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Why Are Cannabis Leaves Turning Yellow?


by Nebula Haze

Are some or all of your cannabis leaves turning yellow? Maybe your leaves also have other symptoms like spots, curling, wilting, brown patches and/or more. Marijuana plants may get yellowing leaves for several different reasons, so it can be hard to figure out the true root of the problem!

Example of yellow leaves on a marijuana plant caused by light burn, not a nitrogen deficiency

Today I'll break down the 10 most common reasons your weed leaves turn yellow, and I'll show you how to make your plant green again!

10 Most Common Reasons for Yellow Leaves

  1. Root pH
  2. Poor Watering Practices
  3. Nitrogen Deficiency
  4. Light Burn
  5. Heat Stress / Cold Shock
  6. Magnesium Deficiency
  7. Iron Deficiency
  8. Not Enough Light (Seedlings)
  9. Bugs or Pests
  10. Bud Rot

When Not to Worry (Pictures of Normal Yellow Leaves)

 

10 Causes of Yellow Leaves (From Most to Least Common)
 

1.) PH is Too High or Low at the Roots

Whether you're growing in soil, coco coir or in hydroponics, probably the most common reason to see yellowing and other nutrient deficiencies is the pH near the roots is too high or too low. Cannabis plants have a difficult time absorbing nutrients when the pH is off, resulting in nutrient deficiencies even if the nutrients are actually there near the roots.

Symptoms

  • Yellow or other oddly colored leaves
  • Spots, stripes or patches
  • Burning around the edges of leaves
  • In fact, basically any nutrient deficiency can be triggered by incorrect pH!

Example of a cannabis plant that has a nutrient deficiency and yellowing leaves caused by the pH being off

How Do Growers Get It?

Growers who don't make sure their pH stays in the right range will often run into nutrient deficiencies, even if they're starting with a pure source of water and good soil!

  • Soil Optimum: 6-7 pH
  • Coco / Hydro Optimum: 5.5-6.5 pH

How to Fix

These symptoms look like nutrient deficiencies, but are actually caused by incorrect pH!

The leaf edges and tips turn bright yellow during a cannabis copper deficiency

Example of a zinc deficiency caused by incorrect pH at the roots

This pot plant is showing the signs of a sulphur deficiency (odd yellowing of leaves starting from the center)This marijuana plant appears to have a potassium deficiency (odd yellowing of leaves starting from the edges) which can look like nutrient burn but is actually caused by incorrect pH

A young cannabis plant with the yellow leaves of a potassium deficiency

This is a deficiency, even though it looks a little like nutrient burn (too high levels of nutrients). The main clue is the yellow striping on the leaves. Another clue is the brown tips are going in further than typical nutrient burn.

The brown edges and tips of these leaves, along with the yellow margins are signs of a cannabis potassium deficiency

Stripes on the leaves (click for close-up) indicates that this isn't a Nitrogen deficiency, even though the symptoms are somewhat similar. In this case the symptoms were caused by the pH being way too high.

The yellow "striping" on the bottom of the plant is one of the biggest signs that this nutrient deficiency with yellow leaves is caused by a pH problem

 

2.) Poor Watering Practices

It's much more common to over-water than under-water cannabis plants, and the symptoms are very similar. Yet in either case the solution is to learn how to water your plants exactly the right amount at the right time!

Symptoms of Poor Watering Practices

  • Water dripping from a faucet animation - the amount of water (and frequency of watering) have a huge impact on your cannabis plants!Droopiness (it's normal for plants to droop a little before the lights go out, but you know the drooping is a problem if it's already happening at the beginning of their "day")
  • Odd problems and symptoms from poor water practices including yellowing and sometimes other deficiencies
  • Overwatering - leaves seem "fat" and swollen with water. Often you'll have a feeling you may be overwatering your plant, especially if it's a small plant in a large container
  • Underwatering - leaves often seem "papery" and thin because they don't have any water inside them. Chronic underwatering leads to overall yellowing and deficiencies.

How Do Growers Get It?

