Moon cactus Gymnocalycium mihanovichii

The species, Gymnocalycium mihanovichii or chin cactus, is a dwarf cactus that is native to Paraguay and Northeast Argentina and is found in rocky areas. It is a member of the cactus family (Cactaceae). It is a slow-growing, globular dark green cactus, 2-3 inches tall, and equally as wide. The genus name, Gymnocalycium, is Greek, meaning "naked calyx." This refers to the flower buds that have no spines or hairs. The species name, mihanovichii, honors Nicolas Mihanovich, a Croatian Argentine businessman supporting Alberto Vojtech Fric who went to Paraguay to study cacti.

In 1948 grafted varieties of the seedlings of this species were introduced by a Japanese cactophile, Eiji Watanbe. They were brought to market in the 1960s and were not initially successful. By the 1980s, the grafted cultivars were very popular and frequently found in garden shops. They have bright-colored tops such as red, yellow, pink, or orange and are known as "moon cactus."  The globular top cactus is a mutant seedling of the Gymnocalycium mihanovichii, and it is grafted to a host cactus which is typically, Hylocereus undatus. The top cactus is known as scion and does not have chlorophyll, and it relies on the host cactus or rootstock for nourishment.

These plants prefer bright, indirect sunlight and well-drained cactus or succulent mix. Water these plants sparingly and allow the soil to dry out completely between waterings. In the winter during its dormant period, reduce watering even further. These plants are not frost tolerant and prefer temperatures between 68 to 86 degrees Fahrenheit  and low humidity. Outdoor plants should be brought indoors before the temperatures drop below 50 degrees Fahrenheit. Fertilize the plant during the growing season only, and pruning is not generally required. The plant can be propagated by seeds or grafting. 

This cactus can be solitary or have offsets. The species is spherical, grayish-green, usually a reddish stem, and 8 narrowly edged, notched ribs. Each areole has 5 to 6 spines that are curved and grayish-yellow. On grafted plants the rootstock is green, ribbed, and has spines, but the mutated top is colorful because it lacks chlorophyll. The flowers are funnel-shaped, pale green to brownish-yellow and bloom at the crown of the plant. Flowering typically occurs in the late spring to early summer. Small, fleshy, grayish-green berry-like fruits appear after the flowers fade and will turn red when matured.

'Hibotan' is a popular cultivar and typically has red stems, but they may be orange, dark purple, yellow or white. The seedlings that lack chlorophyll are grafted to a fast-growing cactus or base plant such as a species of Hylocereus. The base plant provides the necessary chlorophyll for the upper plant. The stems are globose, 4-inches tall, 5 inches wide. They have 8 to 14 deep ribs. Each areole has 3 to 6 dark brown radial spines.The flowers are pale-pink and bloom in the summer. The fruits are grayish-green to pinkish red. 

The grafted plants are unique with vibrant colored tops and are typically used as ornamentals. They are perfect for a beginning gardener due to their easy care and low maintenance.  To add interest to your home or office, consider using one of the grafted plants on a desktop, as an indoor houseplant, or container planting for the patio. 

Insects, Diseases, and Other Plant Problems:  Monitor for mealybugs, spider mites, and aphids. Fungal diseases can occur from overwatering or high humidity. Overexposure to direct sun can cause scorching, particularly in grafted varieties.