Osterhout Milkvetch
This species’ striking white flowers attract bumble bees, honey bees and mason bees as pollinators.
About This Species
Osterhout's milkvetch is a perennial, a plant that lives for more than two years, and is endemic to Colorado, meaning it’s found here and nowhere else on Earth. It’s found at only seven sites near Kremmling and is federally listed as endangered. This milkvetch takes up selenium from the soil and concentrates this mineral in its tissues, resulting in a distinctive odor that reminds some of garlic.
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Physical Characteristics
This taller milkvetch grows 12 to 40 inches tall, with pinnately compound leaves, which means its leaves are made up of smaller leaflets which grow in symmetrical rows on either side of the leaf stem. The Osterhout milkvetch has narrow leaflets; its flowers can be white or cream, and cluster on the ends of the plant’s upright stems from June to August. The resulting seed pods are 1.5 inches long, hang in clusters from the stems, and turn maroon when ripening.
Range
Osterhout's milkvetch is found only within a 15-mile radius (a 700-square mile area) near the town of Kremmling in Grand County. Its elevation range is 7,367 to 8,013 feet.
Habitat
Osterhout's milkvetch grows on the clay soil substrates of Niobrara, Pierre and Troublesome shales. These soils have high concentrations of selenium, which this species can tolerate.
This species is often found growing with big sagebrush (Artemisia tridentata), green rabbitbrush (Chrysothamnus viscidiflorus), broom snakeweed (Gutierrezia sarothrae), winterfat (Krascheninnikovia lanata), Hood's phlox (Phlox hoodii), shortstem buckwheat (Eriogonum brevicaule) and western wheatgrass (Agropyron smithii). Bumble bees, mason bees and bumble bees are this species’ primary pollinators.
Threats to Species
Road maintenance and roadside weed control pose a threat to this species, as it often grows near roadsides. Additionally, off-highway vehicle use outside of designated routes, grazing and trampling by livestock, and dam and transmission line maintenance threaten this species. Black blister beetles feed on the flowers of Osterhout's milkvetch, which prevents the plant from forming fruit and seeds, limiting its ability to reproduce. The species is also highly vulnerable to climate change.