Gymnocalycium (jim-noh-kay-LIS-ee-um) is a popular genus of button shaped cacti. Most are rather small, not exceeding 7 inches in height. However, what they lack in size they make up for in beauty, in both form and flower. As such, these cacti are popular in cultivation
Form:Gymnocalycium are ribbed cacti, but the ribs themselves are slightly tuberculate. This gives the ribs the appearance of chins and gives the cactus its common name, chin cactus.
Colors: Most are a forest green, but some become rust red and even purplish with intense sunlight.
Foliage: All of the foliage of these plants is organized into spines. They have no photosynthetic leaves.
Flowers: Despite variations in vegetative material, the flowers of these plants share a common trait. Each bud is completely naked, lacking both spines and hair. Instead, the buds are scaly and smooth. The open flowers are usually either a pure white or an elegant pale pink.
Light: These plants love lots of light but do prefer some light shade during the hottest parts of the day.
Soil: Pick a gritty, well-draining mix like a cactus and succulent soil (available at garden centers) or mix your own (more info).
Water: Water deeply enough for water to run out the container’s drainage hole. Allow soil to completely dry before watering again. Keep dry in winter.
Hardiness:Gymnnocalycium are not frost hardy and should be brought indoors to a sunny window sill if there is a chance of frost. (What’s my zone?)
Propagation: Many produce new offsets around their bases and can root from these offsets. Some, however, are solitary. Fortunately, they can also be propagated from seeds.
NOTES FROM THE NURSERY
Because of their small size and slow growth, these are commonly used in grafting. Some graphs of this plant are called lollipop cactus due to their shape. We sell a grafted Gymnocalycium called grafted moon cactus. These get grafted on to a rootstalk cactus (typically Hylocereus undatus) because they lack chlorophyll. The root stock does all the photosynthesis while the moon cactus itself can continue to be purple, red, orange or yellow.
In the wild, these plants are native to Argentina, Paraguay, and parts of Uruguay. They can be found at most elevations and tend to grow under other plants as they can tolerate shade and drought. They, of course, like a rockier soil that drains water quickly. Because of these rough conditions, Gymnocalycium tend to be tough and forgiving.