Mammillaria geminispina (2010)

By Tom Degnan (March 2010)

Mammillaria is one of the largest and most popular genera of cacti. Initially described by Adrian Haworth in 1812, Mammillaria is derived from the Latin name “mammill,” which means nipple or teat – referring to the tubercles. Mammillarias can be characterized as cacti that are low-growing, usually globose and distinctly tuberculate, in some instances forming massive mounds.

Without knowing the flower color, my attendance prize plant presents a bit of a challenge to identify. Initial study suggests the plant could be Mammillaria geminispina. For purposes of our newsletter, I will make this assumption. As I gain more familiarity with this plant, I may have to provide the group with an update.

Habitat: M. geminispina has its origins in the central states of Mexico – Hidalgo, Queretaro and San Luis Potosi – growing at an altitude of 1,000 to 1,850 meters. The average temperature in Hidalgo is 61 degrees F. The temperature ranges from a high of 90 to a low of 45 degrees. The average annual rainfall at elevations around 2,000 meters is 23 inches.

Description: It is a densely spined species that can cluster aggressively. The stems on my plant are cylindrical and about 5 inches tall and, due to the dichotomous, forked branch, each stem is just about as wide. The stem has the potential to reach 7 inches.

The tubercles are rounded with latex and the axils (the depression between the tubercles) contain white wool. The axils are also the areas where the flowers originate.

The spines are varied. Some plants have very short spines, while others have long spines. Two to six central spines protrude from the tubercles and can be pure white or white with dark tips. In addition, 16 to 20 chalky white radial spines are approximately .25 inches long.

Flowers are deep pink to carmine red with darker mid-veins and measure .8 inches long and in diameter. The flowers are usually displayed in a ring at the top of the growth of the previous year, but sometime more randomly over the body. If grown from seed, M. geminispina can take seven to eight years before flowering. The fruits are red, and the seeds are brown.

Cultivation: It is a fairly easy plant to grow, but it needs as much light as possible without burning the plant to encourage the heaviest spine formation. During the summer, it is best to keep the plants outside, where the temperature can rise to over 90 degrees F with no harm to the plant. Furnish good drainage that allows ample air to the roots.

They require only a short winter’s rest and should be kept almost completely dry during the winter months. From March, the plant will begin to grow, and watering should be increased gradually until late May, when the plant should be in full growth. Water regularly during the summer so long as the plant has good drainage. During hot weather, you may need to water the plants more frequently so long as the plant is actively growing.

From late September, watering should be reduced to force the plant to go into a state of semi-dormancy. By October, you should be back into the winter watering regime. Feeding may not be necessary at all if the compost in the substrate is fresh. Feed only in summer if the plant hasn’t been repotted recently.

Do not feed the plants from September onwards, as this can cause lush growth, which can be fatal during the darker cold months. Grown specimens have resisted temperatures to 32 degrees F for a short time, but it is best to keep above 40 degrees to avoid risks of rotting.

Propagation: Seeds or by stem cuttings from adult plants.