The moon cactus is a grafted specimen that is almost always grown as a potted plant rather than a garden specimen. The colorful red, orange, or yellow top (the scion) is Gymnocalycium mihanovichii. The lower green cactus host portion can be any number of species but is usually a Hylocereus cactus. It becomes known as a moon cactus when the pieces are joined. The main job of the lower cactus is to display the Gymnocalycium at an advantageous height. These plants rarely last more than a few years, since the upper scion and the lower rootstock portions grow at different rates, which eventually destroys the graft union between the two sections.