So I want to save this moon cactus. The root is rotting and flimsy. I watched a few videos but I don’t know which one is correct. Please help me figure out how to save him.
Very sorry to hear that you think your red moon cactus is in trouble. It sounds like you have removed the plant from the pot to inspect the roots – if they are dark and soggy it may be difficult to save the plant. You could try to cut away the parts that are rotting, let the plant dry out, and then repot in soil formulated for cacti (generally a mix of soil, sand or gravel, and perlite). Ensure that the pot you are using for this plant has good drainage to discourage root rot.
“Red moon cactus” (Gymnocalycium mihanovichii) is a colourful cactus that is native to desert areas in South America. These plants are unable to produce chlorophyll on their own so they are grafted onto another (rootstock) cactus that is able to produce chlorophyll (usually Cereus peruvianus).
Root rot can be caused by a pathogen, overwatering, or poor drainage. In cacti, one of the first signs of root rot is when parts of the plant become yellow or brown and soft. Mushy roots are another sign.
In terms of general care for a cactus, cacti need very little water in the winter. Ensure that the soil is completely dry before watering. As mentioned above, they should be potted in a soil mix specifically designed for cacti. Cacti need sufficient light – at least four hours of direct sun per day. A south facing window is ideal. Toronto Master Gardeners has a “Growing Cacti” guide that you may find useful.
Best of luck trying to nurse your cactus back to health.