Cacti are beautiful plants with many colors, shapes and looks. But if your cactus is turning black that’s not normal.
As such, it is important to observe your plant because a cactus turning black can start from something as minor as a few small black spots. But they will quickly progress and spread.
In this article, I’ll explain why your cactus is black and how to fix it.
Why is your cactus turning black? Cacti usually turn black due to fungal disease. But it can also occur due to overwatering and bacterial infections as well. Less often, temperature changes and underwatering.
Reasons Why Your Cactus is Turning Black
Unfortunately, there are a few reasons why your cactus is turning black. As such, there’s no one single cause nor just one single solution.
Instead, I suggest taking the time to figure out the cause since fixing the problem once you’ve identified it is usually quite straightforward.
Therefore, it is the diagnosis that takes a bit more time.
Below is a list of reasons why cacti can turn black. This will allow you to see which one is actually happening to your plant.
I’ll also explain why this is happening and what you can do to fix it.
Overwatering Can Make Your Cactus Turn Black
Overwatering is the most common cause of plant death. This includes cacti, especially because cacti are designed to survive on less water.
As such, this makes them more susceptible to overwatering than most houseplants.
Since they store moisture, they can go for long periods of dryness and tolerate drought.
This means that is you water too frequently, it increases the chance of overwatering.
That said, overwatering in itself is not what causes your cactus to turn black. However, overwatering is what leads to root rot. And it is root rot that causes the black spots and patches to occur and build up.
This is what makes overwatering very dangerous. It can lead to rotten roots which will eventually kill your cacti if you don’t discover the problem early enough.
How to Fix Overwatering Problems & Root Rot in Cactus
First is avoid overwatering as much as possible. That’s because it can be difficult to save the plant once root rot has set in.
But if there is overwatering and you feel that the soil is wet or mucky, unpot the plant and inspect the roots.
If you’re lucky, root rot has not set in.
Here, you have 2 options.
- Let the soil completely dry out. Then slowly start watering again.
- Repot the plant in fresh, dry potting mix to let it immediately start recovery.
I’m very aggressive in treating overwatering and root rot. So, I always go with option #2. But you can decide for yourself.
If there is root rot, then you need to take more drastic action.
First, cut the affected areas. These are all the black areas and parts.
Then prune the rotten roots and repot the plant in fresh, dry soil. Make sure you use sandy, well-draining soil.
Don’t water the soil and let the cactus stay in dry soil for about a week to let it recover on its own.
Underwatering Can Also Be Why Your Cactus is Turning Black
Another reason for your cactus is turning black is underwatering.
In most cases, underwatering is much less of a problem compared to overwatering. And the cactus is able to recover much faster as well.
Because it can tolerate drought, you can be late or forget watering for a while and the plant will be okay.
However, if you let it go dry for very long periods of time, this will cause the plant to turn black as well.
But unlike in overwatering, it is not root rot that causes it. Instead, dehydration prevents the plant from functioning properly.
Unfortunately, if this issue is left untreated, it will eventually damage the plant as well.
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How to Fix an Underwatered Cactus
Cacti are easy, low maintenance plants because they require little care and can take a lot of neglect.
However, if you haven’t watered your cactus for a while, it will shrivel up. Additionally, it will look sad, very dry and even seem like it will crumble.
The good news is that cacti are resilient.
And they tend to recover faster to overwatering than underwatering.
However, excessive dryness especially for very long periods of time will eventually cause the roots to die as well. So, you want to check for this after you water.
In most cases giving the plant water will allow it to recover.
But don’t just overwhelm it with water in hopes of compensating for the dryness.
Since the plant has been dry for a while, gradually add water to let it get use to absorbing moisture again.
In some cases, the plant won’t recover despite you adding more water.
This usually means something more serious has happening.
One is that roots have already died from dehydration. Therefore, there’s no bringing them back to life.
Be careful not to repot the plant. Or if you do, take extra care. That’s because very dry cacti roots will easy break and tear off since they’re very dry.
If this happens, you’re pretty much killing the plant yourself.
