The moon cactus is an albino plant, which means it has no chlorophyll. Therefore, it relies on the rootstock cactus as a food source. There is a parasitic relationship between the upper and lower portions, and if there is an incompatibility between the needs of the host cactus on the bottom and the scion on top, one or both may die. Like many cacti, these plants prefer a drying period between waterings, even to the point where they slightly wilt. When you water, however, you should water deeply. The plant will noticeably plump up. The cactus mustn’t be exposed to prolonged dampness and standing water, which can cause root rot to develop. Make sure to fertilize the cactus during the growing season for the best results.
Light
The ball tops are tolerant of more shade than many cacti and dislike direct sunlight. By contrast, the stock green cacti on the bottom are often light-lovers. These cacti will do best in an area that received bright indirect light but no more than an hour or two of direct morning sunlight. Too much direct sun will cause the colors to wash out.
Soil
A rich, fast-draining cactus mix with a low pH is ideal. Make sure the soil meets the needs of the host cactus on the bottom.
Water
Allow the soil mix to become nearly dry between waterings, but then water thoroughly. Good drainage is essential since leaving the plant sitting in water can lead to root rot. During the summer months, the plant might need frequent watering, especially if it has been moved outside. Plants in small pots will only need weekly watering. Watering in the winter months is unnecessary, but mist the plant occasionally.
Temperature and Humidity
Ideal conditions for the rootstock and the upper scion portion may not be the same. The upper scion is hardy in USDA plant hardiness zones 11 through 12, while some of the rootstock species (such as night-blooming cereus or blue myrtle) are hardy in zones as low as 8 or 9. During the winter, the recommended temperature range is between 50 and 60 degrees Fahrenheit. It is possible for borderline temperatures to cause the upper portion to die while the rootstock survives. Like most cacti, this plant prefers low humidity levels.
Fertilizer
You do not need to regularly fertilize your moon cactus plant, but you should dose it with a cactus fertilizer every month during its growing season (April to September). Suspend feeding during the dormant winter period.
Types of Moon Cactus
There are many unique varieties of Gymnocalycium mihanovichii cacti, as these are collectors’ plants with a wide following. The appearance can vary widely, depending on what species within the Hylocereus genus has been used as the host, and on the appearance of the upper scion, which is generally a mutated strain of various Gymnocalycium mihanovichii cultivars. They can be red, orange, purplish, yellow, or even white. Older plants sometimes flower with pink blooms during the summer, and many people mistake the colored ball on top for a flower when it is the plant itself.
Pruning
If the lower rootstock portion sends out side shoots, these can be clipped off to the base of the stem, using sharp pruners. Other than this, no pruning is necessary.
Propagating Moon Cactus
Because these cacti are grafted, they are not appropriate for propagation. If you are interested in learning how to graft cacti yourself, it is not difficult, and many species can be successfully grafted. To regraft a moon cactus top: On some older plants, the Gymnocalycium on the top naturally sends out offsets that cluster like satellites around the larger plant. You can remove these and pot them separately as individual Gymnocalycium plants, but it still needs a green cacti host, which supplies the plant with its chlorophyll. If the Gymnocalycium does not have that host, it will die.
Potting and Repotting Moon Cactus
These plants are slow-growing, but they should be repotted every three to four years to rejuvenate the plant with fresh soil. Repotting should preferably be one during the warm growing season. To repot a cactus, make sure the soil is dry before repotting and then gently remove the pot. Knock away the old soil from the roots, making sure to remove any rotted or dead roots in the process. Treat any cuts with a fungicide. Place the plant in its new pot and backfill with cactus-mix potting soil, spreading the roots out as you repot. Leave the plant dry for a week or so to reduce the risk of root rot and then begin to water lightly.
Overwintering
If your potted moon cactus is being grown on a patio, it should be brought indoors as the weather begins to cool to below 50 degrees. Ideal winter temperatures are 50 to 60 degrees Fahrenheit, so patio plants should be given a relatively cool indoor location. Don’t feed these plants during the winter.
Common Pests and Plant Diseases
Cacti in general are free of most common pests, but indoor plants can be affected by spider mites or mealybugs. They can be controlled with neem oil or a chemical pesticide designed for houseplants. The only significant disease is root rot, caused by overwatering or poorly draining soil. If the base of the plant begins to get soft and mushy, withhold all water. You may be forced to regraft the upper scion onto a new cactus and discard the old host cactus.
How to Get Moon Cactus to Bloom
The yellowish-green flowers of the Gymnocalycium mihanovichii scion are indistinct, so there’s generally no reason to coax flowers. In some cases, you may find this plant producing two different types of flowers, since the grafted scion and the lower rootstock are different species with their own flowering patterns. But it is not a problem if the plant has no flowers whatsoever.
Common Problems with Moon Cactus
Like most houseplant cacti, the moon cactus is a tough, easy-to-grow plant, but there are a few issues to watch out for:
Color Is Faded
If the colorful red, yellow, or orange top portion fades, it is usually because the moon cactus is getting too much direct sunlight, which is causing the pigmentation to wash out. Move the plant to a location that gets bright but indirect light.
Plant Begins to Collapse
Overwatering can cause root rot to set in, which will gradually cause the columnar lower host cactus to collapse.
Upper Ball Is Detaching
Because the two cacti species grow at different rates, it’s not uncommon for the graft to separate after a few years. At this point, the best strategy is to separate the top portion and graft it onto another rootstock cactus.
Edges of the Colorful Scion Turn Brown
When the colorful top portion begins to turn brown around the edges, there are two possible causes: the plant is getting too much water, or it is getting too much sun.
