Mammillaria geminispina aka Whitey

Mammillaria geminispina, or twin-spined cactus, belongs to the cactus family. It comes from northeastern Mexico, with a range reaching Veracruz. It grows in dry scrub on rocky, lime-rich hills. Wild plants occur at heights of 3,281–6,070 feet [1,000–1,850 metres]. They form colonies where the site suits them. Past collecting reduced wild stocks, though that pressure has since eased.

This cactus forms a low mound of clustered stems. Some plants stay single, while others make broad clumps. The RHS gives a garden size of 0.3–1.6 feet [0.1–0.5 metres] tall and wide. Its shorter description puts the mound near 0.7 feet [0.2 metres] tall. LLIFLE records larger clumps reaching 3 feet [0.9 metres] across. Short, light green stems hold milky sap. Rounded tubercles bear white wool and long white bristles. Dense, pale spines give the plant a white look year-round. Each spine group has 16–20 short radial spines. Two to six longer central spines project beyond them. These may be white, dark-tipped, or brown, depending on the plant.

Small, bell-shaped flowers range from pink to deep red. The RHS also describes white flowers with deep pink stripes. Flowers tend to form a ring near the stem tip. Some appear farther down the body instead. Chicago Botanic Garden describes bloom from late spring through fall. Its calendar also lists flowers in November and December. Insects visit the flowers and help pollinate them. Small pink-red fruits may share the stems with open blooms. The seeds are brown. Plants raised from seed may need seven or eight years to flower.

Grow it in cactus compost that drains well. The RHS accepts sandy or loamy soil across all pH ranges. Both garden sources list full sun as a position. Under glass, the RHS advises bright light filtered from harsh sun. Keep humidity low and water moderately during active growth. Give balanced liquid feed every two to three weeks then. Keep the plant nearly dry through winter. The RHS rates it H2: it needs protection from freezing. Chicago lists USDA zones 9–12.

Its clustered shape suits pots, patios, and greenhouse displays. No pruning is needed. Propagate it from offsets or sow seed in early spring. The RHS reports few disease problems, but mealybugs may attack. Birds and butterflies visit it, while its spines discourage grazing animals.