Cannabis Leaves Turning Yellow Halfway Through Flowering: Tips On How To Prevent And Fix Them

Watching your cannabis leaves turn yellow halfway through the flowering stage can be alarming, especially when you’re so close to harvest. While some yellowing is normal in the final weeks, premature yellowing should worry you. 

It’s a sign of stress. But don’t panic just yet. By understanding the potential causes and implementing a few key strategies, you can address the issue and keep your plants thriving. In this guide, you’ll learn exactly that. 

Why are Cannabis Leaves Turning Yellow in the Flowering Stage?

When your cannabis leaves start yellowing halfway through the flowering stage, it’s usually a sign that something is stressing out your plant. Here, various factors, from plant diseases and pests to nutrient deficiencies can trigger this chlorosis. More on this later. 

But before we proceed with the guide, it’s important to note that during the last weeks of the flowering stage, some yellowing and leaf drops are normal. However, if you notice yellow leaves outside of this period, it might indicate a problem that needs some fixing. 

As a grower, catching these signs early is crucial to maintaining healthy plants and maximizing their yields. And the first step in solving the issue is to pinpoint the cause. We already alluded to the causes, so let’s take a look at them more in detail. Here are some of the reasons why your cannabis leaves may be turning yellow. 

1. Nutrient Deficiency

One of the most common reasons for leaf chlorosis halfway through flowering is the lack of enough nutrients, especially those that are important during this stage, like nitrogen, phosphorus, and potassium. 

During this stage, your plant needs more of these nutrients, and if they don’t get enough of it, the leaves can start yellowing. So, what can you do about it? 

Solution

Start by giving your plant the right amount of nutrients or use well-balanced fertilizers that are specifically designed for cannabis during the flowering stage. 

Here’s the thing, the nutrients your plant needs change drastically as it transitions into the flowering stage, and you need to be on top of it. Hence, you can find various bloom boosters or flowering-specific nutrients that come in handy here. 

You should also try to figure out exactly which nutrient is missing, so you can give your plant a much-needed nutrient boost during this stage. You don’t want to ignore the problem for too long as it can affect bud development. 

We’ve already made guides on figuring out which nutrient your plant is missing and how to fix it. Use those guides to figure out and fix the exact problem. 

2. pH Imbalance 

The pH level of your growing medium is vital for nutrient absorption. If the pH is too low or high, the roots may find it difficult to absorb nutrients, leading to nutrient imbalances. Fortunately, this problem is easy to solve. 

Solution

Remember, your cannabis plant needs the right pH, i.e., slightly acidic, between 5.8 to 6.2. However, you can maintain between 5.5 and 6.5 in hydroponic systems and 6.0 to 7.0 in soil and still get decent results. Consider investing in a pH pen and keep an eye on the growing medium’s pH. If the leaves are turning yellow and you find the pH to be off, you need to quickly fix it. 

For this, you should rebalance the pH of the nutrient solution and give your plant a thorough flush to make sure no nutrient imbalances are causing the pH change. You can also use pH up/down solutions to make adjustments to the nutrient solution pH. 

3. Light Stress 

Cannabis plants are sensitive to light, and excessive heat, intense light, or prolonged UV exposure can all lead to stress, turning the leaves yellow in return. Yes, plants need a lot of light but more doesn’t always mean better. 

One of the most common problems in cannabis growing is light burn, especially during flowering when growers are usually preparing to bump up their light levels to maximize yield. The right lighting conditions are important for proper growth and flowering, but a lot of light or lights placed very near the plants can cause some serious harm. Light burns can also cause yellowing in leaves. 

Generally, plants experience light burn when they are exposed to extreme light intensity from powerful grow lights such as LEDs or HPS (High-Pressure Sodium) lamps. Such lights can not only make the leaves turn yellow but in many cases, they look bleached or burnt as they are very close to the light source. Not only young plants but even mature plants cannot escape this. Although this is common indoors as there isn't a lot of distance between the lights and the plants, it can occur outdoors as well with intense sunlight. 

Sometimes, you can get confused between light burn and nutrient deficiencies. However, a few differences will indicate that it’s light burn, including: 

• Yellow or white bleached leaves: With nutrient deficiencies, you will notice that the leaves at the bottom turn yellow first. Of course, some nutrient deficiencies cause the lower leaves to turn yellow first, but you can easily differentiate between light burn and deficiencies by just looking at the leaves carefully. You'll see that only the leaves at the top get affected as the chlorophyll breaks down. 
• Burnt Leaf Tips: You can definitely tell that the leaves are burning from excessive light when their tips turn brown and crisp. 
• Leaves Cupping or Curled: To avoid the continuous intense light that is burning them, they curl upwards.  
• Stunted Bud Development: The buds closest to the light source suffer if the light is too intense and the growth comes to a screeching halt as they burn from overhead lighting. This can result in smaller and less dense buds.

