Moon Cactus is a special cactus grown from grafts of two different cacti. The Hylocereus is the rootstock, and the Gymnocalycium mihanovichii is the scion. These two different cacti are grafted together, and their union is collectively referred to as a Moon Cactus. They are also known by Ruby Ball Cactus, Red Cap Cactus, and Red Hibotan, to name a few.
Moon cacti are prized for their unique appearance, and they can be found in many different colors. They are primarily grown indoors in a pot and serve as houseplants requiring little maintenance.
The key part is the colorful scion, which is always a variant of Gymnocalycium mihanovichii species. The rootstock is a choice between many different types of cacti, with Hylocereus being the most common. The rootstock’s two jobs are to prop up the scion to display its unique coloration and provide its chlorophyll capabilities. Gymnocalycium mihanovichii display different colorations than typical cacti because of their lack of chlorophyll. This means that they cannot survive on their own for any length of time; this is where the rootstock comes in. It is a parasitic relationship between the two, with the rootstock usually ending up discarded in favor of fresher stock further down the line.
Moon Cactus Main Characteristics
| Common Name | Moon Cactus, Ruby Ball Cactus, Red Cap Cactus, Red Hibotan, Red Cactus |
| Botanical Name | Gymnocalycium mihanovichii |
| Synonyms | Echinocactus mihanovichii, Gymnocalycium friedrichii |
| Family | Cactaceae |
| Genus | Gymnocalycium |
| Type | Cactus |
| Native Range | Paraguay, Northeast Argentina |
| Foliage | Evergreen |
| Propagation methods | Regrafting, Offsets |
| Sun | Full sun to partial shade |
| Soil | Well-drained |
| Toxicity | Non-Toxic |
Moon Cactus Care
The grafted specimen of Gymnocalycium mihanovichii and Hylocereus are a popular choice for collectors. These plants are relatively easy to care for and have a striking appearance. Care for them like any other cactus despite their unusual nature, and you’ll succeed. Here’s how to get started.
Light and Location
Moon cacti do best when placed in a location that receives bright, indirect light with a few hours of sun each day. So, if you’re growing them indoors (as you should), find a spot near a window where they will receive the right amount of light.
Note that these are two different cacti grafted together; the top portion likes a bit of shade while the bottom wants direct sun. Compromising between the two is necessary for the plant to function.
Watering
Water your Moon Cactus deeply but allow the soil to dry out between waterings. Overwatering results in root rot and death. An excellent way to test is to stick your finger in the soil. If it’s wet, don’t water it yet. Be mindful that, like many others, this cactus doesn’t need watering in the winter; mist the leaves if the cactus looks like it needs some water but nothing more.
Temperature
Moon Cactus prefers intermediate temperatures. Make sure you don’t place it in a spot where it will get too hot or too cold. The room temperature is usually just fine.
Humidity
Gymnocalycium mihanovichii enjoy a dry spell as much as the next cactus, so keeping them in low humidity spots is a given. But remember to mist the plant in the winter as it’s the only moisture it can tolerate during dormancy.
Fertilizer
Feed your Moon Cactus a balanced fertilizer diluted to half-strength every month during the growing season. These plants don’t need it per se, but it will help. Like watering, don’t bother with feeding in the winter as the cactus goes dormant.
Cultivars of Gymnocalycium mihanovichii
There are a few different cultivars of Gymnocalycium mihanovichii, but the most popular is the Ruby Ball Cactus or Red Hibotan. This cactus looks like a round, ruby-colored ball that looks gorgeous mounted on a green stalk. It’s one of the more visually appealing cacti and makes for a great conversation starter.
If you’re looking to add a Gymnocalycium mihanovichii to your collection, be sure to get a grafted specimen as they are much more eye-catching than the regular kind. Your Moon Cactus will thrive with the right light, water, and temperature.
Propagating Gymnocalycium mihanovichii
Due to their peculiar nature, Moon Cacti can’t be propagated the same way as regular cacti. The only real way to propagate them is by grafting the scion on another rootstock. Grafting is easier than it sounds; here’s how you do it:
- Cut off a part of the Gymnocalycium mihanovichii and the top of the fresh rootstock (Hylocereus) with a sharp, sterile knife.
- Join the two cut ends of both cacti so that both of their spines meet. The spines are the distinct white center visible on the cut surface.
- Secure the two cacti with a rubber band that’s not too loose or too tight.
- Wait for about two weeks for the grafting to be complete.
Sometimes the scion produces offsets or “pups” that can also be grafted. The process is the same as grafting the main body of the scion, but instead of using the main body, you’ll be using the smaller offsets it produces.
Potting and Repotting Gymnocalycium mihanovichii
Moon Cactus doesn’t grow too fast, and it will take a few years before it needs repotting. But, even if the roots aren’t spilling out of the pot, repotting can still be beneficial. It will allow the plant to experience fresh soil filled with nutrients.
You can repot these cacti at any time of year, but it’s best to do it when it’s actively growing. When you do repot, be sure to use a pot that’s one size larger than the pot you’re using now. Use a soil mix that drains well, like a cactus mix. Make a hole in the center of the soil and place the cactus in it. Gently pack the soil around the cactus. Water sparingly until the plant is established.
Toxicity
Gymnocalycium mihanovichii is generally non-toxic to both humans and pets.
Common Pests
Mealybugs and scale insects are the most common pests you’ll find on these cacti indoors. If you see any signs, take swift action to get rid of them. Horticultural oils can deal with these pests without damaging the plant.
Common Problems
Washed Out Colors
A common problem is too much light. These cacti need bright, but not direct, sunlight. They can tolerate direct sun, but only a little. If they’re getting too much light, their colors become washed out as a sign of stress.
The Rootstock Is Tilting
The most common problem with Moon Cactus is overwatering. Don’t allow the water to stand in the pot, and be sure to only water when the soil is dry to the touch. Overwatering will cause the roots to rot and the bottom cactus to collapse until it eventually dies.
The Graft Coming Apart
The place where the two different cacti are connected can come under extreme strain due to the two cacti growing at different rates. This is quite natural, and there is nothing you can do except find a new rootstock. The grafted union of a Moon Cactus comes with a strict time limit.
Moon Cactus Indoor Care Guide (Video)
FAQ
Why Is My Moon Cactus Turning Yellow?
A yellowing Moon Cactus is usually a sign that it’s getting too much light. Move it to a shadier spot, and the color should return to normal. It could also result from too much water; check your watering routine.
How Often To Water Moon Cactus?
Watering every week is usually enough, but check the soil moisture to be sure. When it’s dry to the touch, water thoroughly. In the winter, when the cactus is dormant, don’t water at all.
Can I Grow Moon Cactus From Seed?
No, you can’t grow one from seed. These cacti are a result of grafting that doesn’t occur naturally. You need human involvement for this combination to occur.