Fraser Horton
Fraser Horton
Last Updated on March 28, 2021

Cannabis cultivation isn’t hard science, especially if you’ve had some experience with growing a plant. Every grower’s goal is to cultivate strong and healthy crops with high THC and CBD content that will be a delight to consume.

However, no matter how uncomplicated it is, growing cannabis also leaves a lot of room for error, as getting those buds right requires some tinkering. One of the most common issues that marijuana cultivators face is the yellowing of the leaves.

Seeing yellow leaves on their cannabis plants makes many cannabis growers worried that their harvest has been ruined. 

Common Reasons Why Your Cannabis Leaves May Be Yellowing

Yellowing, or chlorosis, is a common occurrence in growing marijuana, usually as the result of the loss of chlorophyll. It can happen to any grower, especially to a novice. There are many reasons why marijuana leaves can turn yellow, so addressing the issue right away will be the best plan of action for your plants’ health.

The yellowing is reversible in most cases, so let’s look at the most common causes, and what you can do to help your plant’s leaves regain their vibrancy.

Inadequate Watering

One of the most obvious reasons has to do with proper watering. Overwatering is more common than underwatering, though they both cause similar symptoms – droopiness and yellowing.

Overwatering most commonly happens when inexperienced cultivators are being too attentive in their watering practices for fear that their plants will dry out. When you overwater, the plants get deprived of oxygen, which hurts the root system. 

The roots will starve, and over time they won’t be able to uptake the needed nutrients from the soil which will cause deficiencies. You’ll know you’ve overwatered your plants when the leaves seem droopy and “swollen” from the extra moisture.

Underwatering doesn’t happen as often as most growers know that hydration is essential for photosynthesis, but sometimes your plants can get dehydrated. If you notice the leaf tips sloping downwards, and the leaves seem thin and papery from the lack of hydration, then you need to water them.

Help your plant:

The solution is to provide your marijuana plants with the right amount of water. The way to do this is to water your plants and wait for the soil to soak up the water before adding any more. You can check the moisture with your fingers or a skewer, and add more water if needed.

Light Deficiency

Adjusting the correct amount of lighting can be challenging for new growers, and even for experienced ones when they’re cultivating a new strain, as each is a little bit different. From choosing the grow lights to choosing the correct distance – it can get confusing.

You can recognize light deficiency if the leaves seem limp and droopy. The leaves of younger plants will be yellowed, while the leaves of mature plants will be darkened.

Help your plant:

If your lights are too weak, it’s best to invest in stronger lights, like high-intensity discharge (HID), metal halide (MH), or high-pressure sodium (HPS) lights. Be careful of placing them too close, though, as that’s not good too.

Light Burn

Light is an essential element for the growth of your plants, so making sure your crops get enough light is beyond important. Indoor marijuana plants are grown under artificial grow lights which need to be carefully picked and skillfully positioned so they can help the plants grow instead of harm them. 

However, grow lights also emit heat, and if you position them too close to the buds and leaves, they can easily get burnt and turn a sad brownish yellow color. Light burn means that your leaves have been stimulated for too long by light, causing them to wither away early.

Help your plant:

Place your hand between the light and the plants – if it’s uncomfortably hot, you should definitely move the grow lights away. Depending on your chosen type of light bulbs, 8-20 inches of distance should be enough.

Heat Stress

Even though cannabis plants like warm temperatures, they’re not immune to overheating, especially when grown indoors. You will notice general yellowing of the leaves, especially at the top, and they may be turned up around the edges. Therefore, controlling the temperature in your grow room and around your plants is especially important to prevent heat stress. 

Help your plant:

First, move the grow lights away from the plants as they also produce heat, and second, make sure there is proper air circulation in the grow room to remove the heated air from around the plants and allow for fresh air to enter. Do this often if your grow room tends to get stuffy.

Inadequate pH Levels

The acidity of the growing medium you use is crucial for nutrient uptake. If the acidity is too high or too low, it will result in a weaker crop and yellowing leaves as a result of nutrient deficiency (more on that below). Whether you use hydroponics or soil, pH imbalances will make your plants’ leaves spotty and patchy with burnt-like edges.

Help your plant:

Using a pH meter to keep track of the acidity is a must. For soil, the pH should be between 6 and 7, while hydroponics requires a slightly more acidic environment – between 5.5 and 6.2. Purchasing nutrient solutions pH-Up and pH-Down from any gardening store is the easiest way to fix imbalances.

Pest Infestations

Pest infestations are easy to spot because you’re most likely to see the pests directly on the leaves. If you notice yellowing along with spots or bite marks, you can bet your plant has had some visitors. However, fungus gnats are the most common reason for yellow leaves as they develop as a result of overwatering and they feed on the roots.

Help your plant:

As a first step, it’s best to restrict watering (until it’s absolutely necessary) so you can prevent the gnats from laying eggs in the soil. And finally, use natural insecticides, like diatomaceous earth or neem oil to get rid of them.

Nutrient Deficiencies

Nutrient deficiencies can be disconcerting for many growers, but luckily, it’s something that you can easily fix once you identify which nutrients your plants are lacking. The best solution would be to add the lacking nutrient slowly and see how the plant reacts (to avoid nutrient burn).

Nitrogen

Nitrogen is one of the most important nutrients for your plant because it has a crucial role in the production of chlorophyll. Nitrogen deficiency, then, will cause a very obvious yellowing of the leaves, especially the mature leaves. Your plants may also flower earlier, but there will be fewer buds.

Potassium

Potassium is also very important as it’s involved in photosynthesis. Potassium deficiency will cause dryness and yellow leaves with brown spots and curled orange-brown tips.

Calcium

Calcium holds the cell walls together, so it’s definitely essential for the plants. Calcium deficiency results in impaired or stunted growth of new parts of the plant such as new leaves or root tips. Lower leaves will show signs of curling and will develop yellow spots.

Magnesium

Magnesium also plays a role in the production of chlorophyll. You will recognize magnesium deficiency by the yellow veins that will appear on the fan leaves. There could also be signs of curling and the leaves could eventually die.

Sulfur

Your plants don’t need large quantities of sulfur, but sulfur deficiencies can happen due to high pH levels. Besides yellowing, the leaves will be delicate and exhibit dryness.

Zinc

Zinc has numerous roles in the cannabis plant, from the production of chlorophyll to carbohydrate and protein synthesis. Zinc deficiency is easy to tell apart as the leaves turn 90 degrees sideways. The veins will become yellow and the leaves themselves will be wrinkled.

Iron

Iron participates in energy production and plays an indirect role in the synthesis of chlorophyll. Iron deficiency is easy to spot because young leaves are the first ones that start turning yellow and eventually, the older leaves start catching up.

Manganese

Manganese helps with nitrogen uptake, photosynthesis, and respiration. Manganese deficiency turns older leaves yellow.

When Are Yellow Cannabis Leaves Normal?

Yellow leaves don’t always indicate there’s a problem with your crops – sometimes it’s a completely normal part of the maturing process. For example, after your plant has grown substantially, the older leaves will naturally turn yellow and be ready to fall out. 

Similarly, when the plant has flowered and is close to harvest, it’s natural for the lower leaves that were in the shade to turn a little yellow. Therefore, when your crops are looking vibrant but have a yellow leaf or two here and there, it’s probably nothing to worry about.

Disclaimer

The information presented on this page is provided as a public service to aid in education and is derived from sources believed to be reliable. Readers are responsible for making their own assessment of the topics discussed here. In no event shall Leaf Nation be held reliable for any injury, loss or damage that could happen if using or abusing drugs.