One thing you don’t want to see is your weed plant leaves turning yellow, but don’t get too nervous if it happens. The yellowing of marijuana leaves can be caused by various reasons so we need to go through the possible causes before we know exactly how to fix it.
What is yellowing?
From a professional standpoint, the yellowing of leaves is called chlorosis. This is a sign that leaves have lost their chlorophyll. If your cannabis leaves are beginning to turn yellow in the last few weeks of bloom, don’t worry—this is normal plant senescence, and like any other plant, by the end of the growing season, the leaves will naturally turn yellow and fall off as the plant transitions its energy to growing flowers and begin to die.
If your plants are still growing and the leaves are turning yellow, cutting those leaves off won’t necessarily fix the problem—let’s find the root.
What can cause leaves to turn yellow?
Underwatering or overwatering plants can cause leaves to turn yellow. We are more likely to over-water than under-water, as we all know cannabis plants are very water-hungry, especially when we use fabric grow bags, which require more water. Too much water can cause your roots to rot, and it can also cause insufficient oxygen in the soil. The solution is simple: reduce your watering frequency.
You can use a hygrometer to keep track of the soil moisture if you’re having trouble. Signs that you are overwatering include fat, sagging leaves, leaves that are darker than normal, and mold growth. If you are underwatering, you will see curling leaves, crunchy leaves, brittle leaves and leaf ends, and brownish-yellow discoloration. If you see these symptoms alongside yellow leaves, try to fix the watering schedule.
Deficiency of light
We said that the essence of leaf yellowing is that the leaves lose chlorophyll, and plants can not perform photosynthesis without chlorophyll. In indoor growing, if your plants are too close to your grow lights your leaves and flowers may turn yellow, but this happens less often and more often when the leaves below don’t get enough light. So low light levels and high heat can both turn leaves yellow (along with making them fall off, look burnt, or actually burn).
Pick the right grow light for your plant and set it in the best position according to the manufacturer’s recommendations. If the visibly lower leaves are yellow, consider adding new grow lights to supplement the light.
pH imbalance is a problem for many growers, which is why a pH meter is a must-have for almost all farmers. For cannabis grown in soil, a pH between 6-7 is ideal. The requirements for hydroponics are more stringent, and 5.5-6.5 is usually the optimal pH for plants growing in hydro.
Improper pH levels can prevent your plants from getting enough nutrients, which is called a nutrient lockout. In turn, the lack of nutrients leads to yellowing and changes in leaf health. This also can happen if you feed your plants too many nutrients, so make sure that you are always feeding your plants the optimal amount by measuring and reacting to your plants’ signs.
You can purchase cannabis fertilizer with the correct pH and use a pH tester to measure the pH of your growing medium frequently. Observe your plants and measure runoff of both pH and total dissolved solids (TDS) frequently so you can know exactly how your plants are doing. If you are facing a nutrient lockout or you are overfeeding, you’ll need to flush your soil thoroughly before you feed your plants again.
Fungus gnats are a common pest that turns leaves yellow. If your leaves are not only yellow but have some spots that resemble serrated edges, like small jagged hole punches in the leaves, then you are unfortunate enough to be attacked by fungus gnats.
There are a number of reasons they get into your tent: you or your pet likely brought it in from outdoors, or there may have been eggs in the soil and overwatering has encouraged them to mature. In any case, yellowing leaves caused by pests and diseases is probably the worst-case scenario.
There are a number of ways you can treat your plant. First, ladybugs make a great predator for any pest that has made its home in your tent. You should also clean the entire tent, including your plants, thoroughly. Look for resources online for how to clean out a tent and clean cannabis plants without exposing them or yourself to harmful chemicals. We have an article on this as well, so be on the lookout for it!
Heat stress is usually related to growing lights. The closer the plant is to the light, the more heat it will feel. Too much heat will not only turn your leaves yellow, but it may also scorch them, leaving them permanently damaged. The easiest way to tell this is to try it yourself: put your hands close to the plant and feel the temperature, if you think it’s too hot the plant also thinks it is too hot.
For plants with yellowed leaves that have been roasted, move them away from light sources and allow them to recover slowly. It’s usually best to check the temperature regularly with a thermometer, and if it’s too high try rotating your plants around your grow tent, raise your lights, or add an air conditioner or an oscillating fan for better air circulation.
There are many reasons why leaves turn yellow, and sometimes you can’t tell exactly how it happens. We want to emphasize that prevention is better than remedy—use a variety of measuring tools to make sure your planting environment is right for your girls to produce the perfect flowers and you’ll be able to avoid yellowing. Treat your plants like your baby and they won’t let you down.
If you have any questions, please don’t hesitate to reach out to us!
And be sure to check out our other blog posts for useful tips on becoming a great grower!
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Used to be really afraid of putting my lights close to my plants when I was growing with HPS but I switched over to LED and thought I wouldn’t have much of a problem but after about a week I noticed my plants were starting to burn it was an unpleasant surprise LOL just a reminder to keep your leds away from the canopy yall!
What is the best way to balance pH? Do I need to flush my plant?
Flushing is probably the best way and it’s not too difficult. Just run RO water through your soil until it comes out clean and your pH is neutral. We have an article on this very topic!
So the lower leaves on my plant are turning yellow while the upper leaves are still green. Do you know what the problem could be? I’m in OR growing white widow in a grow tent. Thanks
This depends: Is your plant near the end of flowering? Then it is normal–actually, it’s a good sign! But if it is in veg you’re likely looking at symptoms of macro nutrient deficiency or underwatering, it’s hard to say. Can you give us some more details?