Although ZZ plants are one of the easiest houseplants to look after, they can suffer from discoloration and black spots on their leaves and stems.
If your ZZ plant is suffering from this issue, read on to find out the possible causes and how to solve the problem!
Black spots on ZZ plants are usually caused by fungal diseases and insect infestations. In addition exposure to direct sunlight, or problems stemming from overwatering can also cause this issue. Black spots due to root rot or overwatering can be fatal for your plant. You need to find the exact causes before taking any treatment actions.
In this article, I’ll walk you through the possible causes, how to identify them, and solutions to the problem.
What Causes Black Spots on ZZ Plants?
There are two types of black spots you may encounter on your ZZ plant. It can be found either on stems or leaves. Let’s find out the causes and solutions.
Black Spots on Stem
If you keep your ZZ plant in a very bright environment or under full sun, it will develop dark, splotchy markings on its stem.
This is normal and is not a signifier of disease or pest activity, but it could suggest that your ZZ plant would appreciate a situation with less light.
If you’ve determined that the black spots on your ZZ plant’s stem are due to exposure to bright light, and you don’t like the markings on your ZZ plant’s stem, try moving it to an environment with less light.
The marks that are already present are unlikely to disappear, but your plant will be less likely to develop more.
ZZ plants are happier in low-light situations than many other houseplants, although they do still need some light to flourish. It is best to keep your ZZ plant in a place where it receives indirect light for at least 12 hours a day.
Avoid keeping your ZZ plant in direct sunlight. In a bright room, a position several feet away from the window is ideal.
If you suspect that the spots on your ZZ plant’s stem are caused by root or stem rot, do not give your plant any water until you have tackled the problem.
If the black spots on the stem of your ZZ plant are right at the base of the stem where it meets the potting mix or if they are slimy or mushy, they could be a sign of stem or root rot.
Both these problems are caused by fungi which are encouraged by bad watering practices.
Stem rot happens when the stem of a plant is constantly wet, either through waterlogged soil or from watering the plant from above.
Several types of moisture-loving fungi, whose spores can be present in the air, in water, or in potting mix, move in and take advantage of the damp conditions, eating away at the stem of your plant.
If you notice signs of stem rot, take action immediately. Re-pot your plant into a very well-draining potting mix, being very careful with the damaged stem.
Check for root rot at the same time, as the two often go hand-in-hand. Cut away any damaged roots with clean scissors.
Place the pot in water for ten minutes or so so your plant can get some water, then place your plant somewhere warm and dry, with indirect light.
Check your plant every day and give it water only when necessary, always watering from below.
Stem rot is a very serious problem and the sad fact is that once you notice it, it’s too late to save your plant.
Far better to prevent stem rot by using good watering practices – always water your plant from below and never allow the potting mix to become waterlogged.
Root rot, like stem rot, is caused by several types of fungi, which are present in the air, earth, and water. They thrive in waterlogged soil and can cause similar black spots on the stem of your plant.
If your ZZ plant is living in a relatively low light environment and has black spots on the stem, stem, or root rot is the likely cause.
The wet environment encourages spores of moisture-loving fungi such as Phytophthora to grow, using the roots of your ZZ plant as food!
As the roots of your plant are attacked, they cannot take in nutrients, causing your plant to become weaker and eventually die.
As with stem rot, it is vital to investigate root rot as soon as you suspect it and treat it immediately if you confirm that your plant has it.
Re-pot your plant using slightly damp (not wet!), very well-draining potting mix, and cut away any rotten or damaged roots with clean scissors.
Remove as much of the previous potting mix as possible from your plant’s healthy roots.
Do not give your plant any more water for a few days, and watch for signs of recovery. Root rot, like stem rot, is very serious and unfortunately, is likely to kill your plant.
Recovery is possible though if the problem is dealt with in time.
The spores which cause both root and stem rot, although everywhere, are only activated in the right environment – when enough moisture is present. So it’s down to you to make sure you don’t provide that environment in your plant’s pot!
Black Spots on Leaves
ZZ plants are happiest in indirect light. In their natural habitat, they grow on the rainforest floor and so receive very little or no direct sunlight.
The leaves and stems of your ZZ plant are very sensitive to light and will easily scorch if exposed to direct sunlight for too long, causing black spots.