Solution 

Correct Light Height: One of the primary causes of light burn is placing the lights too close to the plants. In addition, it makes the plants stretch a lot and the risk of burn increases as they get closer to the light. Therefore, ensure that you follow the recommendations from your manufacturer. Although it will work if you place the lights at least 18-24 inches away from the plants for LED lights, you may need to increase the distance when using HPS lights, depending on their intensity. Lift the light higher as your plants grow.

Check Light Intensity: Extremely strong lights can harm the plants by burning them. This is especially true with HPS lights but modern LED lights may burn the plants too as they are super powerful. To prevent this, use a PAR meter to check how much light the plants are receiving at the canopy level to ensure that your plants are getting adequate light without the risk of being burnt. Dim the lights or move the plants away if you experience too much intensity.

The change in the light cycle: Make sure your plants are getting 12 hours of light and 12 hours of darkness if you're growing photoperiods. For autoflowers, you can continue providing 18 hours. 

Use Training Techniques: Low-stress training (LST) or topping are good simple techniques to prevent your plants from growing too tall. They also help the lower parts of the plant get lots of light, meaning the light distribution will be uniform throughout the canopy. 

Dim your Lights: If you have dimmable lights, consider lowering the intensity to a degree that will not harm the plants. Otherwise, you can lower the wattage of your light until everything is revived.

Trim Wilted Leaves: Trim off leaves that have burned the worst so your plant can spend all of its time and energy on healthy tissue. This is also beneficial for light and air to get to the healthier portion of the plant.

Watch Changes: Always monitor the plants for a few days after you make changes. Do not allow the signs to get worse and maintain healthy new growth. Strike the right balance to prevent further stress on your plants.

Climate Control: Your grow room should have proper ventilation, temperature, and humidity control. If the light burn is made worse by excessive heat from these lights, keeping the environment stable can help the plant recover.

4. Overwatering or Underwatering 

Too many beginners fail to water their plants properly, which is surprising but true. Overwatering or under-watering — both of these things can harm your plant, making the leaves yellow and affecting the plant's health, reducing the yield significantly. 

Overwatering

This happens when you overwater your cannabis plant way too much and its roots are waterlogged constantly. This can suffocate the roots by depriving them of oxygen and cause root rot among other problems, not to mention that it's especially detrimental to the plant during flowering when it focuses all its energy on bud growth.

Some symptoms of overwatering include:

• Wilted-looking yellowing leaves
• The bottom of the plant begins to yellow and then moves upward.
• Growth is slow and bud development is also weak.
• You may notice a rotting smell, indicating root decay.

Solution

• Change your Watering Routine: Typically, the plants need a good watering but you must allow the medium to dry before watering again. This ensures that the roots get oxygen and there is no waterlogging.
• Improve the Drainage: Try to select pots with proper drainage holes so that the excess drains out. Bonus points if you add some perlite to increase the aeration as well. 
• Water Less Often: Some cannabis strains require more or less water during the flowering compared to the vegetative stage, so adjust accordingly. 

Underwatering

As a beginner, you must also learn how to differentiate between overwatering and underwatering in cannabis; underwatering occurs when plants face a shortage of water. So much so that they can't grow normally or conduct their metabolic processes correctly. A lack of water could easily stress the plant and cause yellowing leaves. 

Some symptoms include:

• Curling dry, brittle leaves
• Leaves turning yellow, especially on the lower end of the plant
• Growth becomes sluggish with very small buds, and the plant looks stunted as well. 
• Cracked, parched soil, drawing away from the edges of your container.

Solution

Water More Often: Help keep the soil consistently moist, but never soggy or too dry. You can also feel the top inch of the soil — it should be just damp to the touch. Or, use a moisture meter or stick your finger in the soil to determine when plants need water.

Transfer to More Spacious Pots: If your plants are getting a little too confined, they might take longer than what is necessary for the water to evaporate so transplanting them into larger containers will give you more soil volume that will retain moisture.

Control Environmental Conditions — Monitor the temp and humidity levels of your grow space. This can lead to high water consumption by the plant, as high temperatures and low humidity may be prevalent.

Establish a Regular Watering Schedule: It is important that you set up some sort of watering routine depending on what kind of plants you are growing, their pot size, and the medium in which they grow as well as your local environment.

Good Drainage: Make sure your pots have good drainage, and think about using fabric containers as they normally provide better airflow to the roots than traditional plastic ones.

A correct balance of watering will prevent leaf yellowing from both under and overwatering, keeping your cannabis plants healthy through the flowering phase.

In any case, remember the rule of thumb when it comes to watering. Only water your plant when the top inch of the soil is completely dry. Also, water your plant until around 25% of the water runs out of the drainage hole. 

5. Pests 

Of the many issues cannabis growers encounter, few are as prolific and difficult to deal with as pests and diseases — especially during flowering when a plant's natural defenses might be at their lowest. Left unattended, pests and diseases will cause yellow leaves to form — a sign of reduced bud quality, or worse, total crop loss. Learning how to recognize, prevent, and combat all these threats is key to a successful growing cycle. 

