Ah yes! The age old question that has plagued cannabis growers for generations: Why are my marijuana leaves turning yellow?
I get variations of this question at least three times per week, and the answer usually depends on a number of factors ranging from nutrient deficiencies to lighting issues.
But fear not, fellow grower! Today I will dive deep into the most common causes of yellow cannabis leaves, and I will guide you on how to diagnose, treat and prevent it.
And if you stick around to the end, I will give you a secret trick that will keep your plants healthier than you ever thought was possible.
What are the most common causes of yellow cannabis leaves?
Yellowing cannabis leaves is always the visible symptom of a bigger issue so let’s take a moment to appreciate the fact that your plants “literally” are telling you that you need to give them attention.
The problem is that, because yellow leaves are such a common symptom, you need to diagnose more than the leaf to understand what the underlying issue really is.
The 8 most common reasons your weed leaves are turning yellow are the following.
1. pH imbalance
pH imbalances are fairly common for both beginner and advanced growers and it usually manifests as yellow leaves. Even new growth may come out yellow or with yellow or brown spots. If left untreated, a pH imbalance can cause nutrient lockouts that cause the plant to not get enough of key nutrients which in turn leads to bigger problems.
How do you treat pH imbalance?
Once you know that you are dealing with a pH imbalance, you need to treat the growing medium to get the pH back into optimal range.
Lowering the pH
If the pH is too high and you need to lower it. You can use a “pH down” product that you mix with water and add to the growing medium. You repeat the process in steps until you reach the desired pH.
Increasing the pH
You can increase the pH by using a “pH up” product and proceeding in the same way. Limestone is the most common way of decreasing soil acidity.
How to prevent pH imbalance
To prevent future pH imbalances, you need to regularly monitor your pH levels and make adjustments as you go. It is always easier to prevent the issue than it is to fix it so add a pH meter to your toolbox.
2. Temperature issues
Temperature issues can cause cannabis leaves curling down or fan leaves turning yellow and purple. If you notice symptoms like that you need to check the temp of your grow room to ensure it’s within the optimal range of 70-85°F (21-29°C) during the day and 60-70°F (15-21°C) during the night.
Treating temperature issues
It is easy to treat a temperature issue. You need to adjust your heating, cooling, or ventilation system according to what you want to accomplish.
Don’t forget that the humidity of your grow room plays a role in the temperature too, so you may need to use a dehumidifier (or a humidifier) to maintain ideal humidity levels. 40-60% during the vegetative stage and 40-50% during the flowering phase are good baseline temperatures to aim for.
Preventing temperature issues
If you don’t have a thermometer and a humidity meter in your grow room, get them!
With the right tools, it’s easy to check the temperature of the growing environment so don’t let temperature issues slow your plants and cause leaves to get yellow.
3. Light burns and leaf bleaching
Light burn is a common cause of yellowing cannabis leaves and it is mostly noticeable on the leaves closest to the light source. If the leaves towards the top of the plant are turning yellow, while the rest of the plant remains healthy, it is very likely that you are dealing with light burn.
How do you treat light burn?
Generally speaking, it is easy to treat light burn before it becomes a bigger problem. You can either move the light source further away from your plants or reduce the intensity of the light. If you’re using adjustable LED grow light, you can lower the power output to prevent further damage.
As a rule of thumb, the more powerful the light, the greater the distance it should be from your plants. If in doubt, use a light meter to measure the intensity of the light at different points in your grow space and adjust the intensity accordingly.
4. Under and overwatering
Under or overwatering your plants can cause yellow leaves and it is very common for beginner growers to miscalculate the water needs of their plants.
Over-watering can be diagnosed by looking at secondary symptoms like the cannabis leaves curling down. Under watering will make the whole plant look droopy and the leaves lose their bounce.
How do treat under/overwatering?
Simply adjust your watering schedule to balance out the water in the growing medium. If you’re consistently under-watering, you can increase the frequency or volume of the water that you provide. Do vice-versa if you’re overwatering by reducing the frequency or volume of water.