  • Overwatering is most common with young plants since they still have small, weak root systems
  • You can hurt plants by giving too much or too little water at a time, and you can also cause persistent droopiness by watering too often or too infrequently
  • Bad soil with poor drainage can cause the symptoms of overwatering even if you're watering the plants perfectly!
  • Small plants in big containers are easily over-watered
  • Big plants in small containers are easily under-watered
  • Growers who spend long periods away from their plants and/or don't pay attention to their watering needs are much more likely to run into problems with droopiness!

Example of a watering can for your garden - the amount and frequency of watering spells the difference between healthy and unhealthy plants!

How to Water Your Plants Correctly

Overwatering

Overwatered marijuana plant - pot is too bigOverwatered cannabis seedling has turned almost completely yellow

Droopy seedling was overwatered in a too-big container, so the roots are having trouble getting the oxygen they need to growDroopy seedling was overwatered in a too-big container, so the roots are having trouble getting the oxygen they need to grow

Chronic overwatering can sometimes cause unusual deficiencies even if the pH is spot on, like this plant grown in muddy soil. The biggest sign that these symptoms are caused by overwatering and not pH (or something else) is that the plant is always droopy. 

This apparent cannabis potassium deficiency looks like a pH problem, but it's actually been caused by overwatering

Another example of a deficiency that's actually caused by overwatering (notice how this seedling is also droopy)

Another example of a deficiency caused by overwatering

Chronic Underwatering (Relatively Rare)

Most growers tend to overwater - not underwater - their plants. However, if you're spending long periods away from your plants or the containers are drying up in less than a day or two, it may mean that your plant needs to be watered more often, or be given more water at a time. It's also common to under-water when plants start overgrowing their pots. 

It can be difficult to diagnose chronic underwatering because problems may look like nutrient deficiencies. Your main clue is that plants perk up every time after you water.

Learn more about underwatering

Chronic underwatering can cause cannabis leaves to turn pale and for leaves to turn yellow

 

3.) Nitrogen Deficiency

Symptoms

  • Example of a cannabis plant with a Nitrogen deficiency - notice the leaves are pale and bottom leaves are turning yellow and wilting offPlants tend to be lime green or pale all over, even though the leaves appear healthy without stripes or spots
  • Yellow leaves tend to appear towards the bottom of the plant
  • Yellow leaves feel soft and are easily pulled off (in fact they usually fall on their own). If a leaf feels very stiff or is hard to pull out, that means it is not a Nitrogen deficiency

How Do Growers Get It?

  • Affects plants which have "used up" the nutrients in the soil, which can happen after the plant has been the same container for several weeks or months.
  • Can happen in coco or hydro when the grower isn't providing any extra nutrients (since there is no Nitrogen contained naturally in plain coco or water) 
  • It is very unlikely you have a true Nitrogen deficiency if you're providing your plants with the recommended amount of cannabis nutrients in the water.

How to Fix

This plant is on the verge of a Nitrogen deficiency. This is indicated by its overall pale color, even though all the leaves look really healthy without any spots or stripes. Cannabis leaves should not be lime green or pale, or the plant tends to grow more slowly!

Almost a Nitrogen deficiency - this plant is almost lime green. It's too pale because it needs higher levels of nutrients!

Here's a close-up of a Nitrogen deficient leaf near the bottom of the plant. Nitrogen deficient leaves are soft and look/feel wilted.

A closeup of a nitrogen deficiency

If you have a Nitrogen deficiency, the yellow leaves will start falling off on their own

Cannabis nitrogen deficiency - yellow leaves are piling at the bottom of the plant

Did You Know? Oddly enough, too much Nitrogen can also cause yellow leaves, though the rest of the leaves will be clawed and a deep dark green instead of pale.

Too Much Nitrogen (Nitrogen Toxicity)

A Nitrogen toxicity can also cause certain leaves to turn yellow, but other than that it looks nothing like a cannabis nitrogen deficiency

 

4.) Light Burn

Symptoms

  • Yellowing appears most on the parts of the plant closest to the light.
  • Yellow leaves do not pull out easily, even if the whole leaf is dead
  • Light burn often takes a few weeks to develop, and is most common once the plant is past the 6th week of the flowering stage (when plants aren't making many new leaves to replace old ones). 

