Pediocactus simpsonii aka Plains Cactus

Pediocactus simpsonii is a small cactus from the western United States. It belongs to the cactus family, Cactaceae. Common names include snowball cactus and mountain ball cactus. Its range includes Arizona, Colorado, Idaho, Nevada, and New Mexico. It also grows in South Dakota, Utah, and Wyoming. Kew places it in a temperate climate. It is not native to Britain or Ireland.

Most plants grow as one stem, though some form small clusters. Stems can look like balls, eggs, or flattened eggs. They stand about 0.08–0.49 feet [0.025–0.15 metres] tall. Some reach 0.82 feet [0.25 metres]. Stem width ranges from 0.08–0.49 feet [0.025–0.15 metres]. These figures describe the species, not a confirmed garden size range. Stem size varies across its wide range. Plants may shrink below the soil surface while dormant in winter.

Cone-shaped tubercles grow in spirals around the stem. Round or oval areoles bear hairs and two types of spines. Each areole has 15–35 thin, white radial spines. It also bears four to eleven stiff central spines. These may be straight or curve a little. Most central spines are reddish brown, with cream or yellow bases. A few are black. The dense spines can hide much of the stem beneath them.

The bell-shaped flowers form a small ring at the stem tip. Their colours include pink, magenta, pale yellow, and near-white. Outer tepals can have green-brown stripes down their centres. Their edges may be toothed, finely cut, smooth, or wavy. Plants on eastern slopes tend to have pink flowers. Those on western slopes tend toward yellow tones. Flower colour varies across the species, as does stem size.

In the wild, this cactus grows in grassland, scrub, and woodland. It also occurs in mountain conifer forests and rocky valleys. Some sites have fine, loose soil; others have rocky ground. Its habitats are cooler and wetter than those of many cacti. Larger plants, grass, and fallen plant matter can hide it. The species tolerates severe cold, but the evidence gives no hardiness zone. It remains widespread and abundant, including within protected areas. Mining and changes in land use pose local threats.