The moon cactus consists of two cacti species grafted together. The globular top cactus (Gymnocalycium mihanovichii) is grafted onto a host cactus (typically Hylocereus spp.) to form a grafted specimen. The top cactus (the scion) is a cultivar that lacks chlorophyll, relying on the host cactus (the rootstock) to survive.
Moon cacti are characterized by their brightly colored tops. The cultivar most commonly used as the scion can be red, yellow, pink, or orange.
This cactus grows best as a houseplant or potted patio plant.
grafting: (horticulture) joining the tissues of two plants so that they grow as a single plant.
| Scientific Name | Gymnocalycium mihanovichii |
| Common Name | Moon cactus |
| Plant Type | Cactus, perennial |
| Family | Cactaceae |
| Genus | Gymnocalycium |
| Origin | South America |
| Size | Up to 12” tall, 1-2” wide |
| USDA Hardiness Zones | 10-11 |
| Propagation Method | Offsets |
| Climate | Arid to semi-arid, subtropical |
| Soil Type | Well-draining, rocky |
| Sun Exposure | Partial sun/bright shade |
Moon Cactus Care
Light
The scion (Gymnocalycium mihanovichii) and rootstock (Hylocereus undatus) have slightly different requirements. The scion requires protection from direct sunlight. The rootstock requires partial sun in order to photosynthesize. Place the moon cactus next to a bright window and use a sheer curtain to provide filtered light. It should receive no more than 1 to 2 hours of direct sunlight a day.
Soil
The soil should be loose, gritty, and well-draining. Combine two parts cactus soil with one part perlite and one part coarse sand.
Water
Water sparingly and avoid overly wet conditions. Allow the soil to dry completely between watering. Once dry, wait a further 5 to 7 days before watering again. Water well and allow excess to drain out of the pot. Do not water the moon cactus during the winter – periodically misting is sufficient during the dormant period.
Humidity
Like most cacti, Gymnocalycium mihanovichii prefers low humidity. Most indoor environments provide adequate humidity levels. Avoid placing in a humid location such as a bathroom and provide good ventilation in enclosed spaces.
Tip: If growing cacti in a climate with moderate humidity, incorporate more perlite or coarse sand into the potting mix.
Fertilization
The moon cactus has low nutritional needs. Apply a diluted cactus fertilizer once every 4 to 6 weeks during the growing season from April to September. Do not fertilize from late fall and through the winter.
Tip: Applying fertilizer to dry soil can lead to root burn. Water the cactus a day before fertilizing to protect the roots.
Pruning
Frequent pruning is not necessary. Remove side shoots using a sharp, sterilized knife or pair of scissors.
Moon Cactus Flowering
In its native habitat, Gymnocalycium mihanovichii produces small, tubular flowers in the spring. The grafted cultivars may produce flowers under the right conditions, but it is rare. The moon cactus is more commonly grown for its colorful tops rather than its flowers.
- Provide adequate bright indirect light.
- Feed with a diluted fertilizer in the early spring.
- Maintain a consistent temperature range of 70 °F to 80 °F in the growing season. Avoid temperatures below 50 °F during winter.
- Maintain relatively dry conditions. Overwatering can hinder flowering.
Types of Moon Cactus
There are several cultivars and varieties of Gymnocalycium mihanovichii that can be grown as a grafted moon cactus.
- Gymnocalycium mihanovichii ‘Hibotan’: The most commonly used cultivar for moon cacti. Can be bright shades of pink, yellow, orange, or red.
- Gymnocalycium mihanovichii ‘Hibotan Nishiki’: Contains partial chlorophyll. Can be grown independently.
- Gymnocalycium mihanovichii ‘Variegata’: Variegated pattern in pink, yellow, orange, red, or green. Those containing chlorophyll can be grown independently.
- Gymnocalycium mihanovichii var. friedrichii: Naturally colored variety, containing some chlorophyll. Can be grown independently.
Moon Cactus Grafting
Work on a clean, even surface. Ensure the rootstock is potted and stable.
- Water the rootstock cactus a day or two before grafting.
- Using a sterilized, sharp knife, cleanly cut the top from the rootstock.
- Cut the base off the scion.
Note: Notice the circular, white vascular tissue in the center of the cut sections.
- Place the top onto the base, aligning the vascular circles as much as possible.
- Use a rubber band to hold the two sections together.
- Lightly dust any exposed cut edges with sulfur powder to reduce the chance of fungal infection (optional).
- It may take several weeks to a few months for a grafted cactus to fuse. Signs of successful fusion include new growth and feeling secure to touch.
Tip: Keeping the cacti, surfaces, and tools clean is important when grafting. Sterilize tools with rubbing alcohol before use. Use a spray water bottle to clean any dust or dirt off the cut surfaces of the cacti.
Moon Cactus Propagation
A mature grafted moon cactus will produce offsets (pups), appearing as spherical growths at the base of the scion. In most cases, the moon cactus pups will need to be grafted onto another host cactus – they will not survive independently once separated from the parent plant.
Note: If the scion is a variety that contains some chlorophyll – such as Gymnocalycium mihanovichii ‘Hibotan Nishiki’ and Gymnocalycium friedrichii – it is possible to propagate the offsets independent of a host.
Repotting
Repot every 3 to 4 years in the spring to provide the cactus with fresh soil. Choose a pot slightly larger and with good drainage. Avoid watering for several days after repotting to allow the plant to settle.
Overwintering
Provide a stable temperature range of 50 °F to 60 °F and protect from temperatures below 50 °F. Avoid watering during the winter – mist lightly if the rootstock begins to appear shrivelled.
Problems
- Faded colors: Caused by too much sunlight. To keep the top cactus colorful, protect the plant from direct light. Provide dappled light using a sheer curtain.
- Graft separating: Natural separation caused by the two cacti species growing at different rates. This is likely to happen after 3 or 4 years. Regraft the scion onto a new rootstock.
- Soft, mushy stem: Caused by overwatering. Let the soil dry completely. In severe cases, cut the scion from the host and graft it onto a healthy rootstock cactus.
Pests
Moon cacti are resilient to most pests. They may be susceptible to spider mites or mealy bugs in certain conditions.
- Spider mites: Signs of infestation include a fine webbing over the surface of the cactus. These mites prefer dry conditions. Treat with a mild insecticidal soap. Isolate the affected cactus to prevent the mites from spreading.
- Mealybugs: Appear as white, cotton-like masses. Moist or humid conditions will make the cactus susceptible. Remove with a Q-tip soaked in rubbing alcohol. Spray with a mild insecticidal soap.
Diseases
Like many cacti, overwatering and excessive moisture can cause fungal problems such as root rot. Plant in a well-draining substrate and allow soil to dry completely between watering.
