Why Your Marijuana Leaves are Turning Yellow

If you grow cannabis and experience your marijuana leaves turning yellow, it’s time to do a bit of research into what’s causing it. It is rather alarming to start seeing yellow leaves on your plant, and it isn’t always easy to get to the bottom of the cause.

Luckily, many growers have gone through the same thing, so there is plenty of information out there to help figure it out.

Usually, cannabis leaves turning yellow come from nutrient or environmental reasons, but many possible causes may not be the usual subjects. 

If you’re wondering, “Why are my marijuana plants turning yellow?” then look no further — we have the answers for you. Keep reading for some of the main possible causes — and solutions! — for your weed leaves turning yellow.

Nitrogen Deficiency

One of the most common causes of yellow weed leaves is a nitrogen deficiency in your plants. Even if you’re unsure whether this is the cause in your case, it’s a good place to start, even if only to rule it out and move on to other possible causes.

What Causes Nitrogen Deficiency?

A variety of sources can cause a nitrogen deficiency. It might be that your plant has already absorbed all the nitrogen you gave it. This is highly likely if you’re growing your plants in containers and haven’t moved them from their original container.

A deficiency can also occur in non-soil mediums, especially if the grower (you) forgot to provide extra nutrients. Non-soil mediums, such as coco and hydroponics, do not have naturally occurring nutrients.

It might also just be that you haven’t been giving your plants enough nitrogen and nutrients in general. However, this is very unlikely if you have been following the recommended amounts on your fertilizer or in most online sources.

Nitrogen Deficiency
Nitrogen Deficiency

You can identify a nitrogen deficiency as a pale, lime green color of your leaves. This is especially true of leaves that are closer to the base of your plants. These leaves are not firm and will naturally fall off