Thelocactus macdowellii

Thelocactus macdowellii, or Chihuahuan snowball, belongs to the cactus family. It comes from Coahuila and Nuevo León in northeastern Mexico. Its small range lies in the Chihuahuan Desert. It grows on limestone hills among dry shrubs. These sites sit near 4,920 feet [1,500 metres] above sea level. LLIFLE reports just one known wild location. Illegal collecting may threaten this rare cactus.

The rounded stem reaches about 0.33 feet [0.1 metres] tall and wide. It starts with one head but may form more with age. The stem is pale green beneath its cover of spines. Its ribs break into cone-shaped bumps called tubercles. Each tubercle bears a white, woolly areole. From these grow about 15–25 fine, white radial spines. Two to four central spines are white or pale yellow. RHS lists a maturity time of five to ten years.

Large flowers open near the top of the stem. Their pink or magenta petals surround a yellow centre. Each funnel-shaped bloom can span about 0.2 feet [0.06 metres]. RHS places flowering in spring. LLIFLE gives a season from midwinter to early spring. Most plants have one flush, though some bloom again. The flowers cannot fertilise themselves. The green fruits have a glossy surface without hairs or spines.

Grow this cactus in full sun or light shade. Give it good airflow and low humidity. Use cactus compost that lets water drain freely. Sandy or loamy mixes suit it, across a broad pH range. Rich compost may cause the stem to stretch. Water moderately during growth, from spring through autumn. RHS recommends low-nitrogen feed each month during this time. Keep the plant dry through winter. A cool winter rest can help it flower well.

Sources differ on its tolerance of cold. RHS assigns H2, which calls for frost-free conditions. LLIFLE reports survival down to 19°F [−7°C]. RHS suggests it for homes, conservatories, and greenhouses. Its small size suits growing in a pot. It needs no pruning, and growers can raise it from seed. LLIFLE also lists cuttings as a way to propagate it. Watch for mealybugs, which may attack the plant. Too much water can cause stem and root rot. The supplied sources do not establish edibility or toxicity.