The Moon Cactus (Gymnocalycium mihanovichii) is a composite plant created by grafting two different cactus species together. The brightly colored, globular top, called the scion, is a cultivated form of Gymnocalycium that lacks the green pigment chlorophyll necessary for photosynthesis. This scion is grafted onto a green, chlorophyll-rich rootstock, typically Hylocereus (dragon fruit cactus). This symbiotic arrangement allows the scion to survive by drawing water and nutrients from the photosynthesizing rootstock below. Pups are side growths or offsets that emerge from either the top or bottom portion of this unique plant.
Identifying the Source of the Pups
Pups can arise from the green base (the rootstock) or from the colorful ball at the top (the scion). The visual difference is unmistakable and relates directly to their cellular structure. Offsets from the rootstock are green because they contain chlorophyll and perform photosynthesis. These green growths are clones of the vigorous Hylocereus base. Conversely, pups emerging from the scion match the colorful hue (red, yellow, or orange) of the top cactus, confirming their chlorophyll-deficient nature.
Removing Pups from the Rootstock
Any new growth originating from the green rootstock must be promptly removed to ensure the long-term survival of the colorful scion. The rootstock’s pups aggressively compete for the water and nutrients it absorbs. If left unchecked, this growth will effectively starve the scion, eventually leading to the death of the upper Gymnocalycium portion.
To perform the removal, use a clean, sharp instrument, such as a sterilized razor blade or small knife. Disinfecting the blade with rubbing alcohol prevents the introduction of pathogens. Cut the pup as flush as possible to the main stem of the rootstock, aiming to remove the entire growth point.
Avoid tearing the tissue of the rootstock, as this creates a larger, more vulnerable wound. After the pup is detached, allow the rootstock’s wound to dry out completely for several days. This process, known as callousing, forms a protective, cork-like layer over the exposed tissue, which seals the plant against pathogens and minimizes water loss.
Propagating the Rootstock Pups
The removed green offsets are viable cuttings of the Hylocereus rootstock and can be propagated into full, independent cacti. After removal, the cut end of the pup must be left in a dry, shaded area for five to seven days to allow the wound to callous thoroughly. This critical step prevents the cutting from rotting when it is introduced to the planting medium.
Once a dry, opaque layer has formed over the cut surface, the cutting is ready to be planted in a well-draining cactus or succulent soil mix. A suitable mix contains a high proportion of inorganic material, such as perlite or pumice, to ensure rapid drainage. Insert the calloused end of the pup a few inches deep into the dry soil, making sure it is stable and upright.
Do not water the newly potted cutting immediately; instead, wait approximately one week after planting to give the tissues time to settle without the risk of rot. After this initial dry period, begin watering sparingly, allowing the soil to dry out completely between waterings to encourage root development. These cuttings will grow into the standard dragon fruit cactus, which can be maintained as a standalone plant or used later as rootstock for new grafts.
Caring for Scion Offsets
The colorful offsets that develop on the Gymnocalycium scion do not threaten the health of the overall plant because they draw sustenance from the established scion-rootstock union. For most home growers, the simplest approach is to leave these scion offsets attached, allowing them to form an attractive cluster around the main colorful globe.
If the colorful top becomes too crowded, these offsets can be carefully removed. However, they cannot be rooted on their own due to their lack of chlorophyll. For the scion pup to survive, it must be grafted onto a new, compatible green rootstock, just like its parent. This re-grafting process requires precise, sterile cuts to align the vascular bundles of the scion pup and the new rootstock. While technically possible, this procedure is more complex than rooting a green pup and is typically undertaken only by experienced cactus enthusiasts seeking to multiply their collection of colorful varieties.