Cannabis Leaves Turning Yellow: Causes, Fixes, and What to Do

Cannabis leaves turning yellow is one of the most common issues growers face, and it usually signals an underlying problem rather than healthy growth. While some yellowing is expected later in the flowering stage, early or widespread discoloration can point to nutrient deficiencies, watering mistakes, pH imbalances, or environmental stress. Left unchecked, these issues can slow growth and reduce overall plant performance. This guide explains the most common causes of yellow cannabis leaves, how to identify them based on visible symptoms, and the practical steps growers can take to correct and prevent the problem.

Article Insights

• Yellow leaves are most commonly linked to nitrogen deficiency, but can also result from pH imbalance, watering issues, or environmental stress.
• Lower leaves turning yellow first often indicates nitrogen deficiency, while yellow spots or dots are commonly associated with pest damage.
• Overwatering reduces oxygen in the root zone, while underwatering limits nutrient uptake, and both can cause yellowing.
• pH levels outside 6.0–7.0 in soil or 5.5–6.5 in hydro systems can lead to nutrient lockout and visible deficiencies.
• Some yellowing during the final 2–4 weeks of flowering is typically expected as plants use stored nutrients.
• Early diagnosis based on where and how yellowing appears is key to preventing reduced growth and lower yields.

Why Are My Cannabis Leaves Turning Yellow? Understanding the Basics

Before we cover why cannabis leaves turn yellow, it is important to understand why the leaves are green in the first place. Cannabis plant leaves contain chlorophyll, which is a pigment located in the plant cells. 

This is what allows plants to perform photosynthesis, converting light into sugars and energy for growth and development. Chlorophyll depends on nitrogen to produce those lush metallic green leaves that indicate plant health and vitality. There will be times when the leaves of your plants turn yellow, and this can occur at different growth stages as explained below.

When yellow leaves on a cannabis plant are normal vs a problem

Nitrogen is what gives cannabis plants their green glow, which can range from a darker waxy green to a bright, shiny green. When a grower reduces the amount of nitrogen during the flowering period, fan leaves can begin to turn yellow, turn brittle and fall off the stem when touched. However, other problems can arise during vegetation and early flowering, leading to leaf yellowing, as highlighted below.

• Overwatering and underwatering issues
• pH imbalance and nutrient lockout 
• Light stress, heat stress, and environmental factors
• Root rot and oxygen deprivation in the root zone
• Pests and diseases 
• Poor soil or growing medium 

How does the plant growth stage affect leaf colour changes

During the lifecycle of a cannabis plant, there will be three different stages. These are the seedling, vegetation and flowering period. The following explains the different growth stages and how each stage affects leaf color change.

Seedling stage – 18/6

This covers the first 2 weeks of growth from the day your germinated seeds sprout out of the soil. During the seedling stage, cannabis plants establish a taproot and produce their first set of true leaves. Cannabis seedlings do not require any nutrients at this point, and as long as the soil is healthy, the seedlings will remain green.

• This stage only lasts for 2 weeks, and will be when your plants are most fragile
• Cannabis seedlings do not require high amounts of light intensity
• The seedlings do not need any nutrients and can be given only water

Vegetation stage – 18/6

The vegetation stage is when your seedlings surpass the 2-week mark. Now the plant is converting light energy into sugars for the roots and focusing all its energy on developing root fibres, growing leaves, and developing stems and nodes. It is during this stage that your plants will require the most amount of nitrogen due to the abundance of new foliage.  

• The vegetation stage can last anywhere from 2 to 12 weeks
• Plants are only focusing on developing foliage 
• Cannabis plant training techniques will be applied 

Flowering stage – 12/12

Once you switch your timer from 18/6 to 12/12, your plants will respond to the extended darkness period and transition into flowering. During this stage, cannabis plants will stretch as tall as possible, resulting in an increase in height by 100-300% depending on the genetics. Phosphorus and potassium are increased, whilst nitrogen levels are significantly reduced to near zero, to completely cut off nitrogen before the flushing stage.

• Plants will require phosphorus and potassium to aid in flower development
• Nitrogen levels can be gradually lowered once the transition stage has passed
• A grower should aim to slowly cut off all nitrogen from week 5 onwards
• Leaves will start to turn yellow during the final 4 weeks of 12/12

Flushing stage – 12/12

Flushing means replacing your nutrient solution with only plain, pH-adjusted water. The flushing stage lasts for 14 days and begins 2 weeks before the harvest date. Growers flush their plants to wash away any undissolved salts around the roots and allow the plants to use up any internal nutrient reserves. 

• Flushing also produces a clean, tasty and smooth smoking experience
• Leaves will turn yellow once flushing begins
• Soil growers will need to flush with water that is pH 6.0 – 7.0
• Hydro systems need to be flushed with water that is pH 5.5 – 6.5 

Common Causes of Yellow Leaves on Cannabis Plants

Whilst we often relate yellowing cannabis leaves to a nitrogen deficiency, there can be many reasons why your plants begin to exhibit yellow leaves. These can range from nutrient deficiencies to overwatering, underwatering, pH imbalances, or environmental stress. Below, each potential cause is broken down and explained, with guidance on how to tell them apart.

