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No matter what you are growing, it can be concerning when the leaves begin to turn yellow (chlorosis), especially during a critical phase such as flowering.
It can be super concerning when it happens to a plant that you are growing with a view to harvesting something valuable from it. As with any plant, leaves turning yellow on a cannabis plant could be completely harmless.
However, you should always investigate in case it is something serious that requires attention. In this article, we will look at what causes yellow leaves on a cannabis plant and how to treat them if necessary.
Are Yellow Leaves Always Bad?
As we touched on above, sometimes it is perfectly normal for leaves to turn yellow during flowering. When a cannabis plant begins to flower, the nutrients and energy in the plant are diverted away from the stem and leaves towards the buds.
This diversion of resources causes some of the leaves to die off during the flowering process. This can happen more toward the end of the flowering process when resources are depleted.
Common Causes Of Yellow Leaves
As mentioned above, there are a few different reasons why your leaves might be turning yellow. If they are turning yellow because of the natural flowering process there is nothing to worry about. However, there are other common causes that should be investigated.
One of the most common causes of leaves turning yellow on your cannabis plant is light burn. All this means is that the glow lights that you are using are too bright or turned up too high. The power will end up burning the leaves and causing them to turn yellow.
It is easy to spot when yellowing leaves are caused by light burn as it will affect the leaves that are closest to the lights more than the others. It should also be difficult to pull the leaves off of the plant despite the fact that they are dead.
To fix this issue, simply move the glow lights further away from the leaves or turn them down if they are dimmable. We’ve covered proper light hanging height for cannabis plants in other posts.
Another common problem that can cause the leaves of your cannabis plant to turn yellow is over-watering. This is actually one of the most common reasons for leaves to turn yellow.
Over-watering can be caused by giving the plant too much water in one go, or that you are giving the plant the right amount but too often.
Alternatively, you could be watering the correct amount, at the correct intervals, but the pot or soil that your plant is in doesn’t have good enough drainage.
If the cause of your leaves’ yellowing is over-watering it can present similarly to the yellowing that can occur because of a nutrient deficiency. However, there are a few ways in which you can distinguish between the two.
One of the easiest ways to tell whether over-watering is the issue is the position of the leaves. If they are drooping and look swollen like they are full of water, they are likely over-watered.
You should also pay attention to the behavior of the leaves just after watering. If they droop immediately, they are definitely getting too much water.
This is another simple fix, just reduce the amount of water you are giving the plants. This can be in terms of quantity for each watering or in frequency or waterings. You should also check whether the plants are getting enough drainage.
To identify a drainage problem, pay attention to the time it takes for water to come out the bottom of the pot. If it takes a really long time to come out, the drainage is too poor. Make sure that you are using high-quality soil.
When you water the plants, between 10-20% of the water should come out the bottom. That will let you know when you have given the plant enough water. Remove the excess water from the tray. Checkout our guide to watering cannabis plants.
Insufficient lighting is another issue that can cause yellow leaves for your cannabis plants. However, this yellowing tends to happen in the early stages of a plant’s life rather than during the flowering process.
If your leaves are only turning yellow during the flowering stage, insufficient light isn’t the problem.
There are several nutrients that can cause the yellowing of leaves when they are not present. However, excessive nutrients can also cause yellowing although much less commonly.
There are many charts online that can help you know which nutrients can cause leaves to lighten or turn yellow. Below is a list of the most common nutrient deficiencies and how they present.
- Nitrogen – this is the most common deficiency in cannabis plants. The lower leaves of the plant will begin to yellow during the flowering process. The leaves will be easy to pull off and will likely fall off on their own.
- Magnesium – If your plant is deficient in magnesium, the flesh of the leaves will turn yellow but the veins will stay green. If this is only happening to the lower leaves that aren’t getting light, it is not a deficiency, simply a natural diversion of resources.
