Brown Spots on Cannabis Leaves

Your cannabis plant should be green and rich in chlorophyll for proper growth, immunity, and health. However, if you notice any brown spots, you need to take action.

Brown spots are one of the most common problems cannabis growers face, and they can cause significant damage to the plant. They deteriorate the plant's health by inhibiting photosynthesis, slowing growth, and reducing the overall yield.

Typically, brown spots are caused due to nutrient deficiency, but they can even be a sign of too much light, low pH, or pest infestation. 

In this article, we will show you how to identify what is causing brown spots and how to fix them quickly.

Brown Spots: Causes and Solutions

A healthy cannabis plant growing with appropriate pH levels, light, and environmental conditions, and nutrient solution is rich green in color. But many problems cause the plant to develop brown spots.

For example, nutrient deficiency can turn the leaves yellow and develop brown spots. Or, there may be an issue with the lighting. For instance, too much light or air can cause stress that inhibits photosynthesis. In addition, pests can also alter the leaves' color by damaging the inner tissues.

You must rectify brown spots immediately because they can cause the leaves to wilt and die and slow down bud development in extreme conditions. Below are some of the most common causes for brown spots with easy solutions.

1. Nutrient Deficiencies 

As mentioned earlier, nutrient deficiency is the most common cause of brown spots on cannabis plants. Therefore, if you notice your plant's leaves are turning yellow or developing brown spots, you should first check the nutrient solution.

Each nutrient you feed to your plant affects it in different ways — some help the plant grow a strong stem while others enhance photosynthesis. So, you need to pay attention to each nutrient compound instead of focusing on the nutrient solution as a whole.

Here are some of the most common nutrient deficiencies that can cause brown spots.

• Calcium Deficiency

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Think of calcium as the building block for your cannabis plant — it helps your plant become strong, develop stems, branches, roots, and walls, and resist environmental stress. Calcium is also essential for absorbing potassium.

However, it is a heavy compound that moves slowly through the plant. Thus, calcium deficiency generally occurs during the vegetative stage when the plant develops new leaves.

So, if your plant develops brown spots during its rapid vegetative stage and has other symptoms like weak stems that crack easily or underdeveloped buds, it is most probably due to calcium deficiency. 

Calcium deficiency may also occur if the pH is too low or if there's too much potassium in the growing medium, as it can cause calcium lockout.

• Nitrogen Deficiency

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Nitrogen is another vital macronutrient that helps the plant produce chlorophyll for photosynthesis. Plus, it helps the plant produce amino acids essential for protein production, which is why it is critical during the veggie stage.

Your plant may be experiencing nitrogen deficiency if you notice brown spots on the leaves along with leaves curling and dying off. Additionally, excess nitrogen can also be bad, leading to nitrogen toxicity. 

If your leaves have brown spots or the leaves are dark green — almost black — you need to fix nitrogen levels in the nutrient solution.

• Potassium Deficiency

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The third nutrient compound that can lead to brown spots on leaves is potassium or lack thereof. Potassium is essential for healthy root growth.

If it is not adequate for the plant, your plant may absorb other vital nutrients through the leaves, causing brown spots along with yellow leaves or dry leaf tips.

Potassium deficiency often occurs when there's too much calcium or nitrogen in the grow medium, as they can cause potassium lockout.

The solution to Nutrient Deficiencies

If your plant is experiencing brown spots due to nutrient deficiencies, you need to first check and balance the nutrient solution. Next, we recommend flushing the grow medium with pH-balanced water containing healthy doses of nitrogen, potassium, and calcium.

Some growers also use supplements to grow healthy plants, but sometimes, they can also cause an imbalance in the nutrient levels, leading to brown spots. In such cases, understand how these supplements work and discuss with other growers using similar supplements. 

During the vegetative stage, pay attention to the new leaves. If the leaves show any brown spots, adjust the nutrient solution accordingly. 

You can also use organic matter in the growing medium for better results. Organic products add microorganisms to the medium that break down the nutrients for easier absorption.

2. Low pH Levels

Another most common cause of brown spots on cannabis leaves is low pH levels, regardless of your growing medium. Low pH can affect plants growing in coco coir, hydroponics, and soil alike.

Essentially, when the pH level is low, under 5.5 — aka acidic — it can cause various nutrient lockouts and create an imbalance in the growing medium. 

So, if you spot brown spots on your leaves but the nutrient solution is appropriate for your plant, you need to check the pH of the nutrient solution and soil.

However, checking the pH level of the solution when feeding the plant is not enough as it is not an accurate measure of the pH level in the growing medium. You must check the run-off nutrient solution, too. 

If the solution run-off has lower pH than the one you feed your plant, the pH level at the roots is low. In this case, you must flush your roots and balance the solution according to the run-off pH reading. The ideal pH must be over 5.5 for soilless and over 6 if you're growing in soil growing mediums for healthy growth.

3. Light Burn

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Cannabis growers use massic light panels to increase yield, which is the right thing to do. But if you go overboard with your lighting panels, your plant may experience a light burn and develop brown spots on the leaves. 

Too much light can dry the leaves, leaving them devoid of water and inhibiting photosynthesis.  

Typical light burn symptoms include brown spots and yellow leaf tips and greener leaves at the lower tiers. 

If you notice such symptoms due to too much light exposure, you need to fix your lighting system. 

First, you must ensure your light is located at the right distance from the plant. If it's too close, the heat emitted from it — even from LED lights — can cause light burns. 

Second, you must ensure you are using the correct wattage of lights. An ideal wattage for the cannabis plant is 50 watts per square foot.

Third, remember that plants build resistance to light over time. So, keep the light far when the plant is small and bring it closer to the recommended distance as the plant grows. 

4. Bugs or Pest Infestations

Lastly, some pests can also cause symptoms akin to brown spots. Here are some of the most common pest infestations that can cause brown spots:

• Thrips and Spider Mites: These bugs cause pale or yellow spots on the leaves that darken over time
• Leafhoppers: These bugs cause tiny brown spots on some leaves
• Leaf Miners: These bugs cause long, squiggly lines on the leaves that darken over time
• Broad Mites: These are tiny bugs — invisible to the naked eye — that cause the leaves to blister, twist, or turn glossy, along with brown spots
• Fungus Gnats: These are dark, tiny flies that get attracted to wet soil, causing stress at the root level, which subsequently causes brown spots on the leaves due to nutrient lockout
• Bud Rot: If you notice random brown spots near the bud sites, it is probably due to bud rot

You must keep a close eye on your plant's leaves. If you notice any spots that gradually turn darker, it is a sign of pest or mold infestation. You must fix this immediately by improving the growing conditions, trimming the damaged leaves, and eliminating the pests.

5. Inefficiency Heat and Air Circulation

Inefficient air circulation and heat are other causes of brown spots on the leaves, as these conditions can cause heat stress. On the other hand, too much wind can cause the plant's leaves to dry out, causing brown spots. 

This usually happens when the fans are not working correctly or are too close and directly pointed towards the plant. 

If you notice brown spots on the leaves along with leaf curling, underdevelopment of buds, and symptoms akin to nutrient deficiency, you need to adjust the air circulation. Recheck the fans' position and direction and adjust accordingly to fix this issue.

6. Watering Problems

Brown spots can also occur due to overwatering or underwatering, or if the water is too pure, i.e., R/O purified water. Watering issues can cause problems for the plant, including underdevelopment, root rot, brown spots, etc. This problem mainly occurs in young plants. 

If you can't trace brown spots to any of the problems mentioned earlier, it could be due to watering problems. Here, ensure you don't drown the roots by watering too frequently or let the medium dry out.