Cannabis light burn usually affects the top leaves closest to the grow light instead of affecting the plant evenly 

These cannabis plants are showing the symptoms of light burn - yellow leaves at the top of the plant caused by the grow light being too close

How Do Growers Get It?

  • Light burn is when your leaves are working too hard for too long, causing them to die early.
  • Even if the temperature is in a good range, your plant can still get light burn if the grow light is too close. It's kind of like how skiers can get sunburned even in the freezing temperatures because of all the sunlight reflecting off the snow.
  • Light burn is most common with powerful lights like HPS/LED/LEC.
  • It's also common when switching to new bulbs (which are stronger than old bulbs) or when there is no glass between the bulb and your plants.
  • Some plants are more sensitive than others, and you may have one plant suffering from light burn while the others are fine. That can make it harder to diagnose the problem since some of your plants are thriving in the same environment!

Light burn symptoms can be different from plant to plant, but they always seem to happen mostly to the parts of the plant that are closest to the light

Example of cannabis symptoms caused by light burn. Yellowing top leaves under the grow lights, and the leaf margins often stay great. With light burn, leaves may turn red instead of yellow.

How to Fix

  • The best way to fix light burn is to move you grow lights further away, or bend over the affected plants so they're further from the light. 
  • When in doubt, always follow the manufacturer's recommendations when it comes to how far away to keep your light from your plants! This is especially important for growers who utilize LEDs.

This light burned leaf is not happy...

Example of cannabis plant with light burn. Although this is often confused for a Nitrogen deficiency, if the yellow leaves are appearing at the TOP of the plant, and the symptoms are worse directly under the grow lights, it may be a case of cannabis light burn

 

5.) Temperature Problems (Heat Stress / Cold Shock) 

  • Yellow or burnt leaves near the light
  • General yellowing of upper leaves
  • Leaves start "turning up" at the edges, or forming "tacos"

How Do Growers Get It?

  • If you put your hand where your plants are and hold it there for 30 seconds, is it too hot to be comfortable? If it's too hot for you it's likely too hot for your plants.
  • Although relatively rare indoors since most growers struggle with heat instead of cold, a temperature under 50°F (10°C) can also cause pale or yellow leaves. Some plants will even die if it hits freezing temperatures!

The temperature in your cannabis grow room is important to success

How to Fix

  • You should be able to pass the "hand test" (hold your hand where your plants are and make sure it's not too hot). If it's too hot for you, move the light up and further away from the plants.
  • You should correct the temperature if it's hitting under 60°F at night or above 85°F in the day.
  • Make sure there is good air circulation in the grow space, to prevent hot spots
  • Your plants will be more resistant to cold if you keep their roots warm, so make sure to keep your containers off the cold floor (or if outdoors, you might consider covering your plant roots at night!)!

This plant was decimated by a heat wave - it went through several days of 100°F+ temperatures! Luckily the buds were still great :)

A hot day causes these leaves to shrivel up overnight

Too much heat can cause the edges of leaves to curl upwards and make "tacos".

The yellow leaves and turned up edges of this cannabis plant has been caused by too much heat

Sometimes extended periods of high temperatures causes spots and other odd symptoms in addition to yellowing.

Example of spots and yellowing leaves caused by extended amounts of heat stress to this cannabis plant

Cold Shock

This plant was exposed to temperatures under 40°F (5°C) at night, causing all the newest growth to turn so pale yellow it almost looked white!

Example of a plant that got yellow leaves from freezing night temperatures

 

6.) Magnesium Deficiency

Symptoms

  • Magnesium deficiency causes yellow stripes on the cannabis leaves between the marginsYellowing in between the veins on leaves, often located lower down on the plant.

How Do Growers Get It?

  • A magnesium deficiency is almost always caused by incorrect pH though if you're using heavily purified or soft water (such as RO - reverse osmosis - water) you may need a Cal-Mag supplement to make sure your plant is getting enough magnesium.

