The technical term for yellowing leaves is chlorosis and is caused by a lack of chlorophyll in the leaves. There are quite a few reasons why this may happen. Sometimes it’s natural and other times it’s your plant trying to tell you it’s not happy.
Cannabis plants don’t speak English, but if you pay close attention you can figure out what they’re trying to tell you. Yellow leaves are one way they can tell you there’s a problem.
Let’s figure out what’s causing chlorosis in your plants.
Diagnosing Yellow Leaves
The location of the yellow leaves on the plant is a very good clue to what might be wrong.
When the lower fan leaves are turning yellow, it’s 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 the nitrogen in the lower leaves will mobilize to help new growth.
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 nitrogen.
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.
Improper Watering Habits
When you over-water your plants you prevent the roots from getting oxygen. This will prevent your plants from metabolizing nutrients and cause yellow leaves.
The leaves of an over watered plant will look droopy because they are full of water.
Under watering your plants will cause the leaves to wilt and turn yellow over time.
The solution is simple: adjust your watering habits. If you see over-watering symptoms, water your plants less frequently and vice-versa.
The rule of thumb is to water your cannabis plants once the top inch of soil is dry. Cannabis plants like soil that dries out a little between watering. Finding the right balance will depend on the size of the plant, the size of the planter, and temperature.
“Water your cannabis plants once the top inch of soil is dry.”
pH is Too High or Low
Another common issue is pH imbalance. If the pH of the water you feed your plants is off, they will have a hard time absorbing nutrients through their roots. pH is the measurement of acidity or alkalinity of a liquid, it’s determined by the concentration of hydrogen ions in the water.
If you want to get into the science of what pH is, here’s a great link.
In most cases, your tap water will have a pH around 7.0, but this will vary from location to location. Adding nutrients to your water can change the pH. Keep an eye on the pH of the water you use to feed your plants. You will get the best results from your cannabis grown in soil by maintaining a pH around 5.8 or 6. It makes sense when you realize the pH of rainwater is slightly acidic with a pH of around 6.0.
For hydroponics solutions you will want a slightly more acidic pH of around 5.5
By using a pH level measuring pen or kit you can monitor the acidity of what your feeding your plants. If your levels aren’t in the optimal range, add a base (pH up) or acid (pH down) to either raise or lower the pH.
Incorrect pH is the usual culprit for most cannabis related growing problems.
Are the tops of your plants turning yellow, crispy, and starting to curl? One reason could be light burn.
The leaves closet to the light source will be showing the most severe symptoms. The heat from your lights is frying these leaves damaging them.
Typically, the more powerful, high-intensity lights will result in light burn. LEDs usually give off the least amount of heat.
You will need to raise the lights a few inches to fix the problem. The optimal range is usually one or two feet away.
If your hand is uncomfortably hot where the light hits the leaves, then your leaves are also feeling the burn. The temperature at the canopy (uppermost leaves) shouldn’t go over 30°C (86°F).
Cold Shock or Heat Stress
Next on the list is temperature. Normally, heat is going to be the enemy here, either from the lights or poor air circulation. If you’re growing outside or in your basement, a quick cold spell in the night could harm your plants as well.
For heat stress, the leaves will turn yellow and dry. At the other end, cold shocks, the plant will droop and turn yellow. Either extreme could kill your plants very quickly.
Keep a thermometer (or two) near your plants to check the temp. The ideal range is between 18°C and 28°C. Proper air circulation will keep the temperature balanced throughout the room and help the exchange of CO2 and oxygen.
If the pH is spot on and the plant is being watered correctly, then the most likely problem is nutrients (or the lack thereof). For chlorosis, the usual culprits are iron, nitrogen, magnesium, phosphorus, potassium, sulfur, or zinc.
Take note of where the yellowing occurs on the plant and the leaves. These are your clues to finding out what nutrients are deficient.
Using a fertilizer designed for cannabis is a good first start. Finding one that contains all the essential macro and micro nutrients cannabis needs is the easiest way to avoid deficiencies.
Nutritional needs may vary from strain to strain, but having those base line nutrients will save you a lot of trouble.
This is a reason why many growers love organic living soil. Soil contains all sorts of nutrients that are broken down by roots and microbes within the soil itself. With nutrient rich living soil you may even need to use less fertilizer.
The plant in the photo is showing signs of an iron deficiency (and a few others as well). It was caused by a pH that was too high which locked out several nutrients.
Using too much fertilizer can also cause problems that would result in yellow leaves with burnt looking tips. Fixing over fertilized cannabis plants is simple. Flushing the soil with water will remove the build up of nutrients in the soil.
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.
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Identify and Correct Problems