Cacti are known for their super green appearance so when you notice a cactus turning black then you know something is up with it.
The vast majority of the time when a cactus has turned black in color, there is something seriously wrong with your plant and will need to be addressed immediately.
This is not something you want to wait on to see if it goes away on it’s own.
So thankfully you are here, let’s just hope it’s not too late.
With that said, here are the common causes and solutions for a cactus that has turned black so you can get back to growing and caring for all of your cacti.
Frost damage
A pretty common occurrence with cacti is that they are often left unattended for long periods of time since they are usually pretty resilient in nature.
Unfortunately, cacti do not do well in cold temperatures, especially freezing or near freezing weather since they are more accustomed to dry and warm weather locations.
Depending on your type of cactus, even leaving one out in freezing weather for a few hours or a day can turn black just as fast which is caused by frost damage.
Most of the time if you act fast enough, your cactus will survive if brought indoors in time even if their already is frost damage.
Your number one priority is to get your cactus back to warmer or at least decent temperatures and not near freezing.
With all of that being said, the frost damage will not be going away anytime soon, most likely never until new growth takes over at least.
So you may have to deal with the unsightly appearance for some time or you can just remove the affected parts of your cactus, either way it will still grow just fine once back in proper conditions.
Root rot
Another common cause of a cactus turning black can be attributed root rot which can be caused by overwatering, poor draining soil or a combination of both.
Cacti really don’t like to be watered very often, of course this depends on the specific cactus you have but in general these are pretty drought resistant plants.
If you see your cactus turning black towards the bottom and the area is mushy then you should have a look at the roots.
If the roots are black, brown or soft then odds are you have root rot and you will need to do everything you can to save your cactus.
Solution: Remove all dead or dying roots that are mushy, keep all firm healthy intact roots, rinse the roots and repot your cactus in some fresh soil with a new container if you usually grow in one.
Now only water your cactus when the soil is at least near bone dry and water the soil itself fully and not just the pads of the cactus.
Also remember this time to use some quality soil that drains well.
I recommend this succulent and cactus soil for most types of cacti by the way.
Crown rot
Crown rot just like root rot will directly attack the roots of your cactus which can turn it black but ultimately will destroy your cactus the vast majority of the time.
The thing about crown rot is that it’s a fungus invasion that actually starts in the soil so it’s virtually undetectable unless you check the roots or soil all the time.
It is also largely caused by overwatering so keep that in mind and water sparingly to help prevent it from ever forming.
Unfortunately, crown rot can already be happening even when buying a brand new cactus and will stay in the soil indefinitely.
There isn’t much you can do when crown rot has already occurred as it’s near impossible to remove from the soil so be prepared to discard your cactus.
I’m sorry.
If you want you can try to save your cactus by keeping any healthy roots and replanting in fresh soil but there is no guarantee it will work unlike fighting normal root rot.
Prevention is really the key here so once again make sure you are not overwatering and always use soil that drains well.
Checking the soil and roots every so often wouldn’t hurt either.
Scale pests
There are many types of pests that can latch onto your cactus, but scale bugs in particular can leave behind this honeydew like substance which can cause black sooty mold and leave behind dark spots or just make your cactus turn black with enough of it.
You really don’t want any pests to stay on your cactus, especially scale insects so it’s best to get rid of them before they start doing further damage to your plant.
Solution: To get rid of scale bugs and to prevent that black sooty mold from appearing again here is what you can do. Use a cotton swab or q-tip with some rubbing alcohol and remove any pests you see. Next spray your cactus with some neem oil at least once a week at night or in shade only.
You can usually just use the neem oil if you don’t feel like doing the former as it’s less time consuming doing the latter and you will want to anyway.
Lastly, keep any infected cacti away from all other plants until all the pests are gone to prevent them from latching onto other cacti or plants.