Moon cacti, with their striking, often neon-colored tops, are a popular and unusual sight in many plant collections. If you’ve noticed your moon cactus developing unexpected green growths resembling new stems or “arms,” you’re observing a natural phenomenon. This common occurrence prompts questions about the plant’s health and care.
The Unique Nature of Moon Cacti
Moon cacti, scientifically known as Gymnocalycium mihanovichii, are not single plants but a unique horticultural example where two distinct cactus species are joined. The vibrant, globular top part, called the scion, is a cultivated variety of Gymnocalycium mihanovichii. This scion lacks chlorophyll, the green pigment necessary for photosynthesis, which is why it displays vivid colors like red, orange, or yellow instead of green.
Because the colorful scion cannot produce its own food, it is grafted onto a different, green cactus, known as the rootstock. The most common rootstock used is Hylocereus undatus, also known as the dragon fruit cactus. The rootstock, being green and capable of photosynthesis, provides nutrients and water to sustain both itself and the scion.
Why Your Moon Cactus is Growing Arms
The “arms” emerging from your moon cactus are new growth from the rootstock. This Hylocereus cactus attempts to grow independently, naturally seeking to produce its own stems or pads, a common behavior for its species.
While this growth is a normal biological function for the rootstock, it can impact the grafted plant. Unchecked rootstock growth diverts energy and resources away from the colorful scion. This redirection of nutrients can shorten the lifespan of the grafted moon cactus, as the scion becomes less vigorous.
Managing Rootstock Growth
When the rootstock of your moon cactus begins to grow “arms,” you have two choices for managing this development. One approach is to prune these growths to maintain the aesthetic of the moon cactus and direct the plant’s energy towards the colorful scion. Pruning helps ensure the scion receives adequate nutrients, extending the life of the grafted plant. To prune, use a clean, sharp, sterilized blade to make a precise cut flush with the main stem of the rootstock, being careful not to damage the scion or the graft union.
Alternatively, you can allow the rootstock to continue growing its arms. This will result in the Hylocereus base becoming more prominent, eventually overshadowing the scion as it grows much faster. While this can create an interesting, more natural appearance for the rootstock, the scion may decline over time due to the competition for resources. If you choose to let the arms grow, the rootstock may eventually dominate the plant’s visual presence.
Ensuring the Health of Your Moon Cactus
Maintaining optimal growing conditions is important for the long-term health of your grafted moon cactus, balancing the needs of both the scion and the rootstock. Bright, indirect light is ideal, as direct, intense sunlight can scorch the colorful Gymnocalycium top, while the rootstock benefits from ample light for photosynthesis. Placing the plant near a window with some protection from harsh midday sun works well.
Water your moon cactus sparingly, allowing the soil to dry out completely between waterings. Overwatering is a common problem and can lead to root rot. A well-draining soil mix, such as a cactus-specific potting mix amended with perlite or coarse sand, prevents moisture retention. Ensure the pot has drainage holes to allow excess water to escape. Moon cacti prefer warm temperatures and do not tolerate cold drafts or temperatures below 60 degrees Fahrenheit.