– Gymnocalycium (2006)

By Mike Cushner (July 2006)

Plants of the genus Gymnocalycium are often among the first cacti in the beginner’s succulent collection. They are usually well represented in the offerings shipped to Lowe’s, Home Depot and Wal-Mart for retail sale. In many cases, these cacti are easy to care for and can be successfully grown on a brightly lit window sill.

Their space requirements are often modest because most remain compact. Many often flower while only a few years old and often have flowers as large as the plants’ bodies. If those points aren’t enough to drive the popularity of the gymnos, consider the first time you saw the so-called Moon Cactus displayed for sale. I’ll have more on this oddity later.

From within the tribe Trichocereeae, the genus Gymnocalycium derives its name from the Greek for naked calyx. This refers to the absence of spines or hair on the flower buds. The genus represents 70 to 100 species. A more precise number depends on the constantly evolving botanical classification system.

Gymnos are often referred to as “chin” cacti because of the prominent chin found between areoles. Gymnos’ natural habitats are the grasslands or rocky ground (scree) in several South American countries, most notably Argentina and Bolivia, and also Brazil, Paraguay and Uruguay.

Although some gymnos take on a columnar growth form with age, most of these species tend to remain globular. In habitat, they can undergo severe shrinkage and contraction into the soil in response to prolonged drought. A soaking rain can quickly reverse this process and restore the plants’ shape to the predrought condition.

Despite their typically globular growth form, the gymnos display enough differences to make them interesting to collectors. They can remain dwarf – G. bruchii, with flowers as big as the one-inch plant – or grow to be foot-wide giants like G. saglionis. They can be sparsely spined like G. denudatum, another small plant with large flowers, or densely spined like the G. bruchii mentioned earlier. The flower petals are white, pink, yellow, orange or bright red, and can be solid or striped.

Advice on the culture of these plants is varied. Most sources agree that the gymnos want little or no water when overwintered in a greenhouse. In the hottest part of the summer, they benefit from frequent watering. According to Cacti and Succulents by G. Andersohn, the water should be slightly acidic, as gymnos are among those succulents – including epiphytes, rebutias, lobivias and most cereus – that prefer a slightly acid growing medium.

Advice on light requirements varies among sources. My impression is that the most densely spined gymnos can tolerate the most direct sunlight, but all need some direct sunlight to promote flowering. In The Illustrated Reference on Cacti and Other Succulents, E. Lamb and B. Lamb said the flowers of some species, unless subjected to “very hot conditions,” will not fully open. Of course, most of us need to be cautious when reacclimating our plants to unfiltered sunshine after their winter rest.

The Moon Cactus is a true oddity, but now a very common one. In or about 1940, cuttings from some variegated seedlings were repeatedly grafted until a red, chlorophyll-free cactus, G. mihanovichii var. friedrichii fa. variegata Hibotan Hort, was isolated. Lacking chlorophyll, the scion relied on nutrients provided by the grafting stock.

By grafting offsets and through other means, botanists have made these gymnos available in a spectrum of bright colors wherever cacti are sold. In recent years, Hylocereus trigonus has been the most common stock for grafting. Note that while it is an efficient source of nutrients for the scion, it needs the moisture, winter warmth and a growth medium high in humus typical of an epiphytic cactus.