Brown spots on cannabis leaves are almost always a symptom of underlying problems like nutrient imbalance, pests or environmental stress. Knowing how to identify the root cause and then what to do about it is key to a healthy grow. This article looks at many of the most common causes—calcium, potassium, and nitrogen deficiencies. It also covers pH imbalances, heat/light burn, Leaf Septoria, root issues, and pests—and provides practical solutions to keep your plants healthy and thriving. Contents
As a cannabis grower, noticing brown spots on your plant’s leaves can be alarming. However, brown spots are actually a common issue that most growers will run into at some point. The good news is that identifying the cause of brown spots is usually pretty straightforward. Once you know why they’re happening, it becomes easy to fix the problem and get your plants looking happy and healthy again.
In this comprehensive guide, we’ll explore the most common causes of brown spots on cannabis leaves and give you proven solutions to treat them.
What Causes Brown Spots on Cannabis Leaves?
There are several potential culprits when it comes to brown spots on pot leaves. Here are the most frequent causes:
Nutrient Deficiencies
Nutrient deficiencies are one of the top reasons your cannabis leaves can develop brown spots The three deficiencies most likely to cause spotting are
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Calcium deficiency – Shows up first on upper leaves as random copper, yellow or brown spots. Lower leaves may twist or curl. Fix by adjusting pH to 6.2-6.5 and use Cal-Mag supplements.
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Potassium deficiency – Starts as rusty brown spotting along leaf edges and tips. Fix by using fertilizers with more potassium.
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Magnesium deficiency – Causes leaves to turn yellow between the veins while veins stay green Fix with Epsom salt supplements
Improper pH
If the pH at the roots is too low (under 6), it can prevent proper nutrient uptake and lead to deficiencies Test runoff pH and flush plants if needed to stabilize pH between 6-7.
Overwatering
Excess moisture causes leaves to droop and can create brown spots when combined with cold temps. Allow soil to dry out between waterings and improve drainage.
Light Burn
Too much intense light causes brown/yellow spots near top of plant. Raise lights or use less powerful bulbs to prevent.
Heat Stress
High temperatures cause curling, crispy leaves with brown scorch marks. Improve air circulation and ventilation to cool grow area.
Root Problems
Root rot and other issues can create brown leaves. Use sterile pots/tools and avoid overwatering to prevent root problems.
Pests & Bugs
Bugs like thrips, fungus gnats, mites and more can cause leaf discoloration and spotting when populations get out of control. Inspect for bugs and use natural treatments if found.
How to Fix and Prevent Brown Spots on Leaves
Once you’ve diagnosed the cause of brown spots, it’s time to start treatment! Here are some tips:
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At the first sign of spots, thoroughly inspect plants for pests/bugs. Address any infestations right away before they spread.
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Check pH at the roots. Flush if needed to get back to a normal range.
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Let soil dry out a bit more between waterings. Improve drainage if soil is staying too wet.
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Move grow lights further away and/or use less intense bulbs. Observe if spots worsen or improve.
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Increase air circulation with fans/vents. Maintain ideal temp range in grow area (70-80°F).
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Switch to nutrient formulas designed for cannabis plants. Start at 25% strength and increase slowly.
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Clean pots, tools, and grow area regularly to prevent root problems. Let soil dry adequately before re-watering.
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Remove heavily affected leaves, being careful not to spread pests/spores. Treat with natural fungicides/insecticides if needed.
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Keep a grow log to record when symptoms appear and changes made. This helps identify patterns and dial in your system.
With vigilant monitoring and quick troubleshooting, brown spots can be stopped in their tracks before they get out of control. Consistently maintaining proper pH, nutrients, watering schedule, and environment will go a long way in preventing issues.
When to Worry About Brown Spots
In general, a few brown spots on older fan leaves is not a major concern. But if you notice any of the following, it’s time to take action:
- Spots are rapidly spreading from old to new growth
- Multiple leaves are heavily affected or dying off
- Leaf symptoms combined with drooping or stunted plants
- Issues keep recurring despite your attempted fixes
- Bugs or fungal growth are visible on and around affected areas
Heavily spotted leaves won’t recover. Remove them carefully to prevent further spread. Focus on getting ahead of the problem and restoring conditions that promote vigorous, healthy growth.
Severe or recurring issues may require you to start over with fresh soil/nutrients, sanitize the grow area, treat for root problems, or investigate environmental factors like ventilation or light leaks. Don’t hesitate to solicit help from fellow growers or experts if you need a second opinion diagnosing stubborn brown spots.
Catching and fixing brown spots early provides the best chance of protecting your cannabis crop. Monitor plants closely, document when symptoms first arise, and be prepared to investigate the most common causes. With a few simple adjustments, you can get those beautiful green leaves back in no time. Just stay calm, listen to your plants, and use the troubleshooting tips in this guide to get your grow back on track!
Possible causes for brown spots on cannabis leaves
Brown spots on cannabis leaves can worry even the most experienced grower. These spots indicate underlying issues that need addressing to ensure your plant’s health. Common causes include nutrient deficiencies, pests, disease, and environmental stress. Identifying the specific cause is key to finding the right solution.
Calcium deficiency is probably the most common cause of brown spots on cannabis leaves. Calcium is essential for cell wall structure and growth. When cannabis plants lack calcium, new growth shows brown, crispy spots, usually accompanied by discolored and curled leaf edges. This deficiency usually stems from the incorrect soil pH blocking the uptake of the available calcium in the growing medium or inadequate nutrient supply.
