Why Do Cannabis Plant Leaves Turn Yellow?

August 2, 2019

When most people picture cannabis plants, they imagine the leaves a nice shade of green. Sometimes, however, the leaves of the cannabis plant may turn yellow. This typically indicates some sort of issue. Given the time and money you have invested in growing your plant, it pays to be aware of the most common reasons your cannabis plant leaves have turned yellow and what to do about it.

When Yellow Leaves Are Normal and No Cause for Concern

Before exploring the potentially harmful causes of yellowing leaves on cannabis plants, keep in mind that sometimes yellow leaves are normal and should not worry you.

The very first sets of leaves that your plant grows will likely turn yellow and then die when more start to grow. It is natural for cannabis plants to lose their baby leaves.

It is also common to have a handful of yellow leaves at the point that your cannabis plant is ready for harvesting. As long as you know the yellowing is not from any of the other causes on this list, it is normal.

There may also be natural variations or mutations that involve some yellow on cannabis leaves. This is not usually a concern.

Incorrect pH

One of the most common reasons that the leaves on the cannabis plant turn yellow is due to incorrect pH levels, specifically at the roots. This can be the case regardless of your growing media, including hydroponics, coco coir, and soil. If the pH is not within the ideal range, your cannabis plant will find it too hard to absorb nutrients, leading to a nutritional deficiency. This means that the wrong pH can lead to a lack of nutrient absorption even if the nutrients are right there.

If you are growing in soil, aim for a pH between 6 and 7. If you are growing in coco coir or using hydroponics, aim for 5.5 to 6.5. In addition to yellow leaves, the wrong pH can lead to burning around the leaves’ edges, patches, stripes, and spots, or just general nutrient deficiencies. If you notice those symptoms, check the pH. From there, use one of the many products on the market that helps you adjust the pH of your plant and media.

Incorrect Temperature

Yellowing on the cannabis plant leaves can also occur when your plants are not in the ideal temperature range. You may spot burnt or yellow leaves by the light or just general yellowing by the upper leaves. Some leaves will also likely start curling up around the edges.

Avoid this cause of yellowing cannabis leaves by making sure the area for the plants is not too warm or too cold. Do not let it drop below 50 degrees Fahrenheit (10 degrees Celsius) and make sure you can comfortably keep your hand in the area with the plant without it getting too warm. Ideally, you do not want the temperature to go over 85 degrees Fahrenheit during the day or 60 degrees Fahrenheit at night. You can also reduce the risk of hot spots with good air circulation.

Over or Under-Watering

If your cannabis plant gets too much or too little water, this can also lead to yellowing leaves. You should also look out for deficiencies and droopiness. If the leaves are fat, you likely overwatered and they are swollen. If they seem thin and papery, this indicates under-watering.

Sometimes, the overwatering will be because you physically gave the cannabis plant too much or too little water. To minimize that risk, place the plant in a container of the appropriate size. You should also ensure the plant has proper drainage so the water does not accumulate. You can tell you overwater your plants if they droop after the watering. You can tell you under-water them if they perk up when watering.

Burns From Light

In the case where only some of the leaves on your cannabis plant turned yellow, this may be from a light burn. You can recognize this cause by the fact that the leaves are still hard to remove, even when the entire leaf is dead. Additionally, the light burn will disproportionately affect the leaves that are right by your grow light.

A light burn may cause cannabis plant leaves to yellow if the grow light is set up too close. It can also happen if you change the bulbs. To resolve the issue, move the grow lights or the plants so they are further apart.

Lack of Light

The opposite of the light burn can also cause cannabis leaves turning yellow, specifically in the case of seedlings. When the shell cracks, the leaves will be round and yellow. The green color arrives when the plant gets enough light.

Give your seedlings more light to correct the issue if the leaves remain yellow or do not get darker than pale green. You should also notice that the seedlings seem tall but still have small leaves and there is extra space between each leaf. Resolving this cause of yellow leaves in cannabis plants is as simple as giving the seedlings enough light.

