Southwest Colorado Wildflowers, Astragalus missouriensis and proximus

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 missouriensis
Fabaceae (Pea Family)

Semi-desert. Woodlands, shrublands, openings. Spring.
Blackrock Canyon, Navajo Reservation, Arizona, May 12, 2007.

Astragalus missouriensis and Astragalus amphioxys are quite similar, but A. missouriensis is not as common as A. amphioxys in the Four Corners. A. missouriensis is generally a more robust plant, with larger leaves and flowers and a deeper, more stout root system.

In 1818 Thomas Nuttall named and described this species from a specimen collected by Bradbury.

Astragalus missouriensis
Fabaceae (Pea Family)

Semi-desert. Woodlands, shrublands, openings. Spring.
Blackrock Canyon, Navajo Reservation, Arizona, May 12, 2007.

Astragalus missouriensis seed pods are nearly straight in contrast to the inwardly curving pods of A. amphioxys, and they are about 2 cm long versus 3 cm, and sharp only on one end. See A. amphioxys.

Astragalus proximus
Fabaceae (Pea Family)

Semi-desert. Shrublands, openings. Spring.
De-Na-Zin Wilderness Area, April 23, 2007.

This Astragalus is a study in contrasts: It can be thick and lush as in the photograph at left or it can be sprawling and spindly as in the photograph below. Either way its long and gangly flower stems contrast with its tiny and delicate flowers.

Astragalus proximus is rare in three southwestern Colorado counties and more common but still in need of protection in three northwestern New Mexico counties. Click to read about the plant and its status.

Astragalus proximus
Fabaceae (Pea Family)

Semi-desert. Shrublands, openings. Spring.
De-Na-Zin Wilderness Area, April 23, 2007.

Astragalus proximus was first named Homalobus proximus by Per Axel Rydberg in 1905 from a specimen collected at Arboles, Colorado by Charles Baker in 1899. In 1915 Elmer Wooton and Paul Standley renamed the species Astragalus proximus.