Variously called "Ruby Ball," "Red Cap Cactus," "Moon Cactus" or "Hibotan" (Gymnocalycium mihanovichii var. friedrichii "Rubra" graft), this brightly colored plant results from grafting two different species of cactus together. The top part, the scion, can't live on its own, since it lacks chlorophyll, and it is essentially a parasite on the bottom rootstock plant. To grow the plant successfully you need to consider the growing requirements of both the scion and the rootstock.
The red mutation of the plaid cactus (Gymnocalycium mihanovichii) originated in Japan when a nurseryman noticed two completely red seedling plants among thousands of tiny seedlings. They were immediately grafted onto rootstock, since they wouldn't have lived long on their own without chlorophyll. From these two plants have come the millions of ruby ball cactus grown around the world as a houseplant. The bottom rootstock plant is usually night-blooming cereus (Hylocereus undatus) or blue myrtle cactus (Myrtillocactus geometrizans).
The plaid cactus scion can't tolerate bright sunlight, since it doesn't have the shielding that chlorophyll offers. In addition, the parent species grows as a small cactus beneath sheltering shrubs in deserts of Paraguay and prefers shaded conditions. The night-blooming cereus rootstock also prefers shade, since it is a tropical cactus that grows beneath jungle trees. Blue myrtle cactus grows in direct sunlight but tolerates shaded conditions. For the sake of the scion, even when grown on sun-tolerant rootstock the plant needs bright indirect light.
Watering and Soil
Both rootstocks are vigorous growers, but night-blooming cereus grows fastest, growing more than 40 feet on its own.This rootstock will need more watering than blue myrtle. Grow ruby ball grafts in well-draining cactus mix. Overwatering is more likely for the slower-growing more drought-tolerant blue myrtle. For both rootstocks, watch the top inch of soil. When it is dry, water the plant until water comes through the pot's drainage holes. The time interval varies, depending on the ambient temperatures, the soil mixture and how fast the plant is growing.
The red scion is hardy in United States Department of Agriculture plant hardiness zones 11 through 12. The night-blooming cereus rootstock grows in USDA zones 10 through 11. This combination leads to quick death of both scion and rootstock if exposed to temperatures near freezing. Blue myrtle cactus is hardy in USDA zones 9 through 11. If ruby ball grafts on blue myrtle encounter freezing temperatures, the scion dies and the rootstock survives.
Over time, the tissue between the grafted cacti becomes corky and less viable. Growth of the scion slows or halts, with the scion eventually dying. You can prolong its life by regrafting it. Using a sharp knife sterilized with alcohol, cut the top off a seedling columnar cactus and cut the scion from the old rootstock. Identify the circle of vascular tissue toward the center of the stems of the scion and new rootstock, and press the plants together so the circles 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.
Any time you notice that the rootstock is putting out a branch of its own, immediately remove it. If allowed to develop, the rootstock will give all the food and moisture to its own branch rather than to the grafted scion, and the scion dies. If the branch is young enough, you can simply twist it off or push it off. If it is larger, use sharp clean pruning shears or a sharp clean knife to remove it. Night-blooming cereus is the shortest-lived rootstock and produces more branches.