Whilst we typically associate yellow cannabis leaves with Nitrogen, there are so many variables when growing cannabis and yellowing leaves can be caused by lots of different issues. These range from watering problems to pests and fungi.
In this article, you'll learn about the main causes of yellowing leaves, including over or underwatering, pH issues, nutrient lockout, light burn, temperature issues, root rot, pests, fungi, and Leaf Septoria. By the time you have finished reading this article, you will never need to worry about yellowing leaves again!
Key Takeaways
- Leaf Septoria is a fungal disease that causes yellow spots
- Chlorophyll is the pigment that gives them their green color
- Nitrogen is responsible for the vibrant green colour of leaves
- A lack of nitrogen will cause fan leaves to turn pale yellow and brittle
- Underwatering can be one reason why leaves turn yellow
- Overwatering can also affect nutrient uptake causing yellowing
- pH levels play a crucial role in how plants access nutrients
- Grow room temperature fluctuations can influence yellowing
- Insects can cause leaf damage and suck the sap out of the leaf
What do yellow cannabis leaves mean?
Yellow cannabis leaves are your cannabis plants' way of telling you something is wrong and that the plant is experiencing stress, or lacking nutrients. If the yellowing of the leaves is not resolved early on this will limit the amount of light the plant is able to absorb and can eventually stunt growth.
Cannabis plants produce chlorophyll, which is a pigment that gives them their green colour. Yellowing cannabis leaves indicate they lack chlorophyll and cannot photosynthesise properly.
Act early though and you might be able to fix the issue in just a few short days. The main causes are normally nitrogen nutrient deficiencies, improper watering, temperature issues or pest infestations.
For a first time grower or novice it can be difficult to identify the exact cause, but don’t worry, that is why we’re here to help. Below highlights all the subtle differences between each problem that should help you pinpoint where you might have gone wrong.
Actionable Tip: Act quickly when leaves begin yellowing to prevent reduced light absorption and stunted growth
Lack of Nitrogen
Cannabis plants rely on primary nutrients—nitrogen, phosphorus, and potassium—to support healthy growth. Nitrogen, in particular, plays a key role in chlorophyll production, which keeps leaves green and allows plants to absorb light and store energy. When levels drop, plants can quickly show signs of nitrogen deficiency in cannabis, especially during the vegetative stage.
One of the most common mistakes growers make is underfeeding nitrogen. This causes plants to pull nitrogen from older leaves and redirect it to new growth, leading to yellowing over time. To prevent this, growers can supply nitrogen through liquid feeds, such as fish-based fertilizers, or organic inputs like worm castings.
For best results, use a nutrient solution with an NPK ratio close to 5-3-2 during vegetative growth. Regular top dressing with compost and worm castings every 10 days is also an effective way to maintain healthy nitrogen levels in the soil.
- Nitrogen (N):
- Phosphorus (P)
- Potassium (K)
Secondary Macronutrients
- Calcium (Ca)
- Magnesium (Mg)
Micronutrients
- Molybdenum (Mo)
- Boron (B)
- Zinc (Zn)
- Manganese (Mn)
- Iron (Fe)
- Copper (Cu)
Actionable Tip: Feed plants with nitrogen-rich nutrients (e.g. liquid feed or worm castings) to maintain green, healthy foliage
Over or under watering
If you give your cannabis plants too much water, it can suffocate the roots and prevent them from taking on oxygen which eventually leads to root rot. On the flipside if they receive too little water, they won’t be able to absorb enough nutrients, which can also cause the leaves to yellow.
Watering cannabis plants is a bit of a balancing act and it's really important to learn how to properly water your plants. A simple rule of thumb is to feel the top 3-5cm of soil, and insert your finger down the sides of the pot. If its close to dry then the soil needs watering, if it is moist then leave them and check again the following day.
What to look for:
The leaves of your plants will start to droop slightly. Feel the top layer of soil, if it is very dry, they are probably underwatered; if the soil is damp, they are likely overwatered.