A Deficiency of Nitrogen

Another potential reason for cannabis plant leaves to turn yellow is if the plant does not have enough nitrogen. In this case, you will notice the color is somewhere between lime-green and pale, with yellow leaves by the bottom of your cannabis plant. The yellow leaves, in this case, will be soft and may fall off easily.

Your cannabis plants may experience nitrogen deficiency if you have kept them in the same container for a few weeks or months. In the case of hydroponics and coco coir, the deficiency will occur if you do not actively add nitrogen since those medias do not have any naturally.

To overcome nitrogen deficiency, you can just give your cannabis plant a nutrient that is cannabis-friendly and adds nitrogen. In the case of soil-grown plants, consider transplanting it into a container with new soil.

A Deficiency of Iron

In the case of iron deficiencies, your cannabis plant will only experience yellowing of the newest leaves. These new leaves will grow in fully yellow. Interestingly enough, that yellow usually turns green, starting on the edges of the leaf and working toward the center.

Iron deficiencies are typically due to incorrect pH. Cannabis plants have minimal iron requirements and most water should be able to meet that requirement. This means you should start by checking the pH and correcting it if necessary. If that does not work, give the plant a nutrient containing iron.

A Deficiency of Magnesium

Yet another deficiency that can lead to the yellowing of the cannabis leaves is magnesium. In this case, the entire leaf will not turn yellow. Instead, just the area between the leaves’ veins will turn yellow. This will be more common on the leaves closer to the bottom of the plant.

In nearly every case, incorrect pH leads to magnesium deficiency. It may also be from using soft or heavily purified water. If you notice this type of yellowing, check the pH and correct it if necessary. If the plant still shows magnesium deficiency symptoms, use a Cal-Mag supplement.

Keep in mind that you do not always need to correct this type of yellowing. You could mistake a magnesium deficiency for plants simply getting older. In that case, it will affect just a few limp leaves by the bottom of the plant.

Nutrient Burn

With so many nutrient deficiencies causing cannabis plant leaves to yellow, you may feel tempted to overdo it with the nutrients. Avoid this temptation because giving your plants too many nutrients can also cause yellowing of the leaves or other issues. Be careful, as the long-term nutrient burn cannot always be fixed. If you notice the problem early, however, you can just flush the plants using pure water to minimize the issue.

Bud Rot

In the case of yellow on leaves that only affects a few buds and seems to occur overnight, bud rot might be the culprit. This situation leads to yellowing on just select leaves on the largest buds. The yellow leaves typically fall out easily and you may spot mold grown inside the cola by the leaf’s base.

Bud rot can happen due to cool temperatures, a lack of circulation, or higher levels of humidity. Avoid it by keeping humidity below 50 percent RH during the flowering stage and the temperature above 60 to 70 Fahrenheit at night. You should also ensure your plants get enough air circulation. If the plants get too bushy, defoliate them a bit as well to provide more air to all leaves and buds.

The Presence of Pests

There is a range of pests that can cause the leaves of your cannabis plant to yellow from stress. If this is the case, you will also notice an overall lack of vigor in the plant, bite marks, spots, and may actually see the bugs.

Prevent pests from turning your cannabis leaves yellow and damaging the plants by never adopting clones or visiting another garden unless you fully trust the grower. If you give your plants fresh air, make sure to keep out bugs with a screen.

Don’t Remove All the Yellow Leaves

Since there are various solutions to get yellow cannabis leaves healthy again, there is no reason to prune yellow leaves when you spot them. This is a particularly bad idea for younger plants. Instead, check for the above causes depending on the appearance of the leaves.

The Takeaway

If the leaves of your cannabis plants start to turn yellow, do not take drastic measures. You should be able to resolve the issue easily; you just have to figure out which one of several possible causes is to blame.

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