How to Fix

  • First check the pH
  • If a Magnesium deficiency persists, consider getting a Cal-Mag supplement that is made for plants (you should always add Magnesium and Calcium at the same time because these two nutrients work together closely in the cannabis plant).
  • Learn more about Magnesium deficiencies

With a magnesium deficiency, the yellowing happens between the veins of the leaves, while the veins stay green.

This marijuana leaf is showing signs of a magnesium deficiency (yellow stripes in between the veins)

Example of a marijuana magnesium deficiency

Sometimes Triggered By Old Age / Natural Senescence / Light Deprivation

  • It's actually normal if you only see these symptoms on a few leaves at the bottom of the plant that are no longer getting any light. The plant eventually "gives up" on old leaves if they spend days or weeks without light, which often happens to the lowest leaves at the plant gets bigger. This may look like a magnesium deficiency
  • If this is the case, the leaves often seem droopy, limp and tired. These leaves don't "stick straight out" like normal leaves because the plant isn't wasting resources by putting energy into them.
  • This is most common when using relatively weak grow lights like fluorescent lighting or CFLs, since the light doesn't easily reach the bottom of the plant.
  • Therefore this symptom is only something to worry about if it's happening on leaves that are still getting light, or if you're seeing the symptoms on many different leaves instead of just an occasional leaf here and there.

This cannabis leaf is showing signs of a magnesium deficiency
 

7.) Iron Deficiency

Symptoms

  • Iron deficiencies are unique because the yellowing always affects the newest growth. It does not happen to older leaves that are already green.
  • New leaves usually come in completely yellow
  • Unlike most other nutrient deficiencies that cause yellowing, yellow leaves from an iron deficiency will usually turn green, starting from the outside edges and working inwards.

Example of a cannabis plant with yellow leaves from an iron deficiency

How Do Growers Get It?

  • Unless you are using RO or very purified water, an iron deficiency is almost always caused by incorrect pH. This is because cannabis needs very little iron, and most sources of water already contain trace amounts of iron.

How to Fix

  • If the pH is too high or too low, that is mostly likely the cause of this problem.
  • If using purified water or water that doesn't contain much natural iron, you may need a Cal-Mag supplement which includes iron like Cal Mag Plus or Sensi Cal Mag. You see these three together because Iron, Calcium and Magnesium work closely together in the plant. You never want to supplement your plant with extra iron without also adding the correct ratio of Calcium and Magnesium at the same time, or it may cause other types of deficiencies.
  • Learn more about Iron deficiencies

Iron deficiencies cause the middle and newest leaves to turn yellow, but they will slowly turn green as the plant gets older

Cannabis Iron deficiency - top leaves are bright yellow

Example of a cannabis plant that has yellow leaves as the result of an iron deficiency

 

 

8.) Not Enough Light (Seedlings)

When a shell first cracks, the round leaves inside are actually yellow. They only turn green once the plant starts getting enough light.

Note: Adult cannabis plants without enough light won't grow well, but likely won't have yellow leaves. In fact, adult cannabis plants that are getting relatively low levels of light will actually turn dark green since they aren't using up nutrients for photosynthesis (the extra unused nutrients get stored in the leaves, causing them to appear darker).

How Do Growers Get It?

You know your seedling needs more light when...

  • Seedlings are tall with small leaves
  • There is a lot of stem between each set of leaves (seedlings look "stretchy")
  • Leaves stay yellow or pale green

How to Fix

This seedling is yellow and "stretching" because it needs more light

This stretchy cannabis seedling is yellow and tall because it has not been getting enough light

 

9.) Bugs or Pests

Many different types of bugs or pests can stress your plants, causing them to develop yellow leaves.

Symptoms

  • You can actually see bugs or eggs
  • Yellowing leaves, especially when combined with spots
  • Overall lack of vigor

How Do Growers Get Pests?

  • Track them in from outside
  • From visiting another grower's plants
  • Getting an infected clone or plant (sometimes there's a few tiny eggs you can't see!)
  • Certain things like overwatering, lack of cleanliness and poor air circulation make your garden a better home for bugs, making it easier for an infestation to take hold

Example of a terrible aphid infestation on a cannabis leaf

How to Fix

  • Unless you 100% trust the grower and their growing practices, never ever visit another grower's garden or adopt clones from them. It can be incredibly difficult to get rid of bugs that are already specialized at surviving on cannabis plants!
  • Avoid going straight from outside to your cannabis plants, especially if you've spent time in a garden.
  • Make sure there is a screen to stop bugs if your plants are getting fresh air from outside.
  • Identify your bugs and get rid of them!