How to fix:
To fix overwatering, make sure your pots are draining properly and reduce watering your plants. You need to make sure the soil is drying out enough between waterings to prevent root rot. Lifting the pots before watering will also help you determine how much moisture is inside the growing medium.
If you suspect you have underwatered your plants, you need to give them more water or water them more frequently. As plants get bigger during vegetation and the early flowering stage, they will demand more water to keep up with their growth rate. Using a moisture meter and a watering can can help you to maintain the perfect soil moisture levels.
pH issues or nutrient lockout
pH related nutrient blocking (nutrient lockout) occurs if the pH level in your soil is either too high or too low. When pH levels are not in the right range it prevents your plants from absorbing its nutrients and causes excess nutrients and salts to build up in the soil. Before this problem becomes serious your leaves will begin to turn yellow, due to the fact nitrogen is not available during the lock out, so catch this early and you can fix it.
Actionable Tip: Test soil pH regularly and keep it between 6.0-7.0 to ensure nutrients remain available to the plant
What to look for:
Check the leaves for yellowing starting at the tips and edges. If your plants appear healthy but still exhibit yellowing, test the soil's pH. A pH imbalance often causes nutrient deficiencies, despite regular feeding.
How to fix:
To fix pH issues, test the soil's pH and adjust it to the ideal range of 6.0 to 7.0. Use pH adjusters like pH up or pH down to correct the levels. For nutrient lockout, flush the soil with clean, pH-balanced water to remove excess salts and restore nutrient uptake. Test the pH of the run off at the base of the pots after watering to see if there is an imbalance that needs to be corrected.
Over or under feeding
Overfeeding your cannabis plants can lead to nutrient burn, which causes the tips of your plants leaves to turn yellow or they can sometimes look as though they have burnt. Underfeeding, on the other hand, causes a lack of vital nutrients, which can cause the overall leaf to turn yellow.
What to look for:
Look for yellow leaves with burnt tips (overfeeding) or overall pale yellowing (underfeeding). Overfed plants might also show nutrient burn signs like brown spots, while underfed plants will appear weak and stunted. The edge of the fan leaves will begin to turn crispy and dry.
How to fix:
If you suspect that your plants are suffering from over feeding you need to flush the soil with clean, pH-balanced water to remove any excess nutrients. This will help to reset the nutrient levels in the soil.
If underfeeding is the issue you need to gradually increase the nutrient dosage over the next few waterings. Keep a close eye on how your plants respond and adjust feeding as necessary. Simply increase your nutrient solution by 1ml each time until your plants show they are healthy and happy.
Light burn
Light burn is usually caused when your cannabis plants are placed too close to your grow lights. This causes the leaves to turn yellow or develop burnt spots and can cause a serious amount of stress
What to look for:
Observe the top leaves closest to the light source. If they are turning yellow or starting to develop burnt edges, your lights are either too powerful or too close to your plants. If it is just the leaves at the top of the plant closest to the lights that are displaying these signs while the lower leaves are still green, there is a good chance this is the issue.
Fans blowing hot and dry air at the foliage of your plants will also influence light burn, so make sure your environment is not too hot. Poor extraction will also play a role.
How to fix:
To fix light burn, adjust the distance between the grow lights and the plants, keeping the lights further 12-24 inches away from the top of your plants. Alternatively, if your lights have a setting that allows this you can reduce the light intensity, then use dimmers to adjust the light intensity
Gradually acclimate plants to stronger light sources to avoid shock and burn. Another simple trick, although this should be done in the veg stage only and not flowering, is to use a ScrOG net to prevent your plants from growing too tall. This also increases light distribution which will give you bigger overall yields.
Actionable Tip: Keep grow lights 12–24 inches away from the canopy or reduce intensity to prevent leaf burn
Temperature issues
Extreme temperatures, either too high or too low, will stress your plants and disrupt their metabolic processes; one of the signs of this is yellowing leaves.
What to look for:
Check for curling leaves, yellowing, dry and brittle leaf tissue and slow growth. If it is too hot for your plants this will cause leaves to yellow and curl up, while if it is too cold, they can turn a light yellow and start to droop.
