Coryphantha echinus aka Rhinoceros Cactus

Coryphantha echinus is a small, globular cactus in the family Cactaceae, commonly known as the sea urchin cactus, hedgehog Cory cactus, rhinoceros cactus, or prickly beehive cactus. The species is native to the arid limestone and igneous hills of the Trans-Pecos region of western Texas and the adjacent Mexican states of Chihuahua and Coahuila, where it grows in degraded grasslands and Chihuahuan Desert scrub at elevations from roughly 300 to 1,500 metres. It was first collected by botanist Charles Wright in 1849 and described by George Engelmann as Mammillaria echinus before being transferred to the genus Coryphantha by Britton & Rose.

Plants are usually solitary but can branch with age to form low clumps up to about 80 cm across, with individual stems around 5 cm in diameter. Like other members of Coryphantha, the body lacks ribs and is instead covered in spirally arranged tubercles, each grooved on the upper side. The species is heavily armed: each areole bears 16-30 slender radial spines and typically three (sometimes up to four) longer central spines about 2.5 cm long, giving the plant its sea-urchin appearance. Young plants may lack central spines entirely, developing them only after several years of growth.

The flowers, which appear from April through July at the apex of the stem, are among the most ephemeral in all of Cactaceae. Bright yellow and nearly 5 cm wide, sometimes flushed reddish at the base, they open fully only at midday under brilliant sunlight and wilt within an hour or two. Successful pollination produces green, ovoid, slimy fruits with persistent floral parts that mature two to four months later. The species is slow-growing, often requiring eight to ten years to develop its typical mature appearance.

In cultivation Coryphantha echinus is treated like other Chihuahuan Desert cacti: full sun, very gritty and sharply drained substrate, sparing water during the growing season, and a dry, cool winter rest. It is notably cold-hardy for a cactus, tolerating brief drops to around -16 °C, but it is rot-prone if overwatered or kept wet in cold weather. The species is covered by the family-wide CITES Appendix II listing for Cactaceae, which regulates international trade in wild-collected material; the IUCN has assessed it as Least Concern.