The first step to sort out a calcium deficiency is to adjust your soils pH to 6.2-7.0 (6-6.5 for hydro setups), as this range optimizes calcium absorption in cannabis. Then use a calcium-magnesium supplement or add dolomite lime to your soil to ensure that there is plenty available for your plant to use. Once you have the correct pH range in your grow medium, regularly monitor your plants to maintain balanced nutrient levels and prevent future problems.
Potassium deficiency is another common reason for why cannabis leaves start to show brown patches. Potassium is vital for various plant functions, including water uptake and photosynthesis. When a cannabis plant doesn’t get enough potassium, you’ll notice brown or yellowing of leaves or spots appearing on the older leaves. In addition, leaf edges turn brown and crispy, and the stems themselves can become weak and scrawny.
To fix a potassium deficiency in your plants, use a high-quality potassium-rich fertilizer. Most nutrient companies will list the ratio of nutrients as N-P-K, the letter K representing Potassium (confusing, isn’t it!). The higher the potassium ratio the better. Ensure your soil or growing medium has adequate drainage, as waterlogged conditions can hinder potassium uptake. Check and adjust your soil’s pH to stay within the range of 6.0-7.0, as this is the optimum range for nutrient uptake, particularly Potassium.
Nitrogen is the most important element in chlorophyll production, the green coloring in leaves that allows photosynthesis, and overall plant growth. When cannabis plants lack nitrogen, older leaves turn yellow, particularly the ones nearer the base of the plant. If the deficiency becomes serious the yellowed leaves will develop brown spots, eventually becoming dry and brittle the falling off the plant. Nitrogen deficiency will also lead to stunted plants and significantly worse yield.
Resolving a nitrogen deficiency is quite simple. Firstly, check the pH of your soil and make sure it is in the 6.0-7.0 range. Then increase the availability of Nitrogen in your soil by adding a nitrogen rich fertilizer or feed. Regularly check your plants’ pH and maintain a consistent feeding schedule to prevent future nitrogen related problems occurring.
A low pH level (below 6) in your soil or growing medium can cause brown spots on cannabis leaves. When the pH is too low, some nutrient uptake becomes difficult or impossible for the plant. This will lead to poor growth and damage to existing leaves and stems. Symptoms of low pH include brown spots, yellowing leaves, and stunted overall growth.
If your pH is too low, test your soil or runoff water pH using a reliable pH meter. If the pH is below the optimal range of 6.0-7.0, add a pH up solution or dolomite lime to raise the pH level. Regularly monitoring the pH and adjusting it to stay within this range is critical to ensure your plants can access the nutrients they need. With the pH sorted, maintain a balanced nutrient regimen to support healthy growth.
Light burn is a condition where cannabis leaves develop brown spots due to excessive high-intensity light exposure. When plants are too close to powerful grow lights, the intense light can damage the leaves. This can also happen if indoor grow spaces use poorly fitted reflective surfaces, such as aluminium foil or Mylar that is wrinkled. This can focus light and heat energy into spots on the plant, burning them. Light burn is often accompanied by bleached or yellowing leaves at the top of the plant.
To fix light burn related issues, adjust the distance between your grow lights and the cannabis canopy, ensuring the lights are at a safe distance. Make sure you follow the manufacturers recommendations for light placement, they will know what they are doing! If necessary, use a light meter to measure the intensity and adjust accordingly. Ensure that any reflective surfaces are smooth giving an even reflection of your grow lights. Gradually acclimatize your plants to stronger light levels, and regularly inspect them to prevent future light burn.
Leaf Septoria is a fungal disease that causes brown or yellow spots on cannabis leaves. This disease thrives in warm, humid conditions and can quickly spread, causing the leaves to drop off. Spotting the signs of Septoria early and dealing with it fully are key to avoiding extensive damage to your crop.
To fix Leaf Septoria, remove and dispose of affected leaves immediately to prevent the spread of the fungus. Improve air circulation around your plants by pruning excess foliage and spacing plants adequately. Use a fungicide specifically designed for cannabis to treat the remaining leaves. Maintain a clean growing environment collecting any fallen material or spills, reduce humidity levels, and avoid overhead watering, focusing on the soil surface instead to prevent future outbreaks.
Root issues such as root rot
Root issues, particularly root rot, can cause brown spots on cannabis leaves. Root rot occurs when roots are waterlogged and unable to get enough oxygen. This leads to fungal growth and decay in the roots and the leaves. Symptoms include brown spots, yellowing leaves, and an overall wilted appearance.
If you have root rot, first ensure proper drainage in your growing medium and avoid overwatering your plants as this is very common for many growers. Next remove the affected plant from its pot and trim away any diseased roots. Repot the plant in fresh, well-draining soil or medium, and consider adding perlite for drainage and beneficial microbes or a root treatment solution to promote healthy root growth. Adjust your watering schedule to allow the soil to dry out slightly between waterings and prevent future root issues.
Identifying and Treating Cannabis Diseases #4: Leaf Septoria
FAQ
How do you treat brown spots on plant leaves?
If you see brown or yellow spots that are sunken and growing in size over time, this could be something called anthracnose, a type of fungus. It’s not usually harmful, just unsightly. Solution: Isolate your plant and cut off affected leaves. You’ll probably need to use a fungicide to eliminate the problem.
Do brown spots on leaves mean overwatering?
Browning leaves are typically caused by under watering, sunburn, or overwatering. If the leaf tips are turning brown and crunchy, the soil likely became too dry for too long in between waterings. This can also cause the plant to drop leaves.