Gymnocalycium is the genus made up of 70 small South American globular cacti species known for their flowers. Commonly called the chin cactus, the gymnocalycium genus was first named in 1845. Its species are found wild in the grasslands of Argentina, Uruguay, Paraguay, Bolivia and Brazil. The size of these cacti is about 1.5 to 7 inches (5 to 17.5 cm) in height, with diameter on the largest species reaching 12 inches (30 cm).
Colors can vary widely — from light greens to nearly gray — in gymnocalycium, and their textures can be shiny or mat. Some of the species grow flat to the soil, and other produce small shoots or “puppies” from the main plant. Most of the species have cylindrical stems. The ribs of the cacti create the appearance of chins, giving the plant its nickname.
Gymnocalcyium species are known for their white, cream, pink and yellow flowers. The flowers range in size from 1 to 1.5 inches (2.5 to 3.8 cm). The flowers do not have spines or bristles but are smooth. Species of the gymnocalycium genus flower easily, but plants must receive at least a half a day of bright light to bloom.
Popular for cultivation, plants in the gymnocalycium species are among the most popular houseplants and often part of rock garden displays. Gymnocalycium species often can be found in department stores for a very low price. They frequently are sold as novelty items.
Among the most popular of the cultivated species is the Gymnocalycium mihanovichii, a mutant often called the moon cactus. This species lacks chlorophyll and is colored in brilliant reds, oranges and yellows. It lacks chlorophyll, so it cannot complete photosynthesis. It will die as a seedling unless it is grafted to another cactus that has chlorophyll and is capable of photosynthesis. The display usually is a vertical-growing green cactus on the bottom with a round, brilliantly colored red or yellow cactus on top.
In winter months, gymnocalycium plants should be kept in a warm, sunny area indoors but not in direct sunlight. In the summer, gymnocalycium cacti do best when kept in a sheltered location outdoors, but where they can receive plenty of sunlight. In hot weather, they need water every few days but should be watered only enough to keep soil from drying out during the winter.
These plants are propagated by seed, stem cutting or offset. The cacti are susceptible to mealy bugs and red spider mites, and the sciara fly sometimes attacks seedlings. If over-watered, these cacti are prone to fungal disorders.