If this is limited to a few spots on your plant’s stem, as described earlier in this article, there is no threat to your ZZ plant’s health.
But if your plant’s leaves are scorched brown or black, they will stop working and die off, causing further damage to your plant.
Keep your ZZ plant where it receives minimal direct sunlight, ideally none. Several feet away from the window in a bright room is ideal. For rooms that receive less sunlight, closer to the window should be fine.
The easiest way to kill the ‘unkillable’ ZZ plant is by giving it too much water. Overwatering causes the vast majority of the problems that a ZZ plant can experience, including black spots.
When you give your ZZ plant too much water, the potting mix becomes waterlogged and the amount of oxygen in the mix is reduced.
Why does this matter? It matters because although plants primarily need carbon dioxide, which they take in through their leaves, to photosynthesize, they also need oxygen, which they absorb through their roots.
If soil or potting mix is too wet, not only is the amount of accessible oxygen reduced, but your plant can’t access what oxygen there is as its roots are surrounded by water – essentially drowning them.
As your plant isn’t getting any water, it will show symptoms of dehydration such as black spots and curling leaves. This is why it is so important to check the moisture level of your plant’s potting mix before watering.
In addition to this, the waterlogged potting mix provides the perfect environment for the various fungal infections which cause root and stem rot.
Always check whether your ZZ plant needs water before watering it. Do not be tempted to rely on a watering schedule as the amount of water that your plant needs will vary greatly depending on the conditions.
Instead, get into the habit of checking your ZZ plant every day for signs that it needs water. There are a few ways to check whether your ZZ plant would like some water:
- Pick up the pot – if it feels light, water your plant.
- Use your fingers or a stick to check the top couple of inches of potting mix. If they’re dry, water.
- Notice the color of the potting mix – the drier it is the lighter in color it will be.
- Use a moisture meter to track the moisture level in your plant’s potting mix.
If you have been overwatering your ZZ plant and it is showing signs of dehydration, re-pot it as soon as you can into a drier potting mix.
Check for root rot at the same time and remove damaged roots with clean scissors. Unless the roots are severely damaged, your plant should perk up quickly.
ZZ plants don’t require as much water as many other house plants native to tropical areas. However, they do need some!
The second-easiest way to kill a ZZ plant after giving it too much water is to not give it any water at all!
If you notice dry black or brown spots appearing on your plant’s leaves, especially at the tips and around the edges of the leaves, it could be a sign of dehydration.
Drooping, wilting, and curling leaves are other symptoms of dehydration.
When you think that the black spots on your ZZ plant are down to lack of water before you douse your plant check how much moisture is already present in the potting mix.
If the mix is dry, go ahead and water. If it is moist or wet, the problem could actually be overwatering – which causes similar symptoms to underwatering but is a more difficult problem to treat.
Depending on the conditions, a deep drink every week or so should be enough for your ZZ plant. Set your plant in a tray of water for half an hour or so to allow the water to absorb into your plant’s potting mix, then remove it.
In very hot weather your plant might need more water, make sure to check it every day using one (or more!) of the methods listed above. In winter your plant will need much less water – once a month or so.
Underfeeding or Overfeeding
ZZ plants do not need much fertilizer, and will often cope without any at all. If you want your ZZ plant to look its best though, you’ll want to give it some food.
Overfeeding will cause your plant to develop floppy, yellowing leaves, possibly with brown tips and splotches. This happens because the excess nutrients can scorch your plants’ roots, stopping them from working properly.
To keep your ZZ plant’s foliage lush and healthy, feed it once a month during the growing season – spring and summer.
Stop feeding in autumn and don’t feed your plant again until it starts to put on new growth in spring. Use a general-purpose liquid houseplant fertilizer, diluted according to the instructions.
Dry black or brown spots on your ZZ plant which appear quickly could be a sign that the humidity in your home is too low for the plant to survive.
Plants naturally lose water through evaporation from their leaves, but when the air is too dry it causes them to lose more than they can cope with – causing the leaves to dry and eventually fall off.
ZZ plants are native to tropical areas, and although they can cope with much lower humidity than some other tropical houseplants, they still need a humidity level of at least 40% to be healthy.
Use a humidifier to increase the humidity in your home to at least 40%, 50% would be even better.
You could also use humidity trays under your plants or around the house: fill trays with water and place pebbles inside, big enough to keep your pots out of the water. Put your pots on top and your plants will enjoy their own private humidifier.