How to prevent under/overwatering
Make sure you have a pot and growing medium with good drainage and appropriate-sized containers for the size of your plants. It’s common for growers to skip a pot-size and get “lazy” with transplanting but plants that are placed in too large or too small pots have a much greater likelihood of watering issues.
5. Root rot
If you are a consistent “over-waterer”, you need to watch out for root rot!
Root rot causes cannabis leaves to turn yellow, wilt, and eventually, even die. Scrape off a layer of soil or gently lift the plant from its container so you can peek at your roots to make sure that they are white or light tan in color. Unhealthy roots that are suffering from root rot are dark brown or black with mushy texture and sometimes even a foul odor.
How do you treat root rot?
It’s important to spot rot rot as early as possible because you will need to remove the unhealthy roots completely from your plant. If too much of the root zone has been damaged, you may need to let the plant go 😭
Gently remove the plant from the container and wash off the soil. Use clean scissors or a pruning tool to cut off all the dark and mushy roots. Once only the healthy roots remain, transplant your cannabis plant into a clean pot with well-drained soil.
6. Pest invasion
Pests are easy to spot but can be tricky to deal with. The most common pests that effect cannabis plants are spider mites, aphids, and whiteflies. They can all cause yellow leaves by sucking the sap out of the plant, leading to nutrient deficiencies.
If you notice small insects on your plants or tiny holes in the leaves, it’s most likely a pest problem. Learn how to deal with mites on cannabis plants here.
How to treat a pest invasion
You need to take pest invasions seriously because they can destroy entire crops. Start by removing any heavily infested plants from the grow room. Then remove damaged leaves and apply an organic pesticide or insecticidal soap to the plant (read the instructions carefully).
7. Leaf Septoria
Leaf septoria, aka yellow leaf spot, is a fungal disease that can cause the yellowing of cannabis leaves. It typically starts out as small, circular yellow spots on the lower leaves that turn brown with a yellow halo with time. If left untreated, leaf septoria can spread to the entire plant, causing enough damage to force you to cut it down completely.
How do you treat leaf septoria?
First, you need to remove all the affected leaves so you stop the fungus from spreading. Then you apply a fungicide (usually a copper-based product) to help control the infection.
How to prevent leaf septoria
Fungus likes to grow in wet and humid environments so make sure that you maintain good airflow and proper humidity in the grow room. Try to avoid getting leaves wet when watering and regularly clean and sanitize your tools, hands, and anything else that comes in contact with your plants.
8. Over/Under Fertilization
Over and under-fertilization is another common beginner mistake that can cause yellow cannabis leaves. As you gain experience as a grower, you usually spot symptoms of a nutrient imbalance early but less experienced growers are likely to suffer from this at least a few times in their first grow cycles.
Symptoms tend to start at the top leaves as the tips appear yellow or brown if you have over-fertilized. Under-fertilization on the other hand usually shows up as general yellowing as the leaves fade from a healthy green to a lackluster yellow.
How do you treat fertilization issues?
First, you need to identify whether you have over or under-fed your plants and what essential nutrient is off balance. Then you can get to work to correct the nutrient regimen accordingly.
We go into more detail about how to spot and fix the most common nutrient deficiencies here.
When should you not worry about yellow cannabis leaves?
Yellow leaves are a natural part of a plant’s lifecycle so you shouldn’t panic at the first sign of yellowing. Fan leaves, for example, turn yellow during the vegetative phase and leaves generally turn yellow during the flowering phase because the plant is reallocating energy to produce flowers and seeds instead of photosynthesis.
If the yellowing is limited to the lower, older leaves and the rest of the plant looks healthy, you likely have nothing to worry about since that is normal in the natural life cycle of every plant.
Your secret weapon
Amplify is your secret weapon to succeed as a grower. With a diverse blend of highly concentrated and naturally occurring plant growth-promoting rhizobacteria, the microorganisms in Amplify help increase the uptake of nutrients and water while also helping to protect your plants’ rhizosphere (roots) from harmful pathogenic microbes.
Our full-spectrum microbial inoculant is up to 3X more effective than competing brands, proven to drastically improve your plant’s nutrient uptake throughout the grow, boosting yields, and helping your plants reach their full genetic expression.