One of the first signs that there is a problem with your cannabis plant will be a yellowing of the leaves. This is known as chlorosis.
Chlorosis is caused by many things, but is essentially a symptom, that the plant is not able to produce the chlorophyll, which it needs to make food out of light. As the level of chlorophyll falls, so will the healthy shade of green.
In this guide, we will discuss the causes of different types of chlorosis, how to fix them, and how to prevent the issues arising again.
If your plants are suffering with chlorosis and you need advice fast, feel free to ask for help in our cannabis growers forum
- What is Chlorosis
- Causes of Chlorosis in Cannabis Plants
- How to Fix Yellowing Leaves on a Cannabis Plant
- How to Fix Interveinal Chlorosis
- Over Watering Cannabis Plants
What is Chlorosis
The first signs of most cannabis deficiencies, will begin with yellowing leaves. You see, your plants need chlorophyll, to turn light into sugars via photosynthesis. Since chlorophyll is a green pigment, it gives your plants the healthy green glow.
Chlorosis is the term used for when your leaves, start losing their healthy shade of green, because the level of chlorophyll is falling. If your plant does not have a good amount of chlorophyll inside it, is cannot make the food it needs to grow healthily.
How Chlorophyll works
Chlorophyll is known as a chelate. A chelate is a large molecule, composed of elements like carbon, hydrogen, oxygen and nitrogen.
A chelate also has a central metal ion, in chlorophyll it is magnesium. It also contains four nitrogen atoms bonded to the magnesium ion, in a square type structure.
Chlorophyll is one of the most important chelates in nature. It changes light, into chemical energy in a process called photosynthesis
This process converts carbon dioxide and water, into carbohydrates and oxygen. These carbohydrates can now be used by plants, and the oxygen is released as a waste product.
Why Cannabis Leaves Turn Yellow
Because chlorophyll is made up of many different elements, an imbalance in many nutrients can cause chlorosis.
If your plant does not have the necessary ingredients to make chlorophyll, then eventually, your leaves will start losing colour. The more chlorophyll the plant loses, the less food it can make.
Yellowing leaves on a cannabis plant are the first cry for help that there is a problem on making chlorophyll, and levels are starting to fall below optimal. To be able to identify the cause of the problem correctly, there are a few things to consider.
Causes of Chlorosis in Cannabis Plants
Chlorophyll is made up of a number of elements, but the main 2 are magnesium and nitrogen. When you first see chlorosis on a cannabis plant, it is probably caused by a nutrient deficiency, probably magnesium, or nitrogen.
If your plant is lacking in a major nutrient, it will not be able to make the chlorophyll it needs to photosynthesise. As a plant progresses through its life cycle, the need for different elements, and different amounts will change.
The first and most common cause of chlorosis in a cannabis plant is a nitrogen deficiency:
Because 4 parts of the chlorophyll ion is nitrogen, a deficiency in it will cause your plant to lose its healthy shade of green quickly.
As a plant grows bigger during the vegetative stage, its need for nitrogen will increase, if you plant starts to turn yellow, then a slight increase in the nitrogen content of your feed can help fix the problem.
You can get help fixing a nitrogen deficiency right here:
Because magnesium is also a major part of chlorophyll, a magnesium deficiency will also cause chlorosis in a cannabis plant.
A magnesium deficiency can easily be fixed by using some epsom salts as a foliar feed, ( 1/3 of a teaspoon per litre) or use “cal mag” to increase the magnesium content in the medium.
For a guide on fixing any cannabis plant deficiency, we have a guide to help you pinpoint exactly what’s wrong, and the best method to fix it:
In some cases, chlorosis can be caused by a slight imbalance in pH, causing a lock out of nutrients like zinc, or iron, which are also key in credit to chlorophyll.
It doesn’t matter how much food is in your medium if your ph is off, your plant will not be able to use it.
Many nutrient deficiencies can be put down to an imbalance of pH, and just by flushing and restoring the pH of the medium, you can fix many cannabis plant problems.
Most soils should be treated, and have a good pH buffer, but you should still keep the pH of the soil around 6.5, and for mediums like coco, rock wool, or hydroponics, 5.8 is the optimal level:
Lack of Light
Sometimes, as a cannabis plant grows bigger, the leaves on the lower portion of the plant may struggle to get enough light, especially when growing indoors.
