The Mammillaria mystax is one of the 12 or so Mamm species that are easy to find in any garden center in the United States. According to mammillarias.net, a website devoted to the genus Mammillaria, the species are M. bocasana, bombycina, elongata, haageana, magnimamma, microhelia, muehlenpfordtii, mystax, prolifera, rhodantha, spinosissima, and zeillmanniana.
M. mystax,a Mexico native, needs little water and light shade in Phoenix. It will grow to 12 inches tall. It occasionally clumps, but should remain solitary for many years. In early summer, it produces numerous deep pink flowers in a ring around the cactus stem.
In the photo, the darker green area near the top of the cactus is new growth. The flowers are always located on new growth. Further down on the cactus stem, the dried flower remnants from last summer’s flowers remain.