Too hot
Excessive heat can cause cannabis leaves to turn yellow. High temperatures above 28 degrees Celsius can significantly stress the plants, causing them to lose moisture rapidly due to transpiration and hindering their ability to photosynthesize effectively.
How to fix:
Indoors: To fix indoor temperature issues get yourself a digital hygrometer inside your grow space, and try to maintain a stable environment in your grow room. Try to keep between the range of 70-85°F (20-29°C) during the day and slightly cooler at night. Use fans, air conditioning, or heaters to regulate the temperature. Make sure you have good ventilation to prevent heat buildup.
Outdoors: If you are growing outside, most of what works for indoor grows will not work for you. Instead try to follow these steps:
- Water your plants in the evening an hour after sun sets to avoid water evaporating and burning the leaves.
- If the heat is causing stress you need to do whatever you can to reduce the temperature for your plants, so try to find a way to increase shade during the hottest parts of the day. Shading nets will reduce light intensity and heat. Don’t worry about them not getting as much direct sunlight, the key is to help them survive.
- You can try adding some seaweed kelp extract to your fertilizer. This will help to protect the roots against heat exhaustion.
- Think about what material of pots you use. Fabric pots are great because they help to regulate the temperature of your soil in extreme heat, protecting your roots.
Too cold
When the temperature drops too low, the plant's metabolic processes slow down, leading to nutrient uptake issues which causes the leaves to begin to yellow and droop.
How to fix:
To fix cold issues, use heaters or heat mats to raise the temperature in the grow area. Keep a digital hygrometer in your grow room and try to keep the temperature within the ideal range of 70-85°F (20-29°C) during the day and do not drop below 60°F (15°C) at night.
Insulate the grow space so that it retains more heat and avoid placing your plants near drafty windows or doors. Lifting pots off the cold floor can help prevent cold roots, and If you live somewhere that is always cold, it could be worth using HID lights such as metal halide (MH) or high-pressure sodium (HPS) instead of LED as they naturally give off more heat.
Actionable Tip: Maintain stable temperatures within the ideal range to prevent stress-related yellowing
Root rot
Cannabis plants love oxygen and especially the root zone! The main cause of root rot is overwatering, which prevents oxygen from getting to the roots eventually leading to decay and fungal growth. Over saturated soil will encourage harmful bacteria to thrive which attacks the roots.
What to look for:
Look for yellowing leaves paired with a rotting smell from the soil. Roots may appear brown and mushy instead of white and firm. Plants may also start to wilt despite sufficient watering.
How to fix:
Fixing root rot in organics or hydroponics isn’t easy and the stress involved with the rot itself and what is required to fix the issue can kill cannabis plants. To fix root rot, you will need to remove the plant from its container and gently wash the roots to remove any dead or decayed material. Be careful and try not to damage too much of the roots, as the roots will feel brittle and break away easily.
Prune away any severely damaged roots with sterilized scissors then repot your cannabis plant in fresh, well-draining fresh soil, and make sure the new container has adequate drainage holes. Reduce how often you water your plants and make sure the soil has dried out between waterings to allow roots to search out for oxygen.
Adding beneficial microbes or fungicides to the soil can help prevent further fungal growth. The beneficial bacteria and fungi will fight off the bad bacteria. You can also add amendments such as perlite to your soil to improve drainage.
Pests and fungi
Common pests like spider mites, aphids, and whiteflies, along with fungal infections like powdery mildew, disrupt the plant's health and nutrient absorption, causing your leaves to turn yellow over time.
What to look for:
Inspect your plants leaves for small insects, webbing, or fungal spots. Yellow leaves with tiny dots or spots and visible pests like spider mites or whiteflies indicate pest issues. Fungal infections often show as white powdery patches. Always check the tops and undersides of the leaves and be quick to act if you do notice insects and larvae.
How to fix:
To fix cannabis pest issues, inspect the plants regularly and use organic insecticidal soap or neem oil to treat infestations. You can also introduce beneficial predatory insects, such as ladybugs or predatory mites, to help control pest populations naturally. For fungal based infections like powdery mildew, you will need to remove, and discard affected leaves.
