The ruby ball cactus, also known as the red cap cactus, is a grafted specimen. The colorful red top is a Gymnocalycium, while the lower green cactus could be any one of a number of varieties: Its main job is to display the Gymnocalycium at an advantageous height. These plants are popular in the cactus dish gardens frequently offered for sale. They are not demanding houseplants, but they are not without challenges either.
If there is a disconnect between the light requirements of the stock cactus on the bottom and the scion on top, the challenge is finding the right balance so both will thrive.
Growing Conditions for the Ruby Ball Cactus
- Light. The Gymnocalycium 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 cap begins to wash out.
- Water. Allow the soil mix to become nearly dry between waterings, but then water thoroughly. Immaculate drainage is essential, so never let the pots sit in water. Suspend watering in the winter, but mist occasionally.
- Soil. A rich, fast-draining cactus mix is ideal.
- Fertilizer. During the growing season, fertilize with a cacti fertilizer mix. Suspend feeding during the dormant winter period.
As these cacti are grafted, they are not appropriate for propagation.
If you are interested in learning how to graft cacti yourself, it's not terribly difficult, and many species can be successfully grafted. 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 up separately as individual Gymnocalycium, but they lack the supporting green cacti, which supplies chlorophyll, and they soon die.
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 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.
There are many varieties of Gymnocalycium cacti, which are collectors' plants with a wide following. The common ruby ball grafted plant relies on the G. mihanovichii, which is available in red, yellow, orange or pink. Older plants sometimes flower with pink flowers during the summer, and many people mistake the colored ball on top for a flower, when it's the plant itself. As a point of interest, the ruby ball hybrids cannot produce their own chlorophyll and thus rely on the grafted rootstock to produce chlorophyll and keep them alive.
If you can grow cacti and succulents successfully, you can likely grow the ruby ball cactus without too much trouble. Like many cacti, they 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's imperative that the cactus is not exposed to prolonged dampness and standing water. Never let your cactus sit in a dish of water. Make sure to fertilize during the growing season for the best results.