The ruby ball cactus, also commonly known as moon cactus, is a grafted specimen. The colorful red, orange, or yellow top (the scion) is Gymnocalycium mihanovichii, a name referring to the flower buds bearing no hair or spines. The lower green cactus host portion can be any number of species but is usually a Hylocereus cactus. The main job of the lower cactus is to display the Gymnocalycium at an advantageous height.
These plants rarely last more than a few years, since the upper scion and the lower rootstock portions grow at different rates. This can eventually destroy the graft union between the two sections. However, it is not a difficult matter to separate the scion and graft it onto a new rootstock cactus. The best seasons for planting are spring or summer when faster growth is favored.
|Botanical Name||Gymnocalycium mihanovichii|
|Common Name||Ruby red cactus, moon cactus, ruby ball cactus, red top, red cap cactus|
|Plant Type||Herbaceous perennial|
|Mature Size||Varies, depends on rootstock|
|Sun Exposure||Partial, Shade|
|Soil Type||Rich, fast-draining cactus mix|
|Soil pH||Acidic to neutral|
|Flower Color||Red, orange, or yellow|
|Hardiness Zones||11 to 12 (USDA)|
|Native Area||South America|
Watch Now: How to Grow a Ruby Ball Cactus (Moon Cactus)
Ruby Ball Cactus (Moon Cactus) Care
If you can grow cacti and succulents successfully, you can likely grow the ruby ball cactus without too much trouble. These plants are popular in cactus dish gardens.
The ruby ball 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. It is imperative that the cactus is not 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.
The red 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. Look for a bright area, but not so bright that the color of the top begins to wash out.
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.
Allow the soil mix to become nearly dry between waterings, but then water thoroughly. The cactus should not sit in marshy soil for more than a day or so; good drainage is essential. 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 ruby ball 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. It is possible for borderline temperatures to cause the ruby red portion to die while the rootstock survives. Like most cacti, this plant prefers low humidity levels.
You do not need to regularly fertilize your ruby ball 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.
Varieties of Ruby Ball Cactus (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 actually the plant itself.
Propagating Ruby Ball Cactus (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 ruby ball cactus top:
- Use a sharp knife sterilized with alcohol, cut the top off a seedling columnar cactus, then cut the scion from the old rootstock. You will see a circle of vascular tissue at the center of the stems of the scion and new rootstock.
- Press the plants together so the circles at least partially align.
- Put rubber bands over the scion and the bottom of the pot the rootstock is growing in, holding them together until the tissues grow together.
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 its chlorophyll. If the Gymnocalycium does not have that host, it will die.
Potting and Repotting Ruby Ball Cactus (Moon Cactus)
Repot as needed, preferably during the warm 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.
Pleasant, Barbara. The Complete Houseplant Survival Manual: Essential Gardening Know-How for Keeping (Not Killing) More Than 160 Indoor Plants. Storey Publishing, 2005
Miller, George Oxford, and Paul W. Cox. Landscaping with Native Plants of Texas. Voyageur Press, 2013