Gymnocalycium is a genus of about 70 South American species of cacti. The generic name "Gymnocalycium," from Greek "naked calyx," refers to the flower buds bearing no hair or spines. The members of the genus are commonly known as Chin Cactus.
Their main distribution area of the genus is Argentina, part of Uruguay, Paraguay, southern Bolivia, and part of Brazil. Most species are relatively small, varying from 2 to 6 inches (5 to 15 cm) in size. In cultivation, they are popular for their easy flowering habits, and the flowers are generally brightly colored. Where temperatures fall below 50 °F (10 °C), they must be grown under glass with heat.
Light: Some Gymnocalyciums are shade-seeking in the wild, among shrubs or grasses, while others grow completely exposed. Therefore, some will need light shading from the sun in the hottest months, but to overdo it, will result in the loss of flowers.
Soil: The balance of the potting medium should be sufficient to allow good drainage so that the plants do not sit in soggy soil for more than a day or two after watering.
Water: Watering in the summer months while the plants are growing well can be frequent (weekly for small plants in small pots), but always allowing the soil nearly to dry out before rewatering. Watering in the winter months at all is unwise and certainly not necessary. The difficult times are spring and fall.
Those species which produce offsets can be readily propagated by cuttings. Seeds germinate well when fresh and will keep for a few years if stored in cold conditions.
It is not usually necessary to graft Gymnocalyciums, except for chlorophyll-less or strongly variegated plants. Still, it may be resorted to grow on seedlings of slow-growing species more quickly or propagate more easily solitary or cristate plants. Flat grafting seems to be the most commonly used these days, and it is the easiest to do. The best time for grafting is when both the stock (the bottom part, usually a ceroid cactus) and the scion (the top part, i.e., the Gymnocalycium you are grafting) are actively growing.
Pests and Diseases
Like any other cacti, Gymnocalyciums suffer from the persistent pest, mealybug in both its forms. In addition, Gymnocalyciums will suffer from fungal disorders if overwatered. Still, often only the root system will collapse, leaving you the job of cutting out any rot in the base of the plant and dusting with a fungicide powder before rerooting.