Closeup Photo of Cactus Plant in a PotDiscovering that your cactus turned black can make you worry as it is an uncommon and weird sight. Reasons such as overwatering, high humidity and various diseases can be the cause of your cactus turning black.

Continue reading to find the exact cause of black spots on your cactus as well as learn how to fix it.

Why Is My Cactus Turning Black?

💥 Quick Answer

Some reasons that may be behind your cactus turning black are: overwatering, insufficient light, low temperatures, over-fertilizing, insufficient nutrients, lack of ventilation or disease in the soil. 


Which of these reasons best fit your situation? Let us briefly discuss them all to find out the main cause of your cactus problem.

– Overwatering

If you are growing cactus, overwatering should be the first thing to think of if your plant is having problems. As a desert plant, cactus prefers a dry substrate to a consistently moist one. Hence, overwatering is a common reason for black spots on cactus.

Overwatering causes your cactus to turn black by bursting the cell walls of your cactus. The cactus plant is well-adapted to a dry environment and collects as much water as it can find from the substrate. If the amount of water or moisture in the substrate is much, the plant will collect too much water and this will lead to the cell wall bursting.

Furthermore, overwatering is most probably the reason why the cactus is turning darker in color. Additionally, Other ways to tell that you are overwatering your cactus are root rot and the presence of fungus gnats.

– Not Enough Light

Your cactus turning black on top may be a sign of light deficiency. It is important to take into consideration that cacti love a lot of sunlight and when they do not get enough, they may show various signs including black spots. Furthermore, another way to tell that your cactus needs more sunlight is etiolation.

If your cactus is growing thin and taller than normal while growing in just one direction, it is craving the light. Insufficient light can cause black spots on the upper parts of your cactus as the plant may be trying to adjust to the available light. However, if the light available daily is not sufficient, the plant may develop dark spots, become pale or die.

– Low Temperatures

Cacti usually prefer high temperatures to low ones. Winter and other causes of drastic temperature change can stress your cactus into turning black in some regions. Low temperature can affect your cactus spines as they can either fall off or become darker in color.

On the other hand, cactus spines turning black is not the only way to tell that the temperature is too cold for your cactus. You may also find the plant dying or some branches falling off. If your cactus has black spines and is looking very dry in winter, it could be stressed from low temperatures.

– Over-Fertilizing

Over-fertilizing is another reason why your cactus can turn black as cacti prefer nutrient-poor soil to a nutrient-rich one (the best substrate for cactus is one properly amended with the organic matter without too many nutrients).

If you see your cactus turning black at base after feeding it with fertilizer, you may want to cut back the number of nutrients you feed the plant with.

Furthermore, too many nutrients can kill cactus plants as fertilizer in the substrate can cause dehydration in the cactus. Through a process of osmosis, plants try to balance the water-nutrient ratio in their organs and the soil. If the nutrients are too many in the soil, the plant will start drying out from the top and this can give the base a darker color. 

– Lack of Nutrients

As cacti love nutrient-poor substrates, this does not mean that they do not need nutrients. Note that if there is no nutrient in the soil, your cactus can show signs such as black and falling spines, stunted growth and it can die if you do not properly feed it.

Note that cacti do not quickly show symptoms of nutrient deficiency, that’s why the plant might already be dying before you noticed. For instance, if you have not fed your cacti for a long time, you can conclude that nutrient deficiency is the reason why your plants are becoming black.

– No Ventilation

Ventilation is very important for indoor cacti. Therefore, without proper ventilation, the humidity in the room will go higher than normal and this can harm your cactus. Just as in the case of too much watering, high humidity can kill your cactus but they may first show some black tips in their branches and spines.

The best way to tell that humidity is the reason why your cacti are turning black is if the indoor cacti or others in a crowded garden have more dark spots than outdoor or solitary cacti. Remember that cacti are desert plants, hence they naturally do prefer levels of humidity that are below average.

– Soil Disease

When was the last time you repotted your cactus? Did you plant another cactus in the same substrate where one died previously? Cacti may be hardy plants, but they are surely susceptible to various diseases. One way to tell that your cactus is diseased is when you find fungi growing on it.

Black fungus on cactus as well as your cactus becoming black or dark from the base is a sign that there are harmful pathogens in the substrate. Another common disease that can attack your cacti is root rot and root rot occurs when you overwater the plants.

Now you know so many reasons why your cactus turned black. What do you think is the exact reason your cactus is turning black? Well, it does not matter why the plant is black, you can surely fix it. Continue reading to find out how.

