Let's talk about grafting cacti! Almost every House Plant Hobbyist has encountered a grafted cactus at some point. Colorful grafted "moon cacti" are popular in garden centers and videos demonstrating the grafting process are currently going viral and making their rounds.
So what exactly is grafting, how does it work, and why graft?
At its most basic, grafting cacti is the process of attaching one cactus or a piece of cactus (called the "scion," in grafting terms) to another cactus (the "rootstock") so that the scion can obtain nutrients from the rootstock. Here is how it works.
Cacti have internal structures called "vascular bundles" which are sort of like pipes that transfer water and nutrients up the stem of the plant and distribute it all around. To create a graft, the growing tip of a rootstock (usually a fast-growing and vigorous columnar species) is cut off, revealing its vascular bundle. The bottom of the scion is cut off revealing its own vascular bundle and pressed onto the rootstock and secured, usually with rubber bands. Over time, the wounds heal together and the vascular bundle of the rootstock transfers water and nutrients into that of the scion on top, which then can grow. There are other ways to graft, but this is the most basic method.
So why graft in the first place?
There are a few reasons.
Grafting is often used to increase the growth rate of a scion. This is beneficial for slow-growing species (like Ariocarpus), sensitive species that are hard or impossible to grow on their own roots (like the mutant Gymnocalycium mihanovichii that form the top part of "Moon Cacti"), or to save a species that would otherwise die (like a cactus that is rotting and leaving behind very little tissue to attempt to re-root). Species that are facing extinction in the wild can be propagated by grafting suitable pieces to common rootstocks, thus helping save the species.
A few things to note:
Inter-species grafts are rarely possible. For example, a Euphorbia cannot be grafted to a cactus, and vice-versa.
The closer a scion and a rootstock are, genetically, the greater the likelihood of a successful graft. Pereskiopsis, a leafy cactus, makes an excellent grafting stock for cacti because it is tough and vigorous, but is also thought to the common evolutionary ancestor of all cacti, and thus compatible with all of them.
Occasionally, scions will exhibit weird blended characteristics of their rootstock. These are called "chimera" and can be very interesting and collectible. Myrtillocalycium "polyp" is one such famous chimera.
Assuming a scion is capable of living on its own roots in the first place, they can often be de-grafted and rooted on their own. The rootstock is then likely to produce branches or pups.
As always, happy growing. Feel free to ask questions and find more great content from Michael Arpino over in the House Plant Hobbyist Facebook group!