Mildew spreads incredibly fast, and it is sometimes too late to save a plant unless it is caught early. To avoid any fungal infections, make sure your grow room has good air circulation, and keep the humidity between 40-50% during flowering. You can also apply fungicides or natural remedies like a mixture of water and baking soda to combat fungal growth, however be careful not to get the buds wet when treating the leaves.
Leaf Septoria
Leaf Septoria, also known as yellow leaf spot, is a fungal disease that causes yellow spots to develop on cannabis leaves. It typically thrives in warm, humid conditions and can spread very fast if not addressed quickly.
What to look for:
Check your cannabis plants for circular yellow spots with dark centers on the lower leaves. These spots can spread quickly, leading to widespread yellowing. The fungus typically starts on the lower leaves and moves upward.
How to fix:
To fix Leaf Septoria, remove and dispose of any infected leaves to prevent the spread of the fungus. Improve air circulation and reduce humidity in the grow area by using fans and dehumidifiers. You should also make sure when you water your plants you water directly onto the soil and not the leaves – keeping the leaves dry will reduce the risk of the infection spreading.
Apply a fungicide specifically designed for leaf spot diseases or use natural remedies like a diluted solution of neem oil or a baking soda spray. It is generally good practice to also maintain a good amount of spacing between pots in your grow room to ensure good airflow between the plants.
Actionable Tip: Remove infected leaves immediately and keep foliage dry to stop fungal spread
It is normal for leaves to turn yellow at the mid-end of flowering stage
During the flowering stage cannabis plants begin to switch their focus to producing more buds, so do not need as much nitrogen as they once did and demand high levels of phosphorus and potassium. This causes the leaves of the plant to gradually turn yellow but you don't need to worry as this is completely natural.
You should begin to see older or lower leaves turn yellow and begin to fall off while the buds will continue to develop and swell. This is a normal part of the plant's life cycle and simply helps the plant to produce even bigger buds.
Keeping your cannabis leaves green and healthy isn't rocket science, but it does take a bit of know-how. If you found these tips helpful, give your plants (and us) some love by liking and sharing this article!
FAQ about Yellow Cannabis Leaves
Why are my cannabis leaves turning yellow?
Cannabis leaves turn yellow due to stress or nutrient issues. The most common causes include nitrogen deficiency, overwatering, pH imbalance, nutrient lockout, pests, fungal infections, and environmental stress such as temperature or light issues.
Is it normal for cannabis leaves to turn yellow?
Yes, yellowing is normal during the late flowering stage as the plant redirects nutrients to bud development. However, yellowing during the vegetative stage usually indicates a problem that needs to be addressed.
How do I know if yellow leaves are caused by overwatering or underwatering?
Check the soil moisture. If the soil is wet and leaves are drooping, overwatering is likely. If the soil is dry and leaves appear limp or weak, underwatering is the more likely cause.
Can pH imbalance cause yellow cannabis leaves?
Yes, incorrect pH levels can lead to nutrient lockout, preventing plants from absorbing essential nutrients like nitrogen. This often results in yellowing leaves even if nutrients are present in the soil.
How do I fix yellow leaves on cannabis plants?
To fix yellow leaves, identify the root cause first. Adjust watering practices, correct pH levels, improve feeding schedules, and check for pests or diseases. Acting early gives plants the best chance of recovery.
Do yellow cannabis leaves turn green again?
In most cases, yellow leaves will not turn green again. However, fixing the issue will allow new growth to return to a healthy green colour.
Can pests or fungi cause cannabis leaves to turn yellow?
Yes, pests like spider mites and aphids, as well as fungal infections like powdery mildew or Leaf Septoria, can damage leaves and cause yellowing over time.
What deficiency causes yellow leaves in cannabis?
Nitrogen deficiency is the most common cause of yellow leaves, especially when it starts from the lower leaves. Other deficiencies, such as magnesium or iron, can also cause yellowing depending on the pattern.
