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.
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.
Diagnosing Yellow Leaves on cannabis plants
There are many cause for leaves on a plant to turn yellow. The first step is to narrow down the possible causes by investigating the plant. The first thing we should take note of is the location of the yellow leaves on the plant. Are the yellow leaves at the bottom of the plant or is it the new growth that is being affected?
Lower Cannabis Leaves are Turning Yellow
When the lower fan leaves are turning yellow, it could be a good indicator your plant isn’t getting enough nitrogen. The reason we know this is because nitrogen is considered a “mobile” nutrient. This means that when the plant is deficient, nitrogen in the lower leaves will mobilize to help new growth. This type of yellowing will start at the leaf tips and slowly move down to the stem.
Cannabis roots can only absorb nitrogen when the conditions are right. Incorrect soil pH and soil that is too dry or wet can slow or completely stop the plants’ ability to metabolize nutrients. If there are only a few of these yellow leaves near the base of your plant then the yellowing might also be a natural process of the plant shedding leaves it no longer needs.
New Growth Leaves Turning Yellow
When the uppermost sets of leaves are beginning to yellow the first thing is to check for light burn. Tips of leaves will turn yellow when they are getting to much heat from a light. Place your hand just above the upper most leaves, if it gets uncomfortably warm or hot then your lights are too close.
If you can rule out light burn then it might be a lack of nutrients like iron. Unlike nitrogen, iron is an immobile nutrient. Once iron is absorbed by the plant, it stays where it is. New growth will lack iron and turn yellow as a result. Other times (usually near the beginning of flower) it could be a calcium / magnesium deficiency and adding some additional cal/mag will quickly clear up the yellow.
Cannabis Seedling Leaves are Turning Yellow
If the leaves on your seedling or cannabis sprout are turning yellow it’s usually not a problem with nutrition. Seeds are packed with energy that will let them grow unassisted by nutrients for the first few weeks. The most common reasons your seedling leaves are turning yellow are bad watering habits or not enough light.
By giving your cannabis sprouts too much water the roots begin to “drown”. Cannabis roots need oxygen to facilitate growth, this is why well aerated soil is very important. When there’s not enough water, the roots will dry out and the seedling wilts and turn yellow.
Is your sprout growing tall and skinny and now the leaves are turning yellow? That is usually a sign of not enough light. You may need to upgrade your light source or provide more light for your baby plants.
When Are Yellow Leaves Natural
The cotyledons are the first set of leaves on a seedling. They are flat, broad, rounded, and don’t look like typical cannabis leaves. In most cases, these leaves will turn yellow and fall of a few weeks later.
Leaves that aren’t getting enough light might begin to yellow too. If you can’t find a way to manipulate the light or the plant to penetrate to these leaves simply remove them.
Isolated yellow leaves can show up during the growing process. These are usually caused by the grower accidentally snapping a leaf the week earlier.
During the last few weeks of flower, some leaves will naturally turn yellow as the plants reach their final stage of life. Like leaves changing in the fall, the plant is now focused on reproduction and putting nutrient resources into seeds or flowers. As long as you have an unpollinated female cannabis plant you will only get flowers.