If you’ve been growing cannabis and suddenly spot brown patches on your leaves, I know that sinking feeling you’re probably experiencing. Brown spots can show up for several reasons, from nutrient problems to environmental stress. But don’t stress! We’ll go through all the common culprits and how to fix them, so you can get your plants back on track.
Common Causes of Brown Spots on Cannabis Leaves
Brown spots on cannabis leaves are often the plant’s way of telling you something’s up. Let’s start by digging into the most common causes.
Nutrient deficiencies leading to brown spots
A lot of times, those brown spots are your plant saying it’s missing some key nutrients. Magnesium and calcium are big players here. Without these, your plants can develop brown, rusty-looking spots that start small but quickly take over the leaves.
Calcium and magnesium imbalances
When there’s a lack of calcium or magnesium, cannabis plants struggle. Calcium strengthens the plant’s cell walls, and magnesium is crucial for photosynthesis. A shortage of these minerals leads to brown, crispy spots on your leaves. I like to add a calcium-magnesium supplement (cal-mag) if I see this starting.
Excess nitrogen and phosphorus
Overdoing it with nutrients can also lead to brown spots. If you’re hitting your plants with too much nitrogen or phosphorus, the leaves can develop a burnt, brown appearance. Less is more here, so dial back the feed if you suspect overfeeding.
pH fluctuations and nutrient lockout
Another thing to watch for is pH levels. If the pH of your water or soil is off, your plants won’t absorb nutrients properly, even if you’re feeding them the right stuff. This is what’s known as “nutrient lockout.” I check my pH regularly to make sure it stays in the sweet spot for cannabis, usually around 6.0-7.0 for soil.
Environmental Factors Contributing to Brown Spots
Your environment has a huge effect on how your plants grow. Let’s look at some environmental causes of those annoying brown spots.
Heat stress and light burn
Cannabis plants love the sun, but too much light or heat can cause brown spots, especially if the leaves are getting scorched under strong grow lights. If this happens, you might need to back the lights off a little or improve the airflow.
Poor air circulation and humidity problems
If the air’s too stale or the humidity isn’t right, you can end up with brown spots. Cannabis plants need a constant flow of fresh air, so make sure you’ve got fans going. Keeping the humidity between 40-60% is usually a good call to avoid any moisture issues.
Overwatering or underwatering issues
Watering can be a tricky balance. Too much water can drown the roots, causing brown spots as the leaves start to yellow and die. Too little, and your plants get stressed, which can also show up as brown spots. I always water based on the weight of my pots—if they feel light, it’s time to water.
Pests and Diseases Causing Brown Spots on Cannabis
Sometimes the problem isn’t with your nutrients or environment—it’s bugs or disease. Let’s dive into the pests and diseases that could be messing with your plants.
Spider mites and thrips infestations
These tiny pests are a pain in the butt! Spider mites and thrips suck the sap from your plants, leaving little brown spots behind. I’ve battled them more than a few times, and my go-to is neem oil or insecticidal soap to take them out.
Fungal infections like powdery mildew
Powdery mildew is a common fungus that shows up as white spots on your leaves, which eventually turn brown. Humid environments are a breeding ground for it. A good anti-fungal spray and lowering humidity will help keep it in check.
Bacterial leaf spot disease
Bacterial infections are rarer but can still happen. Bacterial leaf spots usually start small and brown, then spread. You’ll need to prune the affected leaves and use a copper-based fungicide to stop it in its tracks.
How to Diagnose and Treat Brown Spots on Cannabis
Now that you know the causes, let’s get into how to diagnose and treat brown spots.
Identifying nutrient deficiencies from leaf color and patterns
Take a good look at your leaves. Nutrient deficiencies often show in specific patterns—yellowing around the edges, rusty spots, or purpling stems. I check for these symptoms to figure out what my plants are lacking.
Adjusting your feeding and watering schedule
If you’ve nailed down a nutrient deficiency, adjust your feeding schedule. Sometimes just tweaking how often or how much you feed is enough to fix the issue. I’ve found it helps to keep a simple feeding log to track what’s going in and how the plants are responding.
Environmental adjustments for humidity, temperature, and light
Don’t forget your grow room environment. If your plants are getting too hot or humid, brown spots could start showing up. I usually raise my lights or move fans around to make sure the plants get the right amount of heat and air.
Pest control methods: organic vs chemical treatments
When pests are the issue, you’ve got options. I prefer organic pest control methods like neem oil, but if the infestation is heavy, chemical treatments can help. Just be sure to follow the instructions carefully.
Preventing Brown Spots on Cannabis Plants
Prevention is always better than cure. Let’s talk about how to keep those brown spots from ever showing up.
Optimal soil and nutrient management
Good soil and nutrients go a long way in preventing brown spots. I always recommend using a well-balanced soil mix and keeping your feedings light but consistent. Overfeeding can do more harm than good.
Ideal environmental conditions for healthy plants
Keeping the grow room environment stable is key. I like to maintain temperatures between 70-85°F and humidity around 40-60% to avoid any stress on the plants.
Preventative measures for pests and diseases
A clean grow space is the best defense against pests and disease. I make sure to keep everything tidy and regularly inspect my plants for any signs of trouble.
What deficiency causes brown spots on cannabis leaves?
Usually calcium or magnesium deficiency. Brown spots often show up when these nutrients are lacking.
Can overwatering cause brown spots on cannabis?
Yes, overwatering can lead to root problems, which cause the leaves to develop brown spots.
How can I prevent brown spots from appearing on my cannabis plants?
Keep your nutrient levels balanced, avoid overwatering, and maintain good airflow and humidity.
What pests cause brown spots on cannabis leaves?
Spider mites and thrips are the most common pests that cause brown spots.
Should I cut off cannabis leaves with brown spots?
Yes, remove affected leaves to prevent further damage and allow new healthy growth.
