Why do weed plants turn yellow?

You’ve taken great care of your cannabis plants; you water them, feed them nutrient-rich foods, provide the ideal light cycle for their different developmental phases, and prune them just enough to promote light exposure and optimum growth. You’ve done everything right and yet your marijuana plants just don’t look as lively as they should. Specifically, your cannabis plants are showing signs of stress through yellow leaves. Despite

all the love and attention you’ve given them, they just aren’t growing into the bountiful beauties you had hoped.

Fear not fellow cannabis cultivators! Just because your cannabis leaves are turning yellow does not mean they are done-for. In fact, there are steps you can take right now to correct yellow cannabis leaves and prevent it from occurring again. But first, let’s discuss why cannabis leaves turn yellow in the first place

The most common reason plant leaves turn yellow is because of stress. Whether due to inadequate watering, excessive heat, or pest infestations, yellow leaves are a sign of sickly cannabis plants and must therefore be addressed as soon as possible.

To understand the science behind this, we must first look at the contents of a typical leaf and its relationship to the plant’s overall health.

Leaves are made up of plant cells. Plant cells are made up of organelles called chloroplasts. These chloroplasts contain pigments that absorb different wavelengths of light. Though most of these pigments are green chlorophyll, other pigments are also involved. Most notable are the yellow and orange carotenoids that hide under the dominant chlorophyll.

Nitrogen Deficiency

The roll of chlorophyll is to absorb sunlight and transform it into energy via a process called photosynthesis. When plants have ample access to resources, chlorophyll thrives. When plants are stressed, however, the chlorophyll begins to degrade revealing the yellow carotenoids below.

Note that carotenoids cannot directly transform light into energy via the photosynthetic pathway and must pass it onto chlorophyll to finish the job. Therefore, while a minor yellowing leaves will not stop photosynthesis, excessive or unaddressed yellowing can either stunt plant growth or kill the whole thing entirely.

Light Burn

If you notice yellowing toward the top of the plant (specifically, nearest the light source), your plants are likely suffering from light burn. Light burn can happen in temperature-controlled environments as easily as those in high-heat if the leaves get too close to the lights. We liken this to getting a sunburn on the ski slopes.

The best way to correct light burn is to move you plant away from your light source, usually between eight and 20 inches depending on the watt and lighting type.

Make sure to hang your grow lights as per the manufacturer’s instructions. Adjust as plants progress and use a tape measure for precision. These days there are so many new grow light technologies on the market with their own nuances, you really must stick with the manufacturer’s guidelines to position the light in the sweet spot.

Cold Weather

Temperatures below 10°C will cause the leaves of the majority of cannabis leaves to yellow. Some will even take on purple and bluish tones. Sure, this is a fantastic bonus in the final phase of bloom with some strains, but to be avoided early in the grow.

 Indoors simply turn up the heat or add more lights to the grow op and turn a negative into a bumper harvest. Outdoor growers might need to take plants inside at nighttime or at least invest in a basic DIY greenhouse.

Heat Waves

Heat waves can cause cannabis plants leaves to yellow and dry out indoors and outdoors. Temperatures above 28°C prevent cannabis plants from efficiently photosynthesising and cause buds to form more loosely and airy.

Indoor growers need to turn up the air conditioning and ensure intake and outtake fans are powerful enough for the size of the grow room. Again, we strongly advise consulting the fan manufacturer’s website for performance specifications. Outdoors the best growers can do is construct a simple shade and hope the heatwave is short-lived.


Humans aren’t the only life form with a love for cannabis. Many other creatures—large and microscopic—love to chow down on the herb. Fungal pathogens and nematodes often assault the roots, whereas thrips and fungus gnats have developed a taste for succulent weed leaves. Larger animals also take interest in the herb’s earthy tastes, from roaming deer to household pets.

Growers need to make sure they do everything possible to protect their crop from pests and disease. Because so many different species underpin this problem, you’ll need to take a multi-pronged approach. First of all, add mycorrhizal fungi to your soil to defend against fungal diseases and other critters that attack roots.

To defend against above-ground insects, introduce predatory species into your garden and cultivate companion plants that repel these pests. Erect fences and mesh netting to stop deer, birds, and pets from having their way with your weed plants. If your insect problem spirals out of control and you start to come across eggs, apply a foliar spray of neem oil to clean up your crop.

Leaf Septoria

As a prolific fungal disease, leaf septoria often leads to yellow cannabis leaves. This pathogen strikes the leaves of several plant species—cannabis, tomatoes, parsley—where it first presents as black spots. As the disease spreads, these spots swiftly spread from leaf to leaf and cause brown spots, yellowing, and wilting.

Leaf septoria usually ramps up during the early flowering phase. Left untreated, the pathogen can destroy a large amount of foliage. Because cannabis plants depend on their fan leaves to fuel growth and create energy, excess damage can lead to stunted growth and reduced harvests.

 First things first, you’ll need to remove the infected foliage. Clean your pruning scissors and carefully cut away any leaves that display signs of leaf septoria. Removing infected fan leaves will help to prevent the spread, but finishing the treatment with a dose of neem oil foliar spray will help to take out any non-visible traces of the pathogen.

After treating your plants, you should tweak a few environmental factors to prevent leaf septoria from returning. Start by improving air circulation around infected plants. Add fans to your growing room and instal a dehumidifier if your growing space tends to become extremely moist. Outdoors, move your containers to slightly higher ground to expose plants to the wind.

Clean up the base of your plants. Remove any dead foliage that might be carrying spores and add a fresh layer of mulch to prevent spores from making their way into the soil.

Root rot

Yellow leaves are a key sign of root rot, a microbial condition that attacks the root system of plants. When the growing medium becomes too damp from excessive moisture and overwatering, it becomes a breeding ground for fungi, bacteria, and algae. Any of these organisms can begin to attack the roots in these conditions and cause them to start rotting. This leads to yellowing leaves, brown and slimy roots, leaf drop, and an overall droopy and withered look.

There are several ways to combat root rot once it sets in. Start by introducing beneficial microbes, such as mycorrhizal fungi, into the soil to fight the bad guys. Next, you’ll want to take a look at your watering schedule. Hold back on watering and only apply more once the top inch of soil becomes completely dry.

Consider transplanting affected specimens into fabric pots or Air-Pots to increase aeration and boost drainage. Add a heavy dusting of mycorrhizal fungi to the growing medium mid-way through the transplant for best results.

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