When people see these unusually colored cacti, quite often, they are mistaken as flowers. But in fact, they are two different species of cacti grafted together.
The brightly colored cactus that sits on top is actually a mutant variation of a standard cactus. If a mutant pup is created in the wild, eventually it would detach from the parent plant and sadly die as it cannot photosynthesize. It can not photosynthesize because it has no chlorophyll (green pigments), which is essential in the photosynthesis process. So, for a mutant pup to survive, we have to find some other means to give it nutrients. This is where the grafting comes in.
Rootstock plants, which can photosynthesize normally, are what the mutant pups are grafted on to. They can provide enough nutrients for both themselves and their grafted partner, allowing both to survive. Common rootstocks used in grafting cacti include species of Hylocereus, Myrtillocactus geometrizans, Trichocereus pasacana, Harrisia jusbertii, Cereus peruvianus, and many more.
Most of the colorful mutants you will see around are derived from Gymnocalycium mihanovichii and Lobivia silvestrii variegated species. However, the possibilities of grafting are endless and not just limited to mutants.
How to Care for Grafted Cacti
Light: Place these guys indoors or sheltered under verandas or patios as the rootstock is sensitive to the cold and frosts. They do need partial sun, so next to a window is the perfect location if they are indoors.
Water: Very little maintenance is required for these guys to thrive. In fact, one of the main reasons grafted cacti suffer is through overwatering. Water them sparingly, only when the soil is dry (this can take anywhere from a week to a month depending on location, time of year, and environmental factors). The best thing to do is water them thoroughly and then let it drain. If you are using saucers, make sure to empty them after each watering.