Nutrient deficiencies causing cannabis leaves to turn yellow

Nutrient deficiencies can develop due to pH issues or when nutrients are missing from your nutrient solution or soil. Yellowing cannabis leaves are not always due to a nitrogen deficiency; in some cases, the nutrients listed below may be to blame. 

• Nitrogen deficiency causes the oldest fan leaves to turn yellow
• Potassium deficiency causes the tips of the leaflets to turn yellow and brown
• Magnesium deficiency causes green leaves to develop yellow leaf veins
• An iron deficiency causes yellow veins on the newest leaves

Overwatering and underwatering issues

How you water your cannabis plants plays a crucial role in how many nutrients they can access and use. There will be times when you accidentally under- or overwater your plants. 

Underwatered cannabis plants restrict nutrient uptake and, as a result, are forced to use the available nitrogen from fan leaves. This is why you will see the leaves at the bottom of the plant turn yellow first, compensating for the new growth at the top.

Overwatering your plants will produce an oversaturated growing medium that restricts root development, oxygen levels, and the amount of nutrients the roots can access. As a result, nitrogen can be locked out, and the plant sacrifices the nitrogen from the lowest leaves to support new growth.

pH imbalance and nutrient lockout explained.

Cannabis plants require a specific pH range to fully access micro- and macro-nutrients. A pH imbalance prevents nutrients from being absorbed, causing plants to temporarily starve until the issue is corrected. When a cannabis plant cannot acquire the necessary nutrients, it will begin to show a range of deficiencies, with nitrogen deficiencies and yellowing leaves being the main culprits. 

Light stress, heat stress, and environmental factors

Yellowing or browning leaves indicate that your plants are experiencing heat or light stress. When a grow room becomes too hot for a flowering plant, the fan leaves closest to the lights can become dry, brittle and turn pale yellow. 

Leaves under light stress from being too close to the grow light or from excessive PPFD will start turning yellow. The leaves closest to the light will fade from green to yellow first.

Cannabis Root and Plant Health Issues That Lead to Yellowing

What goes on inside the pot beneath what you can see is what dictates how healthy your plants are. The roots of a cannabis plant are what provide the growth above the soil level with all the nutrients they need to thrive during their lifecycle. When cannabis leaves begin turning yellow, it can be due to root rot, pests, or diseases.

Root rot and oxygen deprivation in the root zone

Root rot is a term growers commonly use to refer to the harmful pathogens Pythium, Fusarium, or Phytophthora. This deadly fungus, which lives in low-oxygen, waterlogged environments, will attack the roots of your plant, turning it sick and slowly decaying the root fibres. 

As a result of this process, cannabis leaves will begin to turn pale yellow, starting at the bottom of the plant and working up to the top. If a root rot problem is not quickly resolved, the plant can die.

Pests and diseases that cause yellow leaves on cannabis plants

Sometimes you will notice that even with the perfect diet for your plants, the leaves are exhibiting yellowing. It is only when you take a closer look that you notice it is damage caused on the surface of the leaf by some insect or plant disease. Below is a list of all the pests that can cause yellowing leaves:

• Aphids: Leaves turn yellow due to the aphids sucking sap 
• Thrips: The damage caused by thrips starts with silver markings that turn yellow
• Whiteflies: These will cause yellowing spots on the leaf surface
• Spidermites: The damage appears as an abundance of tiny yellow dots

Pathogens and plant diseases can also be a reason for cannabis plants to display yellowing leaves and decline in health. Below are the different diseases to look out for:

• Fusarium: A harmful fungus that lives in soil and turns the stem and leaves yellow
• Pythium: Also known as root rot, causes older leaves to become yellow 
• Botrysis: Bud rot causes buds to turn brown and mushy, and leaves to turn yellow
• Powdery Mildew: A white, hairy fungus that can lead to yellowing

How does poor soil or growing medium affect plant health

Roots love oxygen, and when a growing medium is dense, thick, and poorly drained, roots are unable to grow and develop freely. This can lead to slow, stunted growth, nutrient lockout, and reduced yield. Poor-quality soil is generally nutrient-deficient and should be amended with worm castings, compost, and a high ratio of perlite.

Yellow Leaves on Cannabis Plant During Flowering: What’s Normal?

There is nothing wrong with having a fully green, vibrant plant throughout flowering, which is common among organic growers. Then again, growers have become accustomed to a nutrient schedule that tapers off nitrogen levels once cannabis plants begin to flower. 

Why yellow leaves on the cannabis plant during flowering can be expected

From my experience, it is wise to lower nitrogen levels between weeks 3 and 5 of the flowering period, and for the final 4 weeks, completely cut off nitrogen. If you notice that your fan leaves are turning yellow much earlier, you are giving the plants too much P and K, which causes a nitrogen deficiency. 

