pure white. ("Flav" is Latin for "yellow".) A. flavus often grows in Selenium soils.

Thomas Nuttall (Harvard Professor, collector, and taxonomist) collected the first specimen of this plant in the Rockies in the mid-1830s and published the description in 1838.

Astragalus flavus
Fabaceae (Pea Family)

Semi-desert. Shrublands, openings. Spring.
McElmo Canyon, Canyons of the Ancients National Monument, March 27, 2005.

Astragalus flavus
Fabaceae (Pea Family)

Semi-desert. Shrublands, openings. Spring.
McElmo Canyon, Canyons of the Ancients National Monument, April 27, 2005.

Astragalus flavus seed pods.

Astragalus pattersonii
Fabaceae (Pea Family)

Foothills. Openings. Spring.
Mesa Verde National Park, May 2, 2006.

Numerous arching red stems and a strong Selenium odor help identify Astragalus pattersonii, but a close look at the flower makes the identification more certain. Notice, in the next photograph, the distinctive fringing of the calyx (the tube surrounding the base of the other floral parts).

Asa Gray named this species from a specimen "collected by Mr. H. N. Patterson [in 1876?]... in the foothills of Gore Mountains, Colorado". (Asa Gray's words as quoted in "T. S. Brandegee's 1876 "The Flora of Southwestern Colorado", part of the Hayden Survey Report.) (More biographical information.)

Click to see masses of Astragalus Pattersonii.

Astragalus pattersonii
Fabaceae (Pea Family)

Foothills. Openings. Spring.
Mesa Verde National Park, May 2, 2006.

Astragalus pattersonii
Fabaceae (Pea Family)

Foothills. Openings. Spring.
Mesa Verde National Park, May 9, 2008.

Click to see masses of Astragalus Pattersonii.

Species present in state and native
Species present in state and exotic
Species not present in state

County Color Key

Species present and not rare
Species present and rare
Species extirpated (historic)
Species extinct
Species noxious
Species exotic and present
Native species, but adventive in state
Questionable presence

Range map for Astragalus flavus

Range map for Astragalus pattersonii