The curious little cacti with the colorful tops have been favorites for office and homes since they became commercially available. Most people who keep moon cacti, however, have no idea that they are grafted cacti composed of two different plants. The colorful top is a cactus that lacks chlorophyll called the chin cactus (Gymnocalycium spp.); the bottom is one of several different cacti varieties. Because these grafted cacti are made using different kinds of rootstock, it is difficult to know what size they would reach at maturation, left to their own devices. They simply do not exist in nature and do not generally live a full cactus lifetime.
Why Graft Cacti
The chin cactus that is the colored top of the moon cactus is often grafted on the top of another cactus because of a genetic mutation that has resulted in its lack of chlorophyll. In a state of nature, these mutant cacti would not live but a few weeks because they would have a difficult time feeding themselves. Grafting them on to a green cactus as a seedling allows them to share food with the green cactus below.
The understocks of moon cacti are usually either from pitaya (Hylocereus spp.) or blue myrtle cactus (Myrtillocactus geometrizans), both dark green cacti that grow larger than their grafted counterparts. Most cacti kept indoors will grow at a slow or medium rate, generally only growing a few inches per year, unlike a cactus in its natural settings.
Pitaya has a fleshy stem that can have three to five sides and can be seen climbing in to tree tops in the wild. They reach upward of 20 feet when left on their own. Pitaya is an important fruit-bearing plant in Florida, with the thornless varieties being preferred to the thorned.
Blue Myrtle Cactus
The blue myrtle cactus enjoys a position among the favored rootstocks for grafting all kinds of cacti. They are large cactus that develop multiple branching sections. A blue myrtle cactus in its entirety resembles a bush that reaches 15 feet high and 10 feet wide. It is a commonly seen cactus in southern Mexico and throughout U.S. Department of Agriculture hardiness zones 9 through 11.