Gymnocalycium stenopleurum
Family: Cactaceae​
​Common Name: moon cactus, lollipop cactus, ruby ball cactus
Botanical Synonyms: Gymnocalycium mihanovichii var. friedrichii, Gymnocalycium friedrichii 
​Origin: Gymnocalycium stenopleurum inhabits xerophilous Chacoan forests within the Cerro León, Boquerón department of Paraguay – and possibly Bolivia – at elevations up to 500 meters. The mutated form is only found in cultivation, and cannot survive in the species’ natural habitat.
Description: G. stenopleurum is a small cactus that develops a globular, solitary stem, grey-green in color, with brownish-red bands. At least 8 deep ribs are present, covered in twisted, needle or awn-shaped spines. Over time, offsets will develop on the head of mature cacti. 
G. stenopleurum differs from G. mihanovichii by the tall, slender ribs; thickening around the areoles; and wide pink flowers, which bloom from mid spring to late summer.
The mutated form is completely without chlorophyll, and therefore will possess red, pink, yellow, orange, or white stems. As a result, mutated G. stenopleurum are grafted onto another cactus – usually Hylocereus sp., which possesses a simple green trunk. Mutated moon cacti often do not grow higher than 4 inches tall.
​
  • Interesting Features:
    • Unfortunately, mutated G. stenopleurum do not survive very long in cultivation, as the upper and lower cacti grow at different rates, eventually destroying the graft. However, it is possible to remove the moon cactus and graft it onto new stock without damaging the original cacti.
    • The relationship between mutated moon cacti and the stock cactus is parasitic, with the lower cacti being deprived of nutrients and water by the achlorophyllous G. stenopleurum.
    • Moon cacti are often grouped under Gymnocalycium mihanovichii var. friedrichii or Gymnocalycium friedrichii, but this naming is controversial, as few differences exist between the pink-flowered variant and G. mihanovichii. Officially, however, all forms are considered variations of G. stenopleurum, a polymorphic species that differs in flower color, needle-shape, and rib depth between specimens.
  • Uses: Although no medicinal or ethnobotanical uses of G. stenopleurum are known, the species is incredibly popular in cultivation, especially in its mutated form. Considered “lollipop cacti”, “moon cacti”, or “ruby ball cacti”, they are unique and easily available in most nurseries. 

Cultivation and Care:
  • Watering: Moderate 
    • The moon cactus should be watered sparingly, as both the graft and original cacti are drought tolerant. In the summer, irrigate weekly, ensuring that soil is completely dry in between waterings. However, in winter, or in overly humid environments, reduce irrigation.
    • To avoid root rot, never allow the species to remain in standing water.
  • Soil: The host cactus usually requires a rich cactus or succulent soil with a low pH and high organic matter content. Soil must be well draining – ensure that the pot also has a drainage hole for the best results.
  • Lighting: G. stenopleurum are more shade tolerant than most cacti, and should be provided with partial shade – do not expose to direct sunlight. However, the host species usually has a higher light requirement, therefore grafted G. stenopleurum should be given more sun exposure than free-living specimens.
  • Propagation: G. stenopleurum can be propagated through seed or offsets. However, propagation may not be possible in the mutated moon cacti, although some specimens will produce offsets that can be repotted. These offsets will also be achlorophyllous, and therefore will require a host cactus in order to survive. 
  • Cold Tolerance: 25°F, USDA hardiness zones 9b to 11
  • Special Accommodations: Because of its parasitic nature, the host cactus will eventually die, and the mutated G. stenopleurum will also perish unless transferred to another cactus.
    • To re-graft, use a sterilized knife to cut the scion from the old rootstock and the upper layer of the new cactus, to expose a circle of vascular tissue in both plants.
    • Align the vascular tissues and wrap both cacti with rubber bands to secure the graft.
    • Once the tissues have fused, the rubber bands can be removed to reveal a newly grafted specimen. 

Sources:​​
  1. Llifle Encyclopedia of Cacti​
  2. Dave's Garden
  3. Cactus Art Nursery
  4. Garden Beast
  5. The Spruce
  6. Mountain Crest Gardens
  7. Living Treasures