Knowlton's Cactus

A tiny cactus, it may reach a maximum size of 55 millimeters tall by 30 mm wide. It is often smaller, growing to the size of a US penny. An individual is considered to be an adult when it exceeds 10 mm in diameter. It is spherical or somewhat cylindrical in shape. The areoles are round and covered in hairs. Each areole has up to 26 radial spines which are red, pink, or white in color. The spines are less than 2 mm long. The plant produces a flower up to 35 mm long by 25 mm wide with pinkish tepals, the outer ones sporting brownish midstripes. The fruit is green, ripening reddish, and measures about 4 mm long. According to a genetic analysis, this species probably evolved as a mutation of the larger Pediocactus simpsonii.

It is endemic to San Juan County, New Mexico in the United States, where there is a single population on a patch of territory measuring about 12 acres (49,000 m2). As of 2008, there are approximately 6100 individuals in this population. The cactus occurs on land very close to the Colorado border; in fact, it has been observed within 30 meters (100 ft) of the state border, but it does not technically occur within the state of Colorado, and is thus an endemic of New Mexico. Its habitat is pinyon-juniper woodland and sagebrush with loamy, gravelly alluvial soils. In some areas the substrate is covered in cobbles. The cactus may grow in the open or in the shade of larger plants.