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
Why Cannabis Leaves Turn Yellow
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.
Example of a cannabis plant experiencing 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.
Diagnosing and Curing Yellow Cannabis Leaves
If your marijuana leaves are turning yellow, don’t panic! This is just your plants’ way of telling you something is wrong. It is up to you to determine what that is so you can treat the problem without making it worse.
The first thing you should do when cannabis leaves start to turn yellow is to measure the pH of your grow medium (soil, water, rice hulls, expanded clay, etc.). That’s because an improper pH balance – whether too high or too low – can actually block nutrient absorption.
After measuring and adjusting pH, take a look at your watering schedule. The most common cause of yellow leaves is either over- or under-watering. Plants that are over-watered will have leaves that seem swollen and droopy while under-watered plants (though much less common) will be thin and frail. Poor drainage can also contribute to overwatering so always grow your cannabis in pots with drain holes.
If the yellowing occurs primarily at the base of the plant, the issue is likely a nutrient deficiency. The most common nutrient deficiency in cannabis is nitrogen, though note that excessive nitrogen can also cause yellowing (plus curled, claw-like leaves). If the problem is caused by a deficiency, slowly increase the concentration of your cannabis-specific fertilizer until new growth appears. If the yellowing is caused by excessive nutrients, flush the root system with pure water then add a half-dose of your fertilizer instead. Magnesium deficiency, characterized by a yellowing around the leave’s green veins, is most commonly caused by an improper pH balance. Use magnesium supplements to correct this issue
Iron deficiency can also cause yellow leaves, though this occurs on new growth only (old leaves remain bright green). Iron deficiencies are also caused by improper pH and can be remedied with iron-fortified fertilizers. When iron is given to deficient plants, the leaves should start to turn green beginning along the edges until the whole leaf is bright green.
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.
Temperatures outside of the ideal range of 70 to 80 degrees Fahrenheit also risk discoloration and curl leaves. These oddities are most common in leaves toward the top of the plant and can easily be remedied with a fan (if too hot) or root insulation (if too cold). We also recommend growing plants off a cold cement or tile floor; use a milk crate or stool to raise them off the ground when necessary.
If your yellow leaves come with spots or bite marks, they’re probably infested with pests. Though you can often see the infestation, this is not always the case – a tell-tail sign (aside from the remnants of the buffet) is a plant that lacks vigor in addition to other symptoms associated with things like overwatering or poor air circulation.
Unfortunately, pests are perhaps the hardest condition to correct, so it’s best to avoid try and avoid them all together. To start, never bring plants or clones from an outside grow into your sanitary grow space and try to avoid entering your grow space directly from the outdoors. Always wash hands, cover your hair and avoid letting pets anywhere near your grow space.
Fungus gnats, which live in wet soil and feed off roots, are the most common pest in cannabis gardens. The best way to rid your garden of fungus gnats is to restrict watering until absolutely necessary (this prevents the gnats from laying eggs in the soil). A general best practice is to only water your plants when the top inch of soil is dry.
Growing your own marijuana is very rewarding, but it can be really nerve wracking, too, especially when those bright green leaves start turning a worrisome yellow. If your cannabis leaves are turning yellow, use these steps to stop the yellowing before it’s too late.
PH is Too High or Low at the Roots
Whether you’re growing in soil, coco coir or in hydroponics, probably the most common reason to see yellowing and other nutrient deficiencies is the pH near the roots is too high or too low. Cannabis plants have a difficult time absorbing nutrients when the pH is off, resulting in nutrient deficiencies even if the nutrients are actually present near the roots.
- Yellow or other oddly colored leaves
- Spots, stripes or patches
- Burning around the edges of leaves
- In fact, basically any nutrient deficiency can be triggered by incorrect pH!
Growers who don’t make sure their pH stays in the right range will often run into nutrient deficiencies, even if they’re starting with a pure source of water and good soil!
- Soil Optimum: 6-7 pH
- Coco / Hydro Optimum: 5.5-6.5 pH
How to Fix
- Use a kit or PH Pen to test the pH of water before you give it to your plants, and adjust if necessary by adding an acid or base to your water
- Learn How to Fix Incorrect pH
These symptoms look like nutrient deficiencies but are actually caused by incorrect pH!