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    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 nuttallianus, Astragalus naturitensis, and Astragalus desperatus are similar to each other and can be very difficult to tell apartA comparison of the photos on this page and those on the Astragalus naturitensis page and A. desperatus page should assist you. Also see A. monumentalis.  

      All four plants have leaves typical of Fabaceae (the Pea Family):  the leaves are cut into numerous, small leaflets.  In the photos below, each of the cluster of leaflets is a leaf.

When in flower or seed, A. nuttallianus and A. naturitensis are difficult to tell apart, but they are even more difficult to tell apart when only the leaves show.  Observing one characteristic of the leaves can help you separate the two species: the leaves of A. nuttallianus branch from long stems.

Astragalus nuttallianus
The leaves of A. naturitensis have no stems or only very minute stems and all leaves emanate from a central point at ground level.  No stems show in this photograph.  (The stem-like growth from which each leaflet grows is not a stem; it is a rachis, part of a leaf.) Astragalus naturitensis
Astragalus desperatus is very similar to A. naturitensis; both have leaves that are practically stemless.  It is the seed pods which most readily Astragalus naturitensisseparate the two species.  At left are the seed pods of A. naturitensis and at right are the seed pods of A. desperatus. Astragalus desperatus
Astragalus nuttallianus

Astragalus nuttallianus variety micranthiformis
Fabaceae (Pea Family)

Semi-desert.  Openings.  Spring.
McElmo Canyon, Canyons of the Ancients National Monument, May 23, 2011.

Astragalus nuttallianus
Astragalus nuttallianus variety micranthiformis
Fabaceae (Pea Family)

Semi-desert.  Openings.  Spring.
McElmo Canyon, Canyons of the Ancients National Monument, April 5, 2005.

This is a very common Astragalus in the lower elevations of the Four Corners region, sometimes carpeting the ground with dozens of plants.  The plant is highly variable in form, ranging from the tiny, prostrate plant pictured at left to plants a foot tall.  Leaves arise from long, twisting stems and leaflets range from 7 to 15.

There are eight varieties of Astragalus nuttallianus; in the Four Corners area our variety is Astragalus nuttallianus variety micranthiformis (small flowered), which was found in 1947 near Towaoc on the Ute Mountain Ute Indian Reservation.  As the range map below indicates, varieties of Astragalus nuttallianus are found in all the southwestern states. (Astragalus nuttallianus should not be confused with Astragalus nuttallii found only in California coastal counties.)

Thomas Nuttall -- teacher, collector, taxonomist -- was a giant of 19th century botany.  He collected the first specimen of this plant "On the plains of the [Arkansas] River" and named it Astragalus micranthus in 1821. Augustin de Condole renamed it to honor Nuttall in 1825.  (Quotation from Intermountain Flora.) (More biographical information about Nuttall.)

Astragalus nuttallianus

Astragalus nuttallianus

Astragalus nuttallianus variety micranthiformis
Fabaceae (Pea Family)

Semi-desert.  Openings.  Spring.
McElmo Canyon, Canyons of the Ancients National Monument, April 5, 2005.

A close look reveals a beautifully bi-colored, tiny flower (about one-third inch long) with an arching, spreading purple banner with a white center.  There are from 1 to 7 flowers per cluster.

 

Pods often curve moderately inward as they dry. 

Astragalus nuttallianusA. nuttallianus pods are sulcate, i.e., they have a groove along a suture. 

The suture on the inward side of the curve is slightly raised.

Range maps © John Kartesz,
Floristic Synthesis of North America

State Color Key

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
Eradicated

Questionable presence

Range map for Astragalus nuttallianus

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