Underwatered Moonlight cactus
Moonlight cactus and other succulents can endure long periods without water, so it’s unusual to find one of these suffering from underwatering. But, if you somehow forgot about your plant and neglected to water it for a month or more, you’ll probably find your Moonlight cactus looking thirsty or with some leaf damage from lack of watering.
It is very easy to identify an underwatered Moonlight cactus. The leaves will look shriveled, dry, and flat. Some may have dried up completely, turned brown and crispy, or dropped off the plant, starting with the lower leaves and moving upward as the dry conditions continue. And of course, the soil will be completely dried out.
If your Moonlight cactus is thirsty and underwatered, give it plenty of water as soon as possible. Submerging the pot entirely in water for about 5-10 minutes is a good way to make sure the soil and plant are rehydrated properly. When you feel a sense of moisture on the surface of the soil with your finger, it means the watering is done properly. If there are dried out leaves still attached, go ahead and pluck them off to make room for new growth.
Overwatered Moonlight cactus
Overwatering is dangerous to Moonlight cactus and can be fatal to your plant if you don’t remedy the situation. Too much moisture over time leads to root rot, which prevents the roots from being able to absorb nutrients and water from the soil. Root rot occurs when wet conditions allow fungi and bacteria to flourish in the soil and feed on roots. When you find that it's overwatered, you'd better change the growing conditions, place it somewhere with more air ventilation and adjust water frequency, for example.
The symptoms of overwatering are yellow, swollen, and translucent leaves that may even burst open from being over-full with water. If the problem continues without being treated, leaves might turn brown or black, and fall off the plant at the slightest touch. Be sure to check the soil to determine if overwatering is the culprit, as some other issues can cause similar symptoms.
It’s a bit difficult (but not impossible) to save an overwatered plant. The key is catching it early before a lot of damage has occurred. If the roots become rotten, it is likely to kill the entire plant. If you suspect you have overwatered your Moonlight cactus, the first step is to remove it from its pot and check the roots and soil.
After removing the plant from its pot, gently remove wet soil from around the roots and then rinse them clean in room-temperature water. This helps with removing fungus that might be lurking in the soil and allows you to get a better sense of how healthy the roots are. If your plant has already developed root rot, you will see roots that are dark brown or black, soft, mushy, or slimy.
If the majority of the roots are already affected by root rot, it may not be possible to save the plant. In this case, it is best to remove any healthy leaves and try to use these to propagate a new Moonlight cactus. Luckily, this plant is easy to propagate even from a single leaf. If, on the other hand, only a portion of the roots have succumbed to rot and other healthy roots still remain, there is a chance it can be saved.
Use a sterilized cutting tool to remove any unhealthy-looking roots. Once you're left with only the firm, pale roots, it’s a good idea to dip them in a fungicide to kill off any remaining spores. After that you can repot your Moonlight cactus in fresh, free-draining potting soil. While this does not always work to save a succulent with root rot, in most cases this plant will be able to make a full recovery and will put out new growth starting in the next growing season.