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U.S. Forest Service

Plant of the Week

Astragalus proimanthes range map. Astragalus proimanthes range map. USDA PLANTS Database.

Astragalus proimanthes flowers Precocious milkvetch (Astragalus proimanthes). Photo by Charmaine Delmatier.

Astragalus proimanthes  and a dime, showing relative size. Precocious milkvetch (Astragalus proimanthes) berries. Photo by Charmaine Delmatier.

Astragalus proimanthes habitat. Precocious milkvetch (Astragalus proimanthes) habitat. Photo by Charmaine Delmatier.

Precocious Milkvetch (Astragalus proimanthes)

By Charmaine Delmatier (2016)

Precocious milkvetch (Astragalus proimanthes) is part of the Fabaceae family, formerly known as Leguminosae. They are mostly herbs, but include shrubs and trees, and all are found in temperate and tropical areas in both hemispheres. They comprise one of the largest families of flowering plants in the world, numbering around 400 genera and 10,000 species. Astragalus is one of its largest genera with approximately 3,000 herbaceous plants and small shrubs with over 350 species. Although our plant is referred to as a milkvetch; the true vetches (Vicia), have similar flowers, and are usually vine-like and often have a terminal stringy tendril.

Astragalus is referred to as a locoweed, however, there are two genera loosely referred to as locoweed, Astragalus and Oxytropis. According to Stanley L. Welsh, Michael Ralphs, and Bryan Stegelmeier in various separate publications; of the 600 toxic locoweeds worldwide, approximately 21 species occur in the United States. Some are more harmful to grazing undulates, specifically domestic cattle and horses. Locoweeds produce a potentially toxic alkaloid, swainsonine, that inhibits the production of crucial enzymes which triggers an unhealthy reservoir of partially-metabolized sugars and disrupts protein synthesis. Grazing animals will lose their appetite, become reluctant to move, and eventually suffer from severe weight loss.

One of the most common questions I get asked in the field is how to tell the difference between Oxytropis and Astragalus? There is a quick field characteristic that you can usually see with the naked eye, but sometimes it will require a hand lens. If the keel on the flower is blunt, then it is Astragalus, however if the keel is narrowed to a slender beak, then it is Oxytropis, but a combination of several field characteristics will further narrow down positive identification between these two, and for many other exciting legumes.

When you want to venture into the vast open spaces of sagebrush and Juniper woodlands, country Precocious milkvetch is located in the far reaches of southwest Wyoming. There are good roads and visiting these areas are a treat and a day’s adventure. In more open areas, Precocious milkvetch is recognizable as a low-growing perennial which forms a loose cushion, and it is profusely covered with whitish hairs giving the entire plant a soft silvery look. These small cushions are constructed of leaves which are divided into three elliptic leaflets, 5-9 millimeters long. Barely showing their face, are the yellow to whitish pea-like flowers, but what makes them so striking is the pink to red-colored veins covering most of the banner petal. The flowers are fairly large for a smaller cushion plant reaching more than half an inch. The pea-like fruit pods are sessile, without stems. All in all, this rarer legume is waiting to be admired by the desert adventurer.

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