- Iron – If an iron deficiency is the issue, it will be the newest leaves on the plant that turn yellow. This makes it an easy thing to diagnose. The leaves should start out yellow and begin turning green from the outside inward.
Under-watering is a much rarer problem than its counterpart. It is very easy to diagnose. If your leaves are drooping but perk up after you water the plant, this is the issue. If you are using the right amount of water but your plant is still limp between waterings, the pot may be too large.
This has slightly been touched on when we mentioned the glow lights burning the leaves. However, if the lights are fine, an ambient temperature in the grow room that is too high or too low for the plants can turn the leaves yellow.
The ideal temperature for your cannabis plants to be kept in is between 60 and 85 degrees Fahrenheit. The temperature should be lower at night, as it would be in nature, but it should never be below 60 degrees.
Incorrect pH Balance
An incorrect pH balance in the roots of your plant is the number one most common reason for yellow leaves. When the pH balance is off, cannabis plants are not able to absorb the nutrients that they need effectively.
If you are struggling with nutrient deficiencies, like the ones mentioned above, the pH balance could be the root cause.
If you find that the pH balance is off for your plants, you should flush the root area with neutral water. Then water your plants with the correct pH. This can be achieved by using pH up/down solutions.
Bud rot is an issue that thrives in the temperatures on the lower end of the appropriate window. Around 60-70 degrees Fahrenheit.
You can identify bud rot by looking to see if there is any brown or gray mold growing inside the cola at the base of a yellow leaf.
The yellowing of leaves is usually the first sign that you will see to indicate bud rot and they will appear quickly. Once bud rot has established itself there is not a lot that you can do.
Therefore, it is best to prevent it from occurring in the first place. Good circulation is the key to preventing bud rot.
Yellow leaves can also be caused by an infestation of pests. You can get an infestation even if you have a completely sealed grow tent due to the risk of contamination on your own clothes and shoes.
Poor conditions can make your plants more susceptible to infestations. Maintaining a clean room and ideal growing conditions can help avoid pest problems.
Troubleshooting Yellow Cannabis Leaves
As we mentioned above, the most common cause of yellowing in cannabis leaves is a nutrient deficiency. However, when you are troubleshooting your yellow leaves, it doesn’t make sense to start with this as a cause because it is so difficult to determine.
You should rule out everything else first.
Step 1: Ensuring Correct Temperature
While incorrect temperature might be one of the least likely causes of yellow leaves, it is the easiest to check for. You should make sure that your grow tent temperature is kept above 60 degrees at night and below 85 degrees during the day.
Step 2: Ensuring The Correct Amount Of Water
This is another easy check. If the leaves are droopy and looking full, you are overwatering. If they are drooping and thin, you are under-watering. Check the section above to see about drainage.
Step 3: Check Lighting
You should check that the glow lights aren’t too close to the plants and burning the leaves, causing them to turn yellow. You can preemptively check by holding your hand near the light to check if it is too hot.
Step 4: Check For Pests
For this step, simply examine your plant closely to see if there are any pests crawling over them. If you want to be very thorough you should check the underside of the leaves too.
Step 5: Check pH Levels
If you have ruled everything else out, you likely have a nutrient deficiency issue. This is most commonly caused by a pH imbalance that can affect the absorption of nutrients.
The best way to test the pH levels of your plant’s roots is to test the pH of the run-off water that collects in the tray. If the pH is off, you should correct it in the manner described above. If it is okay, the issue is nutrient supply.
Step 6: Check Nutrient Deficiencies/Excess
The most common deficiencies are iron, magnesium, or nitrogen. You should test to see which ones your plants are lacking.
You can do this by looking at the leaves as described in the section above. You can then resolve the issue with supplements and trimming the lower branches to free up resources.
Yellow leaves on a cannabis plant aren’t always bad. However, the cause should be investigated so that problems can be addressed quickly.
I’m an avid gardener and cannabis enthusiast. You can usually find me in my garden caring for my plants or at my computer crafting helpful blogs for my readers.