Some common cannabis pests include:

Spider Mites

Spider mites are minuscule sap-sucking arachnids that can destroy cannabis plants rather quickly. They typically hide in the undersides of leaves where they feed and reproduce rapidly.

Some signs of spider mites include yellow or white specks behind the leaves and webbing. If you don’t take care of the problem, the leaves begin to turn yellow and fall off eventually. In the flowering stage, the buds will be covered with webs and will be unable to form properly. 

To prevent spider mites, keep a clean grow area and scout plants regularly. If you find them on the plants, you can release beneficial insects like ladybugs or predatory mites or insecticides suitable for spider mites to manage populations. Also, remove infested leaves and treat plants with insecticidal soap or neem oil. During heavy infestations, you can use a spray of pyrethrin.

Aphids

Aphids are small soft-bodied insects that feed on plant tissue. They produce rapidly and infest plants within days. 

Symptoms of aphids include curling, yellow leaves, and wilting due to the honeydew on the surface of leaves. Dried honeydew also attracts mold.

To prevent aphids, use ladybugs, practice clean growing habits, and avoid excess fertilization because aphids are more attracted to soft growth with lots of nitrogen. To treat plants with aphids, start by spraying the plants with jets of water to dislodge them. You can also treat the plants with neem oil or insecticidal soap. 

Whiteflies

Whiteflies are small, white-winged insects that suck the sap from cannabis plants. They usually reside on the underside of leaves and can easily cause havoc. 

Symptoms include leaves that turn yellow, stunted growth and a sticky substance on the leaf. When these are disturbed, whiteflies will fly up in a cloud off the plant.

To prevent whiteflies, use yellow sticky traps, improve the airflow, and introduce predators such as parasitic wasps.

Fungus Gnats

Fungus gnats are small, dark flies that lay their eggs in damp soil. These larvae feed on plant roots and can be particularly harmful if they are girdling the stem near the cropping line.

It’s pretty easy to prevent fungus gnats as they are typically attracted only to damp soil conditions. Therefore, it’s best to wait until the soil dries properly before watering the plants. You can also add beneficial nematodes to the soil, treat with hydrogen peroxide solution for larvae, or consider a biological control such as Bacillus thuringiensis (Bt).

6. Genetics 

Sometimes, yellowing leaves may not even be a sign of trouble. If you can’t find a particular problem, you can chalk it up to genetics. Some strains inherently turn yellow as they get closer to harvest. 

Just remember to not make a mistake here. Instead, look up your particular strain online and see if yellowing leaves during bloom is part of its genetics. If it is, you don’t need to worry. 

How to Prevent Yellowing Cannabis Leaves During Bloom?

While nutrient deficiencies are the most common cause of yellowing leaves in cannabis, they can also be the most difficult to identify. To prevent this issue entirely, you can provide your plant with the right nutrients and take additional steps to keep them healthy throughout the flowering stage. 

Here are some things you can try. 

• Balanced Nutrient Management 

Instead of simply feeding whatever fertilizer you find in your local horticultural store, you need to take a more active approach to nutrients, and it can be done with balanced nutrient management. 

Here, focus on micronutrients apart from NPK, too. Make sure your plant gets essential micronutrients like magnesium, iron, and calcium, which are also vital for your plant’s overall health. 

You can also incorporate organic soil amendments like compost, bat guano, and worm castings, which offer a slow release of nutrients that can improve soil health and prevent nutrient deficiencies. 

• Regularly Monitor the pH 

You don’t have to wait for the pH to go off to bring out your shiny new pH meter. Instead, use it every couple of days to give your growing medium a check. This will help you fix the pH before it strays too far from the recommended levels. 

Here’s how to check the pH properly — check the pH of both the nutrient solution and runoff water. If it’s different, something is throwing it off in the soil. Here, flush the soil and try again. Sometimes, nutrient buildup in the soil can change the pH, too. 

• Other Tips and Tricks 

Apart from these, you can also try other things to prevent the leaves from turning yellow during the flowering stage, such as:

• Water your plants at the base to minimize moisture on the leaves, preventing fungal issues 
• Maintain healthy roots by ensuring they receive enough oxygen — use air stones or well-aerated soil 
• Use drip irrigation with adjustable flow rates to control the amount of water your plant gets 
• Grow companion plants like basil and marigolds to prevent pests 
• Use neem oil to get rid of any pests — just don’t spray it on the buds 
• Consider giving your plant more CO2 to boost its growth and resilience 
• Maintain a consistent light cycle and make sure your lights are placed at the correct distance 

Summary: Cannabis Leaves Turning Yellow Halfway Through Flowering: Tips On How To Prevent And Fix Them

Preventing yellow leaves during bloom is all about proactive care and attention to detail. From balanced nutrient management to pH monitoring to proper watering techniques and pest control — these measures ensure your plants stay green and healthy. 

So, remember to stay vigilant and address these issues as they arise. You’ll not only save your plant from stress but also maximize your harvest potential. At the end of the day, no one likes losing out on buds, do they?