 
Henry Shaw
Cactus Society
A CSSA Chapter
St. Louis, Missouri
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Along with other articles, columns and club updates, each monthly issue of the Henry Shaw Cactus Digest includes an article or two on members' favorite cactus and succulent species. The articles typically include photos and facts on the plants' natural origins and distribution, growing conditions, common and scientific names, care and cultivation tips, and helpful hints for encouraging flower production. Follow the links below this month's offering(s) to enjoy previous Plants of the Month.
March 2010 -- Geohintonia mexicana
By Eric Driskill
Geohintonia mexicana belongs to the tribe Cacteae and the subfamily Cactoideae. This Mexican genus is monotypic, having only one species in the genus.
Some believe this plant to have originated from an ancient hybrid, possibly between Aztekium hintonii and Echinocactus horizonthalonius. G. mexicana grows alongside A. hintonii on gypsum cliffs and hillsides of the Sierra Madre Oriental in Nuevo Leon, Mexico.
E. horizonthalonius has two varieties: nicholii and horizonthalonius. E. horizonthalonius var. horizonthalonius occurs in the Chihuahuan Desert to San Luis Potosi. With G. mexicana growing alongside A. hintonii and E. horizonthalonius var. horizonthalonius growing through that area and continuing almost another 200 miles further south, it is possible that it is a hybrid between the two.
G. mexicana and A. hintonii were both discovered by and named to honor George Hinton. Hinton discovered A. hintonii in 1990 and G. mexicana in 1991. He asked his friends Charles Glass and W. A. Fritz Maurice to describe both finds.
Soon after announcement of the discovery of G. mexicana, some collectors, excited about a new genus of cacti, sought specimens, often illegally, and the population came close to becoming extinct in the wild. Only the future will tell what population reduction in the wild will do to the long-term fate of this plant.
The blue-green plants are solitary and globose, and at times become columnar. Ribs are well defined from 18 to 20 and lack tubercles. Plants grow to 4.5 inches tall and 4 inches in diameter. Areoles extend along the rib edges. Spines, which are 0.3 to 0.5 inches long, are slightly curved and shed easily. Flowers are pink to magenta and open during the day.
Despite being very slow to grow, the plants are relatively easy to grow. You should provide a well-draining soil. G. mexicana require strong sun and light watering to maintain compact growth.
The plants can be grown from seed, but take a decade or more to reach flowering size. You will, of course, also see grafted plants attain flowering size in a fraction of the time, although in a few years they grow so large as to be hard to recognize, since we are used to seeing even old plants in small pots and taking their time. It may be possible to graft plants for a time and then root them separately to obtain larger plants.
Whatever you do, make sure not to buy a plant that appears to have been field collected. You may have to enjoy a smaller seed-grown plant for a few years, but you will know you aren't contributing to the disappearance of this beautiful plant from its native gypsum perch.
March 2010 -- Mammillaria geminispina
By Tom Degnan
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.
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