The plant commonly known as the Moon Cactus is not a single organism but a composite of two different cacti fused together. The brightly colored spherical top, the scion, is a mutant cultivar of Gymnocalycium mihanovichii that presents in vivid shades of red, yellow, or pink. This stunning coloration is a result of a genetic mutation that causes a complete absence of chlorophyll, the green pigment necessary for photosynthesis. Because this upper portion cannot produce its own food, it must be permanently joined to a green, photosynthesizing base plant, the rootstock, to survive and grow.
Preparing the Plants and Tools
The success of the graft begins with careful selection of both the scion and the rootstock, which must be healthy and actively growing. The colorful Gymnocalycium scion should be firm and free of blemishes, while the rootstock is typically a fast-growing, columnar variety like Hylocereus or Myrtillocactus geometrizans. The rootstock provides the necessary vascular system and green tissue to sustain the non-photosynthetic scion.
Tool preparation is important to prevent the introduction of pathogens into the freshly cut plant tissue. A clean, single-edged razor blade or a sharp grafting knife is required to make precise cuts without crushing the plant cells. All cutting surfaces must be sterilized immediately before use, often by wiping them with isopropyl alcohol to minimize the risk of infection. Securing materials, such as rubber bands or grafting tape, should be gathered beforehand to ensure a swift transition after cutting.
Making the Precision Cuts
The grafting process is essentially a surgical union aimed at aligning the vascular tissues of the two plants. Begin by making a single, swift, horizontal cut across the top of the rootstock, ensuring the surface is flat and perpendicular to the stem. The fresh cut reveals the internal anatomy, which includes the circular vascular bundle, or cambium layer, responsible for transporting water and nutrients.
A second, shallow cut should then be made to bevel the edges of the rootstock, slightly sloping the sides down and away from the central cut surface. This angle helps prevent the rootstock’s outer tissue from shrinking and pulling away from the scion as it dries, which could compromise the union. Immediately before placing the scion, a final thin slice can be taken from the rootstock to expose the freshest tissue possible. The base of the Gymnocalycium scion is cut in the same flat manner, ensuring its own vascular ring is clearly visible.
Joining and Securing the Scion
Positioning the scion onto the rootstock focuses on the immediate connection of the vascular bundles. The small central ring of the scion must overlap with at least a portion of the larger vascular ring of the rootstock. Perfect concentric alignment is not required, especially when the two cacti have different diameters, but a slight overlap at the edges is necessary for the transport of water and sugars to begin.
Once the scion is placed, firm, even pressure must be applied to ensure maximum contact between the two cut surfaces. This pressure is maintained using rubber bands stretched between the pot’s rim and the top of the scion, or by placing a small weight on the scion. The sustained force helps to seal the union, preventing air pockets from forming and promoting the fusion of the meristematic tissues. The binding should remain in place for one to two weeks, allowing the initial bond to form without disturbance.
Ensuring Successful Graft Union
After the scion has been secured, the grafted cactus requires a specific environment to encourage fusion. The plant should be moved to a location that receives bright, indirect light, avoiding direct sunlight that could scorch the sensitive, freshly cut areas. Maintaining high humidity around the graft site, often achieved by placing a clear plastic bag loosely over the plant, prevents the exposed tissues from drying out too quickly.
Withhold watering for the first week to ten days to prevent excess moisture from causing rot at the union site. A successful union is typically observable after two to three weeks, indicated by the scion remaining firm and healthy, without signs of shriveling or discoloration. Once fusion has taken hold, the binding materials can be gently removed, and the plant can gradually be introduced to its normal care routine.