Another great way to increase the humidity level in your home is to bring in more plants! The water that they lose through their leaves adds to the amount of water in the air.
Black Spots Everywhere/Anywhere
‘Sooty mold’ refers to fungi belonging to the Ascomycete, Capnodium, or Saprophytic families which live on the secretions (honeydew) of plant pests such as aphids and scale.
Fungal spores which are present in the air land on the honeydew and produce a dirty, grimy substance. Sooty mold won’t do any damage to your plant, but it looks unpleasant, and the insect infestation causing it could be a threat.
If you notice a dark, powdery substance anywhere on your ZZ plant, it could be sooty mold.
Identify and treat the pest infestation (see below). Wipe the sooty mold and any visible honeydew from the stems and leaves of your plant.
ZZ plants are famous for their toughness. When healthy, they are not susceptible to any diseases. However, if your ZZ plant is not in peak health or is living in an unsuitable environment, it may fall prey to various fungal and bacterial diseases.
If your plant has gradually developed black or brown spots on its leaves, it could be suffering from one of several different plant diseases.
These include fungal infections from the Fusarium, Pythium, Rhizoctonia, and Phytophthora families which generally manifest as root or stem rot, or fungal leaf spot which causes black spots on leaves.
Black or brown spots on your ZZ plant’s leaves could also be a sign of bacterial leaf disease. If you’ve tried a fungicide and it hasn’t worked, bacteria could be the culprit.
There is no treatment for bacterial disease, so the best thing to do is remove any affected leaves and re-pot your plant. If the infection is very severe, it might be better to buy a new plant.
Although fungal diseases can be caused by many different types of fungus, treatment is the same for all of them. Use a horticultural fungicide to treat the immediate infection, and take steps to make your ZZ plant less welcoming for the spores of these fungi:
- Increase airflow around your plant.
- Reduce the humidity (not below 40%!).
- Never wet the leaves of your plant.
- Don’t allow potting mix to become waterlogged.
ZZ plants are not prone to insect infestation, but as with all houseplants, insect attack is a possibility. If you’ve noticed sooty mold, honeydew, or the insects themselves on your ZZ plant, it’s time to take action!
Aphids are a common pest that can cause a lot of damage in large numbers. They use their sharp mouthparts to pierce the skin of plants, then inject a liquefying enzyme before sucking the sap from the plant. It’s nasty, but luckily these soft-bodied suckers are easily dealt with!
Spray your plant and all the aphids you can see with a solution of horticultural soap, or Castile liquid soap, and water. If you use Castile soap, use 2 – 2 ½ teaspoons per gallon of water.
Spray your plant every couple of days until the aphids are gone, and re-spray if you notice their return. You can also use rubbing alcohol to remove soft-bodied insects – just rub them off the plant.
Scale is an irritating pest because often by the time you notice them, they’re stuck tight to the plant and difficult to remove.
Immature scale insects can walk, and spend their time searching for the perfect part of your houseplant to attach to – usually the stem or under the leaves. They then transform into limpet-like creatures which don’t move, sucking nutrients from your plant.
If you notice tiny black or brown dots on the stem of your ZZ plant or under the leaves, it could be scale. Small numbers won’t do any harm, but they spread quickly and could cause problems for your plant if not dealt with.
Scale ‘crawlers’, or immature insects are difficult to spot. If you do find these tiny, brownish, semi-transparent, slowly moving dots, a blast with the soap mixture described above will kill them.
If the insects have transformed into adult scales, the soap spray won’t work.
Use a medium-soft bristled brush such as a nail brush to carefully dislodge adult scale from your plant. Remove badly infested leaves and destroy them, do not put them in the compost bin!
You could also use a horticultural oil spray, which can penetrate the scale’s tough shell.
How to Prevent Black Spots on Your ZZ Plant?
ZZ plants don’t need much to be happy. With the right amount of light and water, they tend to look after themselves. To make sure that your ZZ plant never suffers from black spots, follow these steps:
- Keep your ZZ plant out of bright light.
- Do not overwater your ZZ plant.
- Feed your ZZ plant sparingly.
- Ensure at least 40% humidity.
- Treat insect infestation immediately.
Now you know how to treat and prevent black spots on your ZZ plant! If you have anything to add, let us know in the comments!