Castlewood Canyon State Park, a designated Colorado Natural Area, encompasses 2,303 acres within the northernmost extension of the Black Forest. The Black Forest, also known as the Palmer Divide, refers to an elevated peninsula that juts eastward from the Front Range and divides the drainages of the Platte and Arkansas Rivers. The Black Forest is a unique ecological region because it borders Front Range foothills and plains grasslands ecosystems. Elevations in Castlewood range between 6,200 to 6,600 feet, and Cherry Creek flows through its center.
Vegetation communities at Castlewood include the following:
- Ponderosa pine savanna
- Ponderosa/Douglas fir woodland
- Mixed foothill shrubland
- Mixed grass and short grass prairie
Mixed foothill shrublands occupy drier slopes and openings within the forest community, where Gambel’s oak, mountain mahogany, skunkbrush and snowberry are the dominant species. Along drainages, chokecherry and American plum commonly grow with Gambel’s oak. Grasslands are an interesting association of short grass prairie, tall grass prairie, montane grassland and introduced species. Barren rock outcrops and cliffs support lichen and moss communities.
A variety of riparian and wetland communities are present along Cherry Creek, tributary drainages and on seeps and springs. Wetlands support healthy and diverse plant and animal communities by filtering contaminants and anchoring shorelines. Unique grotto or hanging garden wetlands occupy ceilings and floors of dissolution caverns along the western canyon walls. They contain mosses, liverworts, ferns and vascular plant species representative of the last ice age.
Castlewood supports several rare plants, some "critically imperiled," which have fewer than five populations in the entire state. Richardson’s alumroot, American currant, New Mexico cliff fern, Sprengel’s sedge, giant burreed, grass fern and carrion flower are potentially threatened plant species found in the park.