How To Grow a Moon Cactus – Gymnocalycium Mihanovichii

Even though succulents and cacti are less colorful and dramatic than other plants, they are popular houseplants because they are resilient and easy enough for beginning gardeners to grow successfully. Plus, there are many varieties to choose from. 

If you want to add a little glamour to your cactus garden and have the patience for a cactus that can be a bit of a diva, try a moon cactus. 

Otherwise known as Gymnocalycium mihanovichii, Hibotan cactus, ruby ball cactus, star flowered cactus, or red cap, moon cacti are misshapen, thorny, blob-shaped cacti. They come in various highly pigmented shades, typically bright red, orange, neon yellow, and pink, but occasionally purple or even near-black. 

While you might assume that the green base under the colorful ball is also part of the moon cactus, that is incorrect. Moon cactus cultivars are mutants that cannot survive independently, so they are grafted onto the top of a sturdy host cactus. 

The care required for moon cacti is similar to most cacti, but they can be a little trickier to grow because of being a graft. Some are resilient, while others thrive in identical conditions, maybe surprisingly fussy, and they have a limited lifespan once they wear out their host. 

Still, if you want to add some vivid color to your cacti arrangement, or you like the idea of having your pet mutant, it’s worth taking a chance on a moon cactus.

woman holding a pot of ruby ball moon cactus

Disclosure:It is important you understand that we may receive commissions when you click our links and make purchases. However, this does not impact our reviews and comparisons. All opinions are our own we pride ourselves on keeping our articles fair and balanced. For more info see our disclosure statement.

Quick Guide to Moon Cactus

colorful moon cacti
Sun requirements Bright indirect light
HardinessZone 11-12
ToxicityNontoxic but prickly
Primary growth seasonApril-September
Typical sizesDepending on the host species, it may grow a maximum of 4-12 inches tall
FlowersOccasionally produces pink flowers, mainly when grown outdoors

Moon Cactus Sun requirements

rows of moon cactus outdoors

Since a moon cactus is, again, two cacti grafted together, you’ll be hard-pressed to satisfy its sun requirements. 

The host cactus, often a Hylocereus (a.k.a. dragonfruit), usually likes a lot of sun. However, the colorful Gymnocalycium mihanovichii, the main attraction, cannot tolerate direct sunlight. 

In this case, you should keep your moon cactus in bright indirect light. You could put in a spot near (but not in) a south-facing window or outdoors in an area with spotty sunlight. You may have to move your moon cactus around a bit to find a place where it will thrive.

Hardiness

Moon cactus is drought tolerant but not frost tolerant. Keep it out of temperatures under 40 Fahrenheit. Don’t plant in the ground north of Zone 11. If you anticipate cold temperatures overnight, protect the moon cactus with a blanket. During summer, you can grow them outdoors in a pot. You can then just bring them inside your home when it’s winter. 

Toxicity

Moon cactus is generally considered non-toxic. However, its spines make it a tricky choice for households areas where pets and small children could access it. Moreover, unless you are sure about the host plant’s species, you can’t be sure about the toxicity.

Primary growth season

The cactus is a slow grower, but you can expect to see most of that growth from late spring through early fall. 

Typical sizes

The moon cactus typically only grows an inch or two high, but the height of the rootstock may vary. Usually, the whole plant’s maximum height is in the 4-12 inch range.

Flowers

close picture of moon cactus’ flower

Due to their color, moon cacti are sometimes mistaken for being cactus flowers, when in fact, they are cacti that can produce flowers of their own. These flowers are typically pink, although they can be more red or white. The flowers erupt from the surface of the moon cactus rather than growing from a flower stem. 

Flowers tend to appear only under ideal conditions and rarely when grown indoors. When they flower, it is typically in late spring or early summer.

Growing the Moon Cactus

several potted moon cacti

Understanding your cactus’ origin story is the first step in meeting its needs. And, as adorable as it may be, the moon cactus is best described as a parasitic mutant. 

In South American deserts where it is endemic, Gymnocalycium mihanovichii is green from chlorophyll, but rich green masks other colors. The cultivars sold as houseplants are a mutated version that cannot produce chlorophyll. In the absence of chlorophyll, the underlying color–vivid red, orange, yellow, pink, or even purple–becomes visible.

Because they can’t produce chlorophyll, these beautiful freaks cannot survive in nature. To live, they must draw sustenance from a host plant, onto which they are grafted. The most common choice for the rootstock is a Hylocereus cactus (commonly referred to as dragonfruit). Growing a happy moon cactus means being attentive to the needs of the cactus and the host.

One other thing to keep in mind is that moon cacti often don’t live beyond a few years, even when well cared for. Over time the host plant will eventually weaken from supporting both plants. When it dies, the moon cactus will die too – unless you remove and regraft your moon cactus onto a new rootstock.

Where to Plant

grafted moon cactus outdoors

Because they are not cold-hardy, moon cactus can only be grown outside in Zones 11-12. Even in those zones, you should cover your moon cacti with a blanket, especially if temperatures look likely to drop under 40 degrees Fahrenheit. You can also bring it inside when night falls. 

Indoors, try locating the pot on a shaded sunporch, near but not in a south-facing window, or any other location that is bright and sunny without getting direct sunlight.

Container

four moon cacti in one pot

Like all succulents, the moon cactus needs excellent drainage. Use a pot with a large drainage hole in the bottom. An unglazed pot is ideal, as it will absorb excess moisture from overwatering.

As with any water-sensitive succulent, avoid using a pot that is too large for the plant. Moon cactus don’t mind crowded roots. However, a large container filled with too much soil will retain moisture longer, encouraging root rot. 

