Ruby Ball Cactus (Moon Cactus) Plant Profile

A cluster of flowering ruby ball cactus plants (Gymnocalycium mihanovichii) in pots.

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The ruby ball cactus, also known as the red cap cactus, is a grafted specimen. The colorful red top is a gymnocalycium, which refers to the flower buds bearing no hair or spines. While the lower green cactus could be any number of varieties (usually a Hylocereus cactus). The main job of the lower cactus is to display the gymnocalycium at an advantageous height.

  • Botanical Name: Gymnocalycium mihanovichii
  • Common Name: Ruby red cactus, moon cactus, ruby ball cactus, red top, or red cap cactus
  • Plant Type: Houseplant, perennial
  • Mature Size: Varies, depends on rootstock
  • Sun Exposure: High light
  • Soil Type: Rich, fast-draining cactus mix
  • Soil pH: Acidic to neutral
  • Bloom Time: Year-round
  • Flower Color: Red, orange, or yellow
  • Hardiness Zones: 11 and 12
  • Native Area: South America

How to Grow Red Ball Cactus

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. It has a parasitic relationship. If there is a disconnect between the requirements of cactus on the bottom and the scion on top, one or both may die.

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 is imperative that the cactus is not exposed to prolonged dampness and standing water or else it may develop root rot. Make sure to fertilize the cactus during the growing season for the best results.

Light

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.

Soil

A rich, fast-draining cactus mix with a low pH is ideal. Make sure you meet the soil needs of the host cactus on the bottom.

Water

Allow the soil mix to become nearly dry between waterings, but then water thoroughly. The cactus should not be sitting in a marshy soil for more than a day or so; good drainage is essential. During the summer months, the plant may need frequent watering. Plants in small pots will only need weekly watering. Watering in the winter months is unnecessary, but mist occasionally.

Temperature and Humidity

The ruby ball scion is hardy in USDA plant hardiness zones 11 through 12. Some of its rootstock (such as night-blooming cereus or blue myrtle) is hardy in zones as low as 8 or 9. If the ruby red is grafted onto a plant that remains hardy in those lower temperatures, and it is exposed to those temperatures, the grafted ruby red top will die. The rootstock will likely survive.

Fertilizer

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.

Potting and Repotting

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.

Propagating Ruby Ball Cactus

Being that 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, and 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, but it still needs the supporting green cacti, which supplies the plant its chlorophyll, if it does not have that, it will die.

Varieties of Ruby Ball Cactus

There are many varieties of gymnocalycium cacti, which are collectors' plants with a wide following. Older plants sometimes flower with pink flowers during the summer, and many people mistake the colored ball on top for a flower, when it is actually the plant itself.