Mammilloydia

Mammilloydia is a monotypic cactus genus containing a single accepted species, Mammilloydia candida, the snowball cactus. Franz Buxbaum segregated it from Mammillaria in 1951 on two seed-coat characters: Mammilloydia candida has a perfectly smooth seed surface, while Mammillaria seeds are tuberculate (pitted or warty). A second character reinforces the split: Mammilloydia lacks a perisperm (a nutritive tissue layer present in every Mammillaria seed). Kew POWO continues to accept Mammilloydia as a valid genus on this basis. Many collectors and nurseries still list the plant under the older name Mammillaria candida, which POWO treats as a synonym.

Mammilloydia candida is native to the northeastern Mexican states of Coahuila and Nuevo León, growing on exposed limestone scree and rocky slopes at elevations roughly between 1,500 and 2,200 m. The habitat is semi-arid, with summer rainfall and a cool, dry winter season. Substrate is shallow, fast-draining calcium carbonate rubble with very little organic matter. Populations tend to occur on south and east-facing escarpments where full sun is guaranteed for most of the day.

Plants are globose to short-cylindrical, typically 5 to 14 cm tall and 5 to 10 cm in diameter when fully mature. Growth in cultivation is slow; a seed-grown plant may take eight to twelve years to reach flowering size. Older plants can offset and form small clusters, but most specimens remain solitary for many years. The body is entirely hidden under the dense white spine coat, giving the plant its common name: snowball cactus.

Flowers are campanulate (bell-shaped), small relative to the plant body, and appear in a ring near the apex. Colour ranges from pale pink to cream or nearly white, with a slightly darker midstripe on each petal. Blooms open during the day in spring and early summer. The flower tube is short and the tepals are narrow; the overall impression is delicate against the solid white spine mass. Red berries follow pollination, persisting in the axils for several months.

In its Coahuila and Nuevo León habitat, Mammilloydia candida experiences occasional frost down to approximately −5 to −8°C during dry winter nights. In cultivation under a cold greenhouse or unheated frame, plants handle brief dry cold to around −5°C without damage. Wet cold is more dangerous than dry cold at any temperature; the combination of moisture and frost at the root zone is the main cold-season killer. Growers in USDA zone 9b and warmer can attempt outdoor cultivation in a fully exposed, fast-draining raised bed, but greenhouse overwintering is the safe default.

Plan a 95% inorganic / 5% organic mix with 30% pumice, 15% lava rock, 10% zeolite, 10% granite grit, 20% crushed limestone, 10% horticultural silica, and 5% worm castings; total 100%. The high limestone fraction reflects the species’ confirmed calcicole habitat on Coahuila and Nuevo León limestone scree.

Mammilloydia candida falls under the blanket CITES Appendix II listing that covers the entire Cactaceae family, so cross-border movement of plants or seeds requires CITES documentation. No additional federal species-level protection applies under US or Mexican law beyond the CITES Appendix II listing. Nursery-propagated plants are legal to buy, sell, and grow in the United States, the United Kingdom, Canada, the European Union, and Australia, provided appropriate documentation accompanies any international shipment. Wild collection is prohibited.