Astragalus microcymbus

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Skiff milkvetch (Astragalus microcymbus) is a rare, flowering plant in the legume family that is found only in Colorado. It grows in Gunnison County and near the northwest edge of Saguache County. It was discovered in 1945 by Rupert Barneby. The plant is considered Critically Imperiled by NatureServe and is a Tier 1 species in Colorado’s Rare Plant Addendum, meaning it faces a high risk of extinction.

Description: This small, perennial herb usually reaches up to about 30 cm tall and can spread along the ground. It has pale-purple to off-white flowers arranged in loose clusters of 7–14, blooming from May to early July. Leaves are 2–4 cm long with leaflets 3–9 mm long. The boat-shaped fruits are up to 8 mm long. The plant can produce seeds in its first year and may continue reproducing for up to ten years, but it can also go dormant for several years. Pollinators may help seed production, and seeds are likely dispersed by wind and rain.

Habitat and range: Skiff milkvetch lives in a very small area of sagebrush steppe in the Mountain-Prairie region, within two drainages—South Beaver Creek and Cebolla Creek—at elevations around 2,400–2,500 meters. Its range covers roughly a 3.5 by 10 mile area, with some populations 10 miles apart. It grows in gravelly, well-drained soils, sometimes on rock outcrops, and is often found among sagebrush.

Threats: The species faces habitat loss from urban development and increasing recreation near Gunnison. Invasive cheatgrass threatens its ecosystem, and grazing by deer, elk, and livestock can damage plants. Herbivory by small mammals and climate change with hotter, drier summers also threaten reproduction and survival. The plant exhibits a mast-seeding pattern, producing many seeds in some years and few in others.

Conservation and research: Since 1995, the Denver Botanic Gardens has monitored skiff milkvetch populations and studied its life history, pollination, seed banks, herbivory, and environmental factors. The Bureau of Land Management lists it as a special status species, guiding land use and restoration in its range. It remains a candidate for federal endangered status, reflecting ongoing concerns about its future.