When yellowing during flowering signals a problem

When your plants are flowering, they will require nitrogen to support the development of all the new leaves forming at the internodal growth sites. Your plants should remain green and healthy until you choose to lower nitrogen and increase phosphorus and potassium with flowering boosters. 

When cannabis plants start yellowing quickly, it can indicate nitrogen deficiency. Yellowing plants can bounce back with the right nutrients; however, during flower development, this is not ideal and can set you back, significantly lowering yield. 

How nutrient needs change in the flowering stage

Nutrient brands will develop liquid nutrients tailored to each stage of a cannabis plant’s lifecycle. Vegetative nutrients will always be higher in nitrogen than in P and K. Flowering nutrients will be the opposite, with nitrogen levels much lower. 

If you think about how many new leaves your plants produce once they have stretched and begin flowering, then you know how important a steady supply of nitrogen is. Cannabis flowering boosters are typically used during weeks 5, 6, and 7 of flower production, followed by a flush using plain water.

How to Fix Yellow Leaves on Cannabis Plants

Yellowing leaves may not look good, but you can restore plant health to normal under the right conditions. The most important factor is that you can diagnose what actually caused the yellowing in the first place. Below explains the different ways to help fix the problem.

How to diagnose the exact cause step by step

Knowing the root cause is the first step; from there, you can prevent it from happening again. 

Overwatering Pants

Overwatering causes the leaves of a plant to droop, and over time, they fade from light green to yellow. If the pot always feels heavy and takes a long time to dry out between waterings, reduce the volume of water by 50% and only water when the soil or growing medium feels dry and light to lift.

Underwatering Plants

When a cannabis plant does not have access to water in the soil or growing medium, it will slow down growth and restrict the amount of water and nutrients it can suck up during the process of transpiration. Not only are the plants deficient in water, but they are also starved of primary nutrients N, P, and K. Water your plants more frequently, using half the water you normally use.

pH Imbalance

You would be amazed at how important it is to grow within the correct pH range, and how quickly nitrogen can be locked out of the plant. If you are using soil and organic nutrients, then using a pH pen, you need to check that the pH solution is between 6.0 and 7.0. Hydroponic growers require the pH levels of the nutrient solution to range between 5.5 and 6.5 for optimal nutrient uptake and availability.

Environmental Stress Factors

Heat stress and light stress will cause cannabis leaves to turn yellow. Adjusting the distance or intensity of your grow light can make a big difference. Lowering the grow room temperature and improving air flow will also prevent fan leaves from becoming dry, yellow and lifeless.

Root Rot 

The reason why growers can experience root rot is that the soil or growing medium has become a breeding ground for harmful pathogens. Anaerobic bacteria and fungi rely on a high humidity and low oxygen environment to thrive. 

Poor drainage, overwatering and high soil temperatures are the main reasons for root rot. Inoculating your growing medium with beneficial bacteria and fungi is an excellent way to kill the pathogen.

Pests

The yellowing caused by pests and insects is not the same as a shift in pigment colour, and will be the result of physical damage caused when using the leaf as a food source or sucking the sap. It is wise to check the tops and undersides of your cannabis plants daily during the vegetative and flowering period. Use insecticidal soaps to wash the leaves of any leaves containing insects or larvae.

Adjusting nutrients, watering, and pH levels

As long as you follow the recommended nutrient bottle guidelines, you should be able to provide your cannabis plants with a well-balanced diet of micro and macro nutrients. If plants are incorrectly watered, or the pH of the nutrient solution is above or below the recommended range, cannabis plants can experience nutrient lockout or deficiencies.

• Always follow the nutrient brand’s feeding chart or guidelines
• Hydroponic growers should use an EC pen to determine nutrient strength
• A digital pH pen is the most accurate way to test pH levels
• Using a measuring jug allows for a constant volume of water to be used
• Cannabis plants should be watered when the pots are light to lift up
• Adding 50% perlite to the growing medium helps improve drainage 

Fixing environmental issues like light and airflow

When your grow lights are too close to the top of the canopy, plants can experience heat and light stress. This issue is actually easy to solve and involves raising the lights by 12-18 inches until you can hold the back of your hand under the light without it feeling hot.

A good amount of fresh air and air flow is essential when growing cannabis. You have the option of using an extractor capable of moving more air out of the grow space, or to add oscillating fans to keep the grow room temperature cooler, and allow hot stagnant air to be dispersed out much faster. Poor air flow is the number one cause of cannabis bud rot.

When to remove yellow leaves and when to leave them

Insects are attracted to yellow leaves, which is why sticky tape for catching flies is produced in a bold yellow colour. A yellow leaf represents a sick plant that would make an ideal habitat for pests and insects. I would advise removing any dry and brittle leaves that have turned yellow from your plants, especially if you are still in the vegetative stage. 

Fan leaves can also contain resin, so for the sake of growers who collect their leaves for making hash or extracts, do not remove any leaves until harvest time, unless you have a dedicated trim bin that the leaves can go into sooner.