How To Fix it (And Ways To Prevent It)

Here are some ways to fix your cactus:

– Remove the Cactus From Its Substrate

The reason why removing it from the substrate is important is because you can now examine the root, properly fix the cactus, and expose the cactus roots to oxygen. Don’t worry, removing cacti from their substrate will not harm them so long as you handle them with care.

If you are growing your cactus in a pot, you should gently tap the pot to loosen the soil. You can use your hands (wear hand gloves) to remove the plant from its substrate. Please handle the cactus with care so that you do not break the roots.

– Cut the Region Below the Black Tip

The first thing to do is to cut off the black tip. You should cut off the stem 1-2 inches lower than the dark or black region. This will ensure that the remaining parts of the plant are healthy. You can use a garden knife to cut the plant.

Remember that you should gently handle the plant as it has spines that can hurt you. Also, rough-handling your cactus can lead to its death. Make sure that you use a clean or sterilized knife so that you do not transfer pathogens from the knife into the cactus.

If more than one cactus is turning black, sterilize the knife after each cut before moving to the next plant. If your cactus is becoming black from the base, do not cut the entire plant. Instead, try to examine the roots.

– Examine Your Cactus Roots

You should examine your cactus roots for damage. Examining the roots is very important if you want to know the exact cause of your cactus turning black. Check for damaged roots or roots that appear darker than others.

Bad roots can be stinky and you can find some fungi growing on them. You should prune off these roots and rinse the remaining healthy roots with clean and running water. Do not soak the root of your plant in water as you may be killing it.

– Leave Your Cactus To Dry Out for a While

Do you think that you have fixed your cactus? Well, you are almost through. Before transferring your cactus back into a suitable substrate, wait until the root system is fully dry and the cut on its branch is dry. Remember that you should keep your cactus exposed for 24-48 hours.

Make sure that the temperature is not too cold, else you may be killing the plant. When the plant is dry, you should transfer it into a suitable substrate. Remember that cactus substrate is mostly sandy and rocky with some amounts of organic materials.

– Properly Water Your Cactus

After transferring your cactus into a suitable potting mix, wait a day or two before watering the plant. If it is winter, you should water it once after transferring it and never again until spring. For warmer months, wait until the soil is almost completely dry before watering the plant again.

It is a bit risky to water your cactus by misting as water droplets can get trapped between the spines. If you must water by misting, mist water on your cactus in the morning so that the sun can evaporate the remaining droplets.

Note that watering problems are very common amongst cactus growers, so you should pay close attention to your plant watering needs. If you feel that your cactus needs water but you are not so sure, you should wait a day or two and then water the plant. You cannot overwater it this way.

– Isolate Sick Plants

You must isolate your sick cactus, if only one plant is black or has dark spots, remove it from your cactus garden and fix it before transferring it back into the garden. This will help prevent other plants from turning black if the initial plant is black because of a disease.

– Keep Your Plants Where They Will Get Enough Sunlight

Here is a cool tip to help you grow your cactus successfully: Cacti prefer sunlight to water. Instead of providing more water to your cactus, give it access to more sunlight. Many cactus growers successfully solved their cactus problems by shifting the plant a bit further into the sun.

If you are growing your cactus indoors, make sure that it receives six or more hours of direct light daily. Sunlight is very important and you cannot successfully grow a young cactus to maturity with only grow lights. Always remember that your plant will love more sunlight.

If you are living in subtropical regions or places where the afternoon sun is very hot, you can shield your cactus from the sun; however, make sure that it receives the morning sun.

– Grow Your Cactus in Suitable Temperature

The optimum temperature to grow cactus is 75-80 degrees Fahrenheit. You should try not to grow your cactus below 65 degrees Fahrenheit except the species or variety is cold-tolerant. In winter, take your plants indoors and place them close to the window or under grow lights.

– Use Cactus-Specific Fertilizer

One easy way to help prevent over-fertilizing as well as not giving enough nutrients to your cactus is to use cactus-specific fertilizer. Note that you should amend your cactus substrate with organic matter. Moreover, your cactus will appreciate some extra food if the substrate is nutrient-depleted.

You can fertilize your cactus lightly while watering it. To amend the substrate with organic matter, wait until spring and mix at least ten inches of the substrate with organic matter such as rotted manure or compost.

– Take Your Plants Outside

Catus Plants in the Backyard

Remember that cactus plants do not need an environment with high humidity. If the room is mostly humid, you can use a dehumidifier to reduce the humidity. A better approach would be to take the plant outside if the temperature is just right so that your cactus can get fresh air and wind.

Do not take your cactus outside if the temperature is very low or it is constantly raining outside as you may be killing it. In such cases, you should use a dehumidifier in the room.

Congratulations! You can now fix your black cactus.

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