Why & How to Stop My Weed Plants Turning Yellow?

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Cannabis leaves turning yellow

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?

Improper watering

Why are the leaves on my weed plant turning 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

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.

Pests

Why are my weed plants turning yellowFungus 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

Cannabis leaves turning yellowHeat 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.

Tips to prevent cannabis leaves from turning yellow

Ensure Proper Watering

  • Since overwatering or underwatering can stress the plants and lead to yellowing leaves, we only need to water the plants when the top layer of soil feels dry to the touch to ensure proper drainage to prevent waterlogged conditions.

Monitor pH Levels

  • Maintain the pH of the growing medium within the appropriate range for cannabis cultivation (typically between 6.0 and 7.0 for soil, or 5.5 and 6.5 for hydroponic systems).
  • pH imbalances can interfere with nutrient uptake and cause nutrient deficiencies, leading to yellowing leaves.

Provide Sufficient Light

  • Ensure the plants receive adequate light during the vegetative and flowering stages. Insufficient light can cause yellowing and other signs of stress.
  • If growing indoors, use high-quality grow lights and adjust their intensity and distance from the plants as needed.

Wrap-up

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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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

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FAQ about Cannabis Leaves Turning Yellow

Improper watering

Underwatering or overwatering plants can cause leaves to turn yellow. 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.

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).

pH imbalance

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.

Pests

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.

Heat stress

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.

Nitrogen Deficiency

One of the most common causes is a nitrogen deficiency. Nitrogen is a key nutrient for vegetative growth and healthy green leaves. As plants enter flowering, their nitrogen needs decrease, which can lead to an N deficiency if not adjusted properly. Leaves will yellow starting from the bottom up.

Improper watering

Both overwatering and underwatering can cause leaf yellowing. Overwatering leads to oxygen deficiency for the roots, causing leaves to droop and yellow. Underwatering stresses the plant and makes it cannibalize nutrients from fan leaves.

pH imbalance

If the pH of your soil or hydroponic solution is too high, it can cause nutrient lockout where the plant cannot properly absorb certain nutrients like iron, manganese, and zinc. This leads to interveinal yellowing.

Light Burn

If your grow lights are too intense or too close to the canopy, it can create yellow spots and browning on the leaves closest to the lights. This is light burn damage.

Root Problems

Unhealthy roots due to issues like root rot, heat stress, or circulating nutrients improperly can cause overall plant stress and yellow leaves.

Natural Yellowing

Some yellowing of fan leaves is natural as flowering progresses and the plant diverts nutrients toward developing dense buds instead of leaves.Improper watering

 

VIVOSUN Team

VIVOSUN Team

The VIVOSUN Team, established by VIVOSUN, specializes in tropical plant and cannabis cultivation research. With extensive experience in indoor growing equipment operation and research, they lead innovation in the field. Through articles, workshops, and industry forums, they share practical insights to empower growers and shape the future of indoor cultivation practices.