The ruby ball cactus, also known as the red cap or Moon 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.
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.