Cannabis fan leaves turning black (Causes and Solutions Guide)

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Written By Professor

Regardless of the cannabis growth stage, fan leaves turning black is a serious warning sign that your plants are in tremendous pain. While back fan leaves can be fatal, it does not always mean a death sentence for your plants. But diagnosing the cause of the leaves turning black can be tricky because it could happen for numerous reasons.

While there are many reasons why leaves turn black, it is usually related to poor cultural and environmental care-taking factors. Identifying the symptom cause can help you act accordingly to either treat or at the very least stop the blackness from spreading further. Usually, the symptoms can be treated by making adjustments to the grow room environment or caretaking regime. But in rare cases, it can be transmissible and infect any plant that is in close proximity, including other rooms.

Knowing the difference between bacterial infection and environmental stressors can prevent your plants from going into a complete tailspin and die.

Today we are looking at how to identify the different reasons why cannabis fan leaves turn black to help you quickly get your plants back on track.

Cannabis fan leaf turning black
Cannabis fan leaf turning black

1. Overwatering

Giving your ladies too much water can cause all sorts of serious negative consequences including turning leaves black. Typically once fan leaves turn black because of too much internal moisture rot has already taken control and the damage is irreversible. Luckily the rot does not affect the entire plant overnight and allows the opportunity to take quick action to readjust the care-taking course.

Quality pot soil prevents overwatering

To prevent fan leaves turning black, use quality pot soil that has good drainage properties. Unlike humans, plants will drink water till nothing is left instead of drinking until hydrated. Using random outside soil with poor drainage properties will result in more available water for the plant to absorb.

Before leaves turn black from overwatering, the plant will experience other symptoms that can help the grower identify the problem before plant rot has set in. While overwatering symptoms can closely resemble nutrient deficiency, there are two notable signs to help diagnose the problem.

Signs of overwatering

Signs of overwatering
Signs of overwatering cannabis plants

Once plants absorb too much water the leaves start to droop and swell from excess liquid. The swelling happens because the plant absorbs water quicker than its ability to transpire moisture back into the atmosphere.

Although overwatering is not the only reason preventing the plant to breathe, it does set in motion a chain reaction that accelerates the process.

Another sign to look out for happens directly after watering your plants. If fan leaves start swelling immediately after watering this is a sure indicator that plant rot will set in and turn the leaves black if watering maintenance is not adjusted.

Treating overwatering symptoms

Once overwatering is diagnosed as the culprit, the treatment is pretty straightforward. Stop watering your plants until the leaves start to wether and curl for dehydration. You can verify that these symptoms are actually from dehydration instead of overwatering by sticking a finger about knuckle deep into the soil. If the soil is moist, you need to wait longer until dried out. Once you verified that the pants are experiencing mild dehydration it is time to give your ladies a drink of water.

Be careful not to give too much water at a given time. The easiest way the estimate the right amount of water the plants need is by following this simple procedure. The first step, verify that you are using quality pot soil and use its saturation ability to calculate watering needs. Proper pot soil drains about 25% of the water to the overflow tray which indicates that the soil is 100% saturated with water.

In the second step, measure the amount of water used per plant and increase the measure until 25% are collected in the overflow tray. Once the soil is 100% saturated, remember to remove the liquid from the overflow tray to prevent the plant from absorbing it.

While overhydration can be easily treated, plant rot from high humidity is permanent and can result in total plant death. Removing open pools of water from a grow tent is critical to help reduce humidity without the use of a dehumidifier.

2. Fertilizer

cannabis leaf nutrient burn
cannabis leaf nutrient burn

The most common response for fan leaves turning black is to use fertilizer to combat the symptoms because they can closely resemble a nutrient deficiency. However, making a wrong diagnosis can be fatal to your plants. Usually, bad outcomes like plant death are many harmless problems working together to create a perfect storm.

High temperature and humidity accelerate the plant’s metabolism and result in the cannabis absorbing more water than it can handle.

While overhydration is mostly harmless, aborting too much nutrient-rich water from fertilizer can cause serious problems like nutrient burn and blacken fan leaves.

3. Bacteria Infections

cannabis leave bacteria infection creating dead spots

Bacteria infection is extremely dangerous to all plants and can wipe out your entire crop. If a bacterial disease breaks out in your grow room it can quickly spread throughout the entire crop if unnoticed.

Following best practices can drastically lower the risk of bacterial infection. However, sometimes it’s more tricky to prevent because the infection can come from the farm you buy clones.

Although clones can be a high risk for bacteria, the biggest contributor to bacterial infection is cross-contamination between plants from pruning. Much like humans, open wounds on a plant are the ideal entry point for harmful diseases. When pruning your ladies always use sharp and sterile shears or sculpting blades to prevent cross-contamination of bacteria.

Using sterile equipment when doing plant surgery is an effective precaution but should also be accompanied by good housekeeping practices. Keep your grow space clean by removing any open water pools like spillages or overflow water from the pot tray. Stagnating open water sources in a grow room is the perfect environment for pests and diseases.

Should I Cut off Leaves that turned black because of bacteria?

Leaves that turn black from bacteria are usually past the point of saving but there is some quick action you can take to help contain the spread. Once leaves turn black from bacterial infection it is saved to assume that serves no purpose the to plant development anymore and can be removed to prevent further spread. It is also a good idea to isolate the infected pant from the healthy ones to prevent cross-contamination.

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