Soil

Moon cactus needs well-draining soil. You can use a well-draining succulent or cactus mix or make your potting medium by adding some coarse sand, perlite, pumice, or similar gritty amendments. 

Gymnocalycium Mihanovichii Maintenance and Repotting

woman transplanting cacti

Moon cacti stay very petite and don’t mind crowded roots. That means repotting is usually only necessary to freshen up the potting mix after a few years when its nutrients have been consumed. 

Roots growing out of the drainage hole are another excellent sign your moon cactus is ready to move up to a slightly larger container. Either way, you shouldn’t need to repot more than once every few years. 

Here’s a video about repotting a moon cactus: 

Repot your moon cactus in the spring, at the beginning of its growing season. Water it a couple of days before repotting to hydrate the roots and make them more resilient to transplant shock.

Moon cactus don’t require any other maintenance. However, if you want to keep a more regular circular shape, you can trim off any pups that grow on the sides of your mother plant. Use a sharp, sterile blade or simply twist off the pups. You can graft the pups onto new rootstock. 

Watering Gymnocalycium Mihanovichii

It is essential to avoid overwatering your moon cactus. These plants are very drought tolerant, so overwatering can cause leaf drop, root rot and put the plant at greater risk of opportunistic pests. 

You can avoid overwatering your moon cactus by watering it only when it needs it. Do not water unless the plant appears to have been dry for about a week, or if, when you poke a finger into the soil at the plant’s base, the soil is bone dry to your first knuckle. When you do water, use the “soak and dry” method: 

  1. Confirm that the soil is fully dry to at least an inch of depth. 
  2. Place the pot in a sink and gently water the plant until water streams out of the drainage hole in the pot’s bottom. 
  3. Wait a few moments for the water to absorb throughout the soil as much as possible.
  4. If you see dry patches, add a bit more water to make sure the potting mix is completely saturated. 
  5. Let the pot drain well so that it does not accumulate water in its saucer.
  6. Move the plant back to its home. 
  7. Let the potting mix dry thoroughly before watering again. 

You should only water a mature moon cactus during the growing season (April-September), not during the winter. A young, tender plant may want very sparing waterings over the winter.

Fertilizing The Moon Cactus

woman adding fresh potting mix

Moon cactus have low feeding needs. It may not need any at all, assuming it is planted in reasonably fresh potting mix. However, you can use a liquid cactus fertilizer, preferably organic, diluted to half strength. Apply this mixture no more often than every other month during the growing season, April to September. 

Pests and Disease

Mealybugs and scale are the two most common problems with the moon cactus (other than root rot or other issues related to overwatering). In either a case, you can pick the bugs off individually or kill them with a bit of isopropyl alcohol on a q-tip or cotton ball. For a nasty infestation, you could use alcohol in a spray bottle. 

Propagation

You can’t propagate the moon cactus the traditional way because it cannot survive on its own. They must be grafted to a rootstock. However, you can create new moon cacti using an offset (or “pup”) from a mother plant and a new host cactus. 

These instructions assume you are using an already-rooted piece of Hylocereus rootstock, ideally from a fruit-producing mother tree rather than an offset. However, it is possible to do the graft and plant the host at the same time. You can cut the spikes off your host cactus to make it easier to handle, but this is optional.

How To Graft Moon Cactus 

picture of a moon cactus

1. Water your moon cactus

The first step is to water your moon cactus but not on the same day of the grafting. Water your moon cactus three days before the procedure.

2. Sterilize a razor or sharp knife 

After three days, get a razor or sharp knife – you’ll use it to cut the rootstock. Sterilize it with alcohol or hydrogen peroxide before proceeding to the next step.

3. Prepare your host cactus

Prepare the Hylocereus by slicing off the top. You can cut the rootstock short or leave it a bit taller, but around four to six inches works well. Make sure the cut area is very smooth and level. 

Bevel the cactus’ edges. In other words (assuming you’re using Hylocereus), trim off the ends off of its three fin-like projections. Start with your knife at the outer edge of the core – leave a flat top the size of a coin. Cut at a downward angle, away from the core, like you were carving a blunt-tipped spear. 

You should see the cambium and vascular ring in the center of the exposed area. Beveling is an important step. Beveling reduces trapped moisture that can cause the graft to rot. Plus, when the cut ends of the Hylocereus dry out, they may swell a bit, and that can move the moon cactus out of position before the graft takes. 

4. Prepare your scion

Choose a pup from the side of the mother moon cactus and remove it. Due to the spikes, the easiest way is usually to grip it firmly but gently with tweezers and twist it until it detaches from the mother. 

Using your sterile razor or knife, remove a thin slice from the pup. Make sure to cut the side where it was attached to the mother plant. You should see the cambium and vascular ring in the center of the exposed area.

5. Graft the cact 

Place the moon cactus baby on top of the host. Match up the vascular ring of the moon cactus precisely with the vascular ring of the host cactus. Then, apply some pressure. 

Use rubber bands, string, or tape to secure the moon cactus in place, maintaining pressure between the host and scion. 

After several weeks, the two cacti should have fused, and you can remove the string or whatever you used to secure the graft.

The Wrap Up

No other houseplant compares the dramatic, spiky, near-neon moon cactus – their beauty does come with downsides. While low-maintenance, they can be fickle and are destined to wither and die after a few years without regrafting. Plus, they require you to consider two plant’s needs instead of just one. Despite all that, owners of the moon cactus will tell you they are worth the effort. 

About The Author

Aaron is the founder of and Essential Home and Garden. He likes to spend his spare time with his family, and doing DIY projects in the home and garden.