These must be one of the most common cacti found in my local garden center, and since I don't think my garden center is all that unique, I'm assuming they can be found everywhere cacti are sold. The ruby ball cactus, also known as the red cap cactus, is actually 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. As a result, these plants are popular in the cacti dish gardens frequently offered for sale. As far as growing them goes, they are not especially demanding, 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.
The Gymocalycium 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 cap begins to wash out.
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.
A rich, fast-draining cactus mix is ideal.
During the growing season, fertilize
with a cacti fertilizer mix. Suspend feeding during the dormant winter period.
As these are grafts, they are not really 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 will naturally send out offsets that will cluster like satellites around the larger plant. You can remove these and pot them up separately as individual Gymnocalycium, but they will obviously lack the supporting green cacti.
Repot as needed, preferably during the warm season. To repot a cacti, make sure the soil is dry before repotting, 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, then begin to water lightly to reduce the risk of root rot.
There are multitudes of varieties of Gymnocalycium cacti, which are collector's 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 will sometimes flower with pink flowers during the summer, and many people mistake the colored ball on top for a flower when it's the actual plant itself. As a point of interest, the ruby ball hybrids cannot produce their own chlorophyll, and thus rely on the grafted root stock to produce chlorophyll and keep them alive.
If you can grow cacti
successfully, you can likely grow the ruby ball cacti 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 sitting water. Never let your cactus sit in a dish of water. Lastly, make sure to fertilizer during the growing season for the best results.