Location: Conejos County
Size: 4305 acres
Landowner: Bureau of Land Management and The State Land Board
Of the approximately 3000 plant species native to Colorado, perhaps 10 percent are considered rare. Such plants are often restricted to a particular type of habitat, and if this habitat is uncommon or declining, this has direct impact on the rarity of the plant species itself. Nearly half of Colorado's Designated Natural Areas (25 of 60) include at least one rare plant species, many of which are endemic to limited regions of the western United States. One such Natural Area is Rajadero Canyon in south central Colorado. Located in Conejos County, this area in the eastern ridges and canyons of the San Juan Mountains protects part of the largest known Colorado population of a rare plant species, the Ripley milkvetch (Astragalus ripleyi). The name "Rajadero" possibly derives from the Spanish "rajador" – wood splitter, or "rajadura" – cleft, fissue, or crack.
Ripley milkvetch is a member of the pea family (Fabaceae), and has characteristic pod-like fruits. The plants grow up to four feet tall, with numerous stems that are a distinctive reddish purple at the base. It blooms from late June through late July, with pale yellow flowers that hang down from the stem. The global range of this species is less than 600 square miles between Terrace Reservoir, Colorado and Tres Piedras, New Mexico. Within that range, the plant is restricted to a narrow belt of volcanic soils less than ten miles wide.
Ripley milkvetch is considered a sensitive species by both the Forest Service and the BLM. The species was at one time considered a potential candidate for listing as threatened or endangered under the federal Endangered Species Act, because where we find it now suggests that it may have been eliminated from much of its historic range. Ripley milkvetch populations are maintained by a delicate balance of natural processes, including fire, deer, and harvester ants. Most of the Ripley milkvetch’s preferred habitat of bunchgrass grassland with scattered shrubs, ponderosa pine, fir and aspen trees has been altered through a combination of historic heavy grazing (conversion to grass-poor shrubland), fire suppression (conversion to dense forest), and exotic species introduction (roadside plantings of smooth brome and yellow sweet clover which have spread).
The Rajadero Canyon Natural Area was designated in 1996, and is managed by the Bureau of Land Management as an Area of Critical Environmental Concern. This Natural Area provides high quality habitat for Ripley milkvetch, and is an outstanding example of relict montane grassland and ponderosa pine savanna plant communities in its own right.