From the Boulder, CO Daily Camera.
Pediocactus simpsonii, Mountain Ball Cactus, Photo by Stephen Jones
We once saw more than 60 blooming in one spot near the Meyers Homestead Trail at Walker Ranch Open Space. The blossoms close on cloudy days, so go out in the middle of a sunny day to find them.
The globular cactus plants are especially abundant on dry, gravelly ridges where they receive the most intense light, but they also thrive on rocky soils in foothills canyons and have even been found on mountain passes.