Black spots on ZZ plants are caused by overwatering. Overwatering can happen when too much water is given to the plant too frequently, if the plant is growing in poor soil that does not drain well, if the pot is too large for the plant or if it is sitting in water in a pot tray.
ZZ plants grow from rhizomes which hold water well. They can be easily overwatered causing stem damage and black spots.
This article will explore what causes black spots on ZZ plants stems and the easy steps you can take to solve this problem.
What causes black spots on ZZ plant stems
Black spots on ZZ plant stems occur when the plant gets too much water. The reasons the ZZ plant gets too much water can include the soil quality, pot size, water being captured in pot trays or watering too frequently.
Too much water
The top reason why a ZZ plant is overwatered causing black marks on stems is when we water too regularly. While the ZZ plant looks tropical it actually can survive long periods of time without water. Watering deeply every 4 weeks is usually enough to keep the plant happy. Any more than this can rot the rhizome and roots and the stems can turn black.
Poor potting soil can cause overwatering of ZZ plants. Poor quality potting soil or if you have used garden soil to pot up your ZZ plant it may not drain freely. Soil that holds too much water can rot the rhizome and cause black spots on stems.
Pot is too large
ZZ plants transplanted into pots that are too large can get too much water and develop black spots on their stems. A large pot will hold a large amount of potting soil which can stay wet, cold and cause the ZZ plant to absorb too much water.
Pots sitting on trays
Another cause of the ZZ plant getting too much water is when the pot is left sitting on pot trays. If the plant is watered and the excess is not allowed to drain out it can cause the potting soil to stay wet and the plant to absorb too much water. This will cause black spots on stems and plant damage in the long run.
How to fix black spots on ZZ plant stems
There is a way to fix the problem of black spots on ZZ plant stems. Here are my top 5 ways to fix black spots on ZZ plant stems.
1. Leave the plant alone
For a mild black stem problem you can solve this problem essentially by leaving the plant alone. Leave it on your shelf, bookcase or in a corner to dry out and recover from too much water. Make sure it is not sitting in water in a pot tray or self watering pot and leave it for 4 weeks before watering again.
If the plant is only slightly overwatered it will recover and the stems will return to the healthy green color.
2. Repot the ZZ plant into fresh soil
For more severely affected plants with larger black spots on their stems it is best to repot them. Tip the plant out of the pot and get rid of the soil. Choose a clean pot with drainage holes and use fresh soil to repot the plant.
3. Water the plant deeply – but only once
After repotting your ZZ plant water it to settle the soil in. This is all the plant will need for 4 weeks so only do this once. Let the excess water drain out before putting it back on its pot tray or inside an outer pot without drainage holes.
4. Fix or remove damaged stems
The next step to help your ZZ plant to recover is to remove any damaged stems. Snip off any stems that are completely broken or are brown and bending. It is best to snip these off ½ an inch above the soil level.
5. Move the ZZ plant into a shady spot
The final step is to move your ZZ plant into a spot out of direct sunlight. Keep your ZZ plant protected while it recovers and give the stems the chance to recover. ZZ plants love full shade and will thrive in a corner where most other plants will not.
Black spots on ZZ plant stems | Summary
Black spots on ZZ plant stems is common and caused by too much water. Taking some time to get the water balance right when you first get a ZZ plant is important. Start off by watering every 4 weeks, always let the plant drain well in a sink or outside and then sit it on a dry pot tray.
ZZ plants have large rhizomes for their size. They look like potatoes and will hold water to keep the plant alive for a long time without extra water. If you start to see black spots on the stems it is a good indicator that the plant is getting too much water.
Stems with black spots can recover as long as the amount of water is reduced. Repot your ZZ plant if the soil is very wet and damaged and move it to a smaller pot.