Fungal and Bacterial Diseases Can Make a Cactus Turn Black
Another reason for your cactus to turn black are infections. These fall under fungal and bacterial diseases.
The two are similar but very different.
That’s because they are infections caused by pathogens. But the pathogens that cause them are all different. Therefore, you need to figure out which bacterial or fungal infection is causing the issue.
And from there, you’ll be able to fix the problem.
The three most common diseases that cactus suffer from are crown rot, bacterial necrosis and phyllosticta pad spotting.
Each has its own symptoms and will affect the plant differently.
Unfortunately, some of them have no cure.
Incorrect Temperature Conditions Causes Black Spots on Cactus
Cactus turn black when the temperature gets too cold.
Unfortunately, this is a bit harder to tell since different cacti have varying temperature tolerances. Some have issues with temperatures below 40 degrees Fahrenheit.
But there are cacti varieties that are cold hardy and can take winter weather.
As such, this varies significantly.
So, black spots in your cactus may or may not be caused by temperature problems.
That said, most cacti prefer temperatures between 70 to 95 degrees Fahrenheit. Although they also don’t have problems with hotter weather as they’re accustomed to the desert.
Another thing about desert climates is that the night gets significantly cooler than the day.
As such many cacti actually enjoy night time temperatures between 40 to 55 degrees Fahrenheit.
This means it is very important to know what kind of cactus you have and what its temperature tolerance levels are.
How to Fix Temperature Issues in Cactus
In case your cactus is in fact turning black due to cold conditions, the best fix is to move it to a warmer location.
You can use a digital thermometer to figure out what the current temperature is. And find a more suitable spot to keep the plant.
Luckily, the plant will be able to recover on its own once you’ve done this.
As such, all you need to do now is wait. It will take a while before the damaged tissues recover.
How to Treat a Cactus Turning Black
Now that you know the potential causes of cacti turning black, you can narrow them down to identify the root cause.
The final step is no treat your cactus.
Isolate Your Cactus
As with any plant that suffers possible infection, always isolate the infected plant immediately.
This prevents what ever disease it is from spreading to your other houseplants.
Additionally, check the other plants that used to be near or beside the affected cactus. You want to make sure that they did not catch anything.
If they did, quarantine them as well. Otherwise, they’ll contaminate the rest of the nearby plans.
Remove Affected Areas
Now it is time to prune the affected areas.
Here, you want to get rid of everything that’s been damaged. So, anything with black or brown will go.
Make sure to use a sterile knife or pruning shears. You can sanitize the blades with rubbing alcohol or hydrogen peroxide.
Cut all the way as long as you see black or brown areas.
Note that the internal areas will also get affected since fungal disease can reach deep into the tissues. So, prune off everything that looks brown or black.
This means you could end up to quite a small plant.
Nevertheless, this has to be done. And removing all the damaged areas will give the remaining plant a better chance to survive.
This is important since the black spots and areas will continue spreading if you don’t remove them.
Replace the Soil
Because overwatering can lead to fungal infections and some diseases come from the soil, it is important to not only replace the soil but to discard it properly.
In this part, you’ll essentially be repotting your cactus to help it recover.
The important things here are to:
- Remove all the soil from the cactus and its roots.
- Clean the pot to make sure there are no remnants of the soil or pathogens.
- Have fresh, dry potting soil on hand.
- Discarding or throwing away all the soil carefully.
In addition to repotting, you’ll want to remove all the soil from the plant. This way, no pathogens (if there are any) will follow the plant.
You’ll also want to clean the pot to get rid of remnants of the infection there.
Once you’ve done all these steps, you can repot the cactus in fresh, dry, well-draining soil and allow it to begin recovery.
Prune Some More If Needed
Finally, you need to monitor the cactus for the next few weeks.
That’s because there may be some remnants of the problem. This can either be from affected areas that weren’t completely pruned, the pot or some soil still harboring the disease or original issue.
If this is the case, black and rotting areas will resurface within a few days up to about a couple of weeks.
Hopefully, nothing does. And your cactus will begin growing again.