Although pediocactus simpsonii
has a wide geographic distribution, from Oregon and Montana south to Arizona and New Mexico, the cactus is still relatively uncommon, growing in high elevation locations, often in small, localized areas, frequently obscured by grass and fallen leaves. The plant may form clusters but usually remains solitary, starting as a neat globe, becoming elongated and cylindrical when aged (up to 10 inches tall).
The 15 to 35 white radial spines are thin and short, while the 4 to 11 central spines are thicker, longer (up to 3/4 inch), and brown or yellow at the tips; all grow from pronounced tubercles, arranged in spirals. Spine cover tends to be dense, nearly or completely obscuring the green stems, though in some specimens the spines are much shorter, and fewer in number.
The plant's appearance changes greatly in different locations, and several sub species have been proposed, but distinguishing features are so variable across the 900 mile range that precise identification is difficult.