How to Care for a Moon Cactus

We are over the moon with this low-maintenance succulent! This is the moon cactus, scientifically known as Gymnocalycium mihanovichii, and they are great for beginner gardeners. In this guide we’re going to go over basic light soil and watering needs for this plant, as well as general health, common problems, and more so that you can keep these brightly colored beauties thriving.

What is a Moon Cactus?

A moon cactus isn’t just one cactus; it’s a sort of symbiotic Frankenstein's monster of the succulent world. The colorful top portion of the plant is the Gymnocalycium and it is attached to the top of, most commonly, a Hylocereus cactus, otherwise known as a dragon fruit cactus. The top piece, called the scion, lacks chlorophyll, and relies on the lower plant, the rootstock, to survive! We’ll go over more about the grafting process later in this blog.

Perfect Lighting for Moon Cactus

Find a spot by a window for your moon cactus that gets bright light and is in sight, so you can enjoy its vibrant colors while ensuring healthy growth. If you notice your cactus leaning towards the sun after a while then that means you should give it more light, and if you notice your cactus getting dry brown sunburns then move it away from the window to reduce the intensity of the light.

Best Soil for Moon Cactus

The moon cactus needs a well-draining soil that has a good handful of perlite or pumice added to it so that it does not become waterlogged. It is especially prone to overwatering and rotting so be sure to use a well-draining soil medium.

How to Repot a Moon Cactus

To repot your moon cactus, find a pot with drainage so that excess water can escape and your plant can breathe. With this plant you’ll want to pick a pot that isn’t much larger than it’s previous container, as this will help prevent excess soil from retaining too much moisture and possibly causing your cactus to rot.

Fill that pot about halfway with well-draining soil, pressing down firmly. 

The spines of this plant aren’t too sharp but it still might help to use gloves to more confidently handle this plant. Gently remove your moon cactus from its previous container and start to massage the old soil as best you can to give the roots the freedom to move around and grow. Place your cactus in the new pot and fill in soil all around, tucking it in to make sure it’s nice and secure and won’t topple over. We like to use top dressings as a final touch to add more stability and complete the look!

How/When to Water

The Moon cactus can be a little fickle when it comes to watering. You’ll want to make sure that the plant feels a bit soft and has some give to it when feeling it with your hand, and to make sure that the soil has been completely dry for several days. Test this with a finger or a tool like a wooden skewer to see if any damp soil sticks. You may also notice that the weight of the plant is much lighter when the soil is dry.

When it’s time to water, do several rounds of watering to make sure that the soil is completely saturated, checking that any excess water is draining out of the bottom of the pot. Once you’ve done this enough, give the soil time to completely dry out before watering again. And remember: when in doubt, underwatering is better than overwatering, especially with moon cactus! Since this plant is so slow growing, you’ll want to water it way less frequently.

Common Moon Cactus Problems and Solutions

A healthy moon cactus will have a firm green rootstock, a rigid structure, and vibrant colors on the scion or “bulb”. These plants are slow growers and will not grow taller, but rather the scion will grow bigger and form new offsets around it. If you do notice growth from the rootstock it is best to remove it as soon as possible to redirect the energy of the plant back towards the scion. 

It is totally normal to have a bit of scarring at the base of the scion where the grafting union was formed, kind of like a surgery scar that has healed over. This will not affect the health of the cactus. 

The most common problem for this cactus is rot due to overwatering. The bottom portion will turn brown and mushy and the plant will topple over. You will most likely have to start over, or remove the top portion and regraft it onto a new plant. This happens sometimes so don’t become discouraged! Learn by reducing watering, and trying again.

Moon Cactus Pest Control

The moon cactus can be plagued by small pests like mealybugs and scale that like to hide between all of the nooks and crannies of the plant. Every now and then take a look between the small buds growing off the top of the cactus, and if you see any bugs then treat them with isopropyl alcohol.

Is a Moon Cactus Safe or Toxic?

The moon cactus is non-toxic to humans and animals, however we would recommend some caution around its spines for the sake of small children and pets.

Ideal Temperature & Humidity

The moon cactus prefers a warm dry climate, so it will do best inside your home on a bright windowsill. Generally you won’t want these plants to be outdoors; due to their variegated tops they burn very easily in full sunlight. They also can be damaged by frost and do poorly in temperatures lower than 40° Fahrenheit, so it’s best to enjoy these indoors.

How to Propagate a Moon Cactus

Propagating a moon cactus is a bit more challenging than your average cactus or succulent but it’s not impossible. Remember, a moon cactus is made by grafting the colorful, round scion onto a different species of cactus capable of sustaining it with energy and water. Since the cactus cannot survive on its own it needs to be grafted onto a sturdy rootstock. Dragonfruit, san pedro cacti, and blue candle cacti are a few common options, and in this example we’re going to be using a blue candle cactus to graft onto.

Prepare the rootstock by cutting off the top with a sharp, clean tool. Note the circular pattern at the center of the opened stem: that's the vascular bundle, a fancy term for the veins which transport nutrients throughout the cactus! This is where you’ll place the scion. 

Remove the pup from the parent plant by twisting it like a screw until it comes off or by cutting. Then make a cut so that the flat part can be flush with the rootstock. Place the pup onto the center of the rootstock and secure it with a bandage, grafting tape, or rubber bands. Then give it time to form a union. Once the two are fused together you’ll see new growth on the scion and know the grafting was successful! Be sure to remove any offsets from the rootstock as they emerge. 

Moon Cactus Blooms

A moon cactus will usually bloom in late spring or early summer after its winter dormancy period. The “bulb” of a moon cactus may produce pink or white, long-stemmed, daisy-like flowers around its crown.

Conclusion

We hope that you enjoyed learning about the Moon Cactus and can understand why it’s such a beautiful and popular plant!  By taking things slow and observing its growth you can maintain a healthy and vibrant Moon Cactus in your own garden.

Key Takeaways

• A moon cactus is composed of two plants - the colorful top portion (scion) is the Gymnocalycium and it is attached to the top of another plant (rootstock).
• Remember: bright light and in sight!
• The moon cactus needs very well-draining soil, with thorough but infrequent waterings.
• A common problem for this plant is rot due to overwatering. Fix this by grafting, or start over with a new plant and try again.
• The moon cactus is non-toxic to humans and animals.
• This cactus does not do well outdoors, so it’s best to keep these inside.
• Propagating moon cacti can be done through grafting, where the top portion (scion) can be attached to the bottom portion (roostock) of another plant.