One of the most common pests that can cause yellowing without really any other symptoms is fungus gnats. These tiny winged creatures hang around your wet topsoil, and are most likely to appear if you're overwatering your plants. Although the adults don't attack your plants, their larvae feast on the roots, which can eventually cause yellowing, especially on small or weak plants.

A bad fungus gnat infestation can damage or even kill your plant! 

 Cannabis leaves damaged by fungus gnats

Another common pest that may cause overall leaf yellowing is spider mites!

Close-up of spider mite bites on a marijuana leaf

But any time a plant has an infestation, you may notice the leaves start yellowing regardless of the type of bug. You should be very concerned if you also see spots!

Click here to find out which bugs are attacking your cannabis plants!

 

10.) Bud Rot

If yellow leaves appear overnight on just one or a few of your main buds, inspect the areas closely! Sometimes this is caused by bud rot at the base of the leaves.

Symptoms

  • A closeup of bud rot - usually the leaves nearby will turn yellow overnight - this is sometimes you're only warning sign!Yellow leaves on select parts of the biggest buds
  • Yellowing often appears overnight
  • Yellow leaves usually easily fall right out
  • At the base of the leaf you can see white, gray or brown mold growing on the inside of the cola

How Do Growers Get It?

  • Humidity above 60% RH
  • Lack of air circulation/breeze
  • Cool temperature - bud rot thrives around 60-70°F
  • Bushy plant (too many leaves) in a small space like a grow tent
  • Outdoors in rainy, cool or humid weather

How to Fix

  • Keep humidity under 50% RH if possible
  • Keep the temperature above 65-70°F at night if possible
  • Make sure there's lots of air circulation around all the colas and through the plant
  • Defoliate a very bushy plant, especially if it's getting close to harvest time
  • Learn how to prevent and treat bud rot!

One of the first signs of bud rot is often yellow leaves where the mold is taking hold

 

Sometimes Yellow Leaves Are Normal!

Sometimes marijuana leaves turn yellow for totally normal reasons, including....

First Leaves Turn Yellow - Normal

After your plant has grown a few sets of leaves, it's very normal for the first few sets of leaves to turn yellow and die, especially if they're not getting light anymore. You will almost always lose the round cotyledons, the single-finger leaves, and the three-finger leaves (first three sets of leaves).

This vibrant young cannabis plant is healthy and growing over an inch a day

This fast, healthy young vegetative cannabis plant is growing over an inch a day

However if you look closely at the bottom of the plant, you can see the three bottom sets of leaves have turned yellow and are dying. This is normal! The plant does not hold onto these baby leaves for long!

Single-Finger Leaves (plus the tiny round cotyledon leaves)

Example of the first two sets of leaves dying (single finger and cotyledon leaves) as plant gets older - this is completely normal!

Three-finger leaves

Example of three-finger leaves dying naturally as the plant gets older - this is normal!

When just first 3 sets of leaves turning yellow like the example above (leaves with three fingers or less), it's not something to worry about as long as the rest of the plant is green, healthy and growing fast!

You don't normally see these in pictures because most growers remove them :)
 

Plant is Ready to Harvest - Normal

Often plants will have a few yellow leaves by harvest time! This is completely normal and nothing to worry about as long as you've ruled out bud rot! 

It's normal for some of your cannabis leaves to turn yellow as the plant approaches harvest - this is normal yellowing!

 

Mutation - Cosmetic (Usually Not Harmful)

Occasionally you may see mutations or natural variation that results in parts of leaves being yellow. The general rule of thumb with any unusual leaf symptom is if the rest of the plant is green, vibrant and healthy, there's usually nothing to worry about.

Example of a two-tone marijuana leaf - a common mutation

See more common cannabis mutations!

 

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10-Step Quick Start Guide to Growing

 


 

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