Your plant will sense that these leaves are costing more energy to keep alive, then they are producing. This will cause the plant to absorb what nutrients it can from the leaf, and then let it fall off.
This is a natural process for the plant, and you shouldn’t interrupt it. It may not look pretty, but there is a reason the leaves are doing what they are doing, let the plant do its work.
To prevent further leaves being lost, you can either increase the lighting below the canopy, or, do some trimming and training to your plants to allow more light to get to the lower leaves.
How to Fix Yellowing Leaves on a Cannabis Plant
Chlorosis in a cannabis plant is most likely caused by 2 things, a nutrient deficiency, or a pH imbalance.
Just like any nutrient deficiency, you need to check:
The pH of the medium
The EC of your reservoir or run off
Check your pH
The pH of your medium is key to a healthy plant diet. If the pH is not in the correct range, the nutrients your plant needs will not be available. This is a likely cause for your plants problems. Check this first, and if it is out of range, restore the levels by giving the plants a flush and refeed.
Monitor Your EC
When growing cannabis with salt based nutrients, it is easy to tell if your plant is being fed too much or not enough.
Monitor the EC of your feed, before you feed it to your plants. Then monitor it again, by collecting run off ( water coming from the bottom of the pot after feeding) or by checking the EC of your reservoir.
If the EC is higher then the feed going in, you’re over feeding and need to reduce the EC on the next feed. If the EC comes out lower, you’re under feeding your plant, and this is the cause of your deficiency.
If your EC is falling, you need to increase to amount of feed your plant is getting especially of nitrogen. This can be done by increasing the amount of “grow” nuits you give to your plant.
For more help on EC and how it works, see our guide here. It is the best way to monitor the diet of your plants, and give them what they need, when they need it.
Interveinal Chlorosis on Cannabis Leaves
In some cases, chlorosis will not affect the whole leaf, but more spread out from the leaf veins.
Interveinal chlorosis is a symptom of a micronutrient problem, and your plants are lacking in zinc, iron, sulphur or magnesium.
A micronutrient issue is likely to be caused by a pH imbalance rather than a lack of it in the medium (if feeding correctly). Unless your plant is being massively under fed, the most likely cause of interveinal chlorosis is a calcium and magnesium deficiency.
Fix Interveinal Chlorosis
But if you adjust your pH so it is at the correct level, and by slightly increasing the EC of your feed, you can quickly overcome most nutrient deficiencies that cause interveinal chlorosis.
For a quick fix, you can treat your plants with a foliar feed of epsom salts. This will deliver a good dose of magnesium directly to the plants cells.
Only foliar feed a plant in veg though! If you’re heavy into flower, then you can use “cal mag” in your feed to increase magnesium content in the medium.
Over Watering a Cannabis Plant
If your roots can not get a good supply of air, the uptake of nutrients will be affected. Over watering a cannabis plant can also cause chlorosis.
If you over water your plants, not only will the root zone be restricted of air, but the plant itself will have a high water content, which will interfere with how the plant cells function.
This can have an effect on the production of chlorophyll, and your plants can soon show signs of chlorosis.
Ensure you medium is drying out in between waterings. The pot should feel light, and the top 2 inches of your soil should feel dry if you put your finger into it.
If there is too much water, air can not flow into the medium easily, and this can cause bad bacteria to start growing. The bacteria will eat at your roots, and alter the pH causing more problems.
Make sure you are watering your plant correctly, and always remove run off after watering them!
Hydroponic growers growing in full water systems must ensure their root zone is getting a good supply of air, this can be done by using air stones in the reservoir.
If your plants are showing signs of being over watered in hydro, then there may be a problem with your airstones and you should check for blockages or consider adding another pump to the reservoir.
It is hard to over water in coco, as it has very good drainage. If your plant is showing symptoms of over watering in coco, it is likely down to the runoff not being removed after feeding.
It is crucial when growing in coco that you get at least 10% run off with every feed, and run off must be removed after draining.
Thanks for reading! You can find more information on anything related to growing in our cannabis grow guides. All of our guides are written by experienced growers, and members of our cannabis growers forum. If you have any questions, sign up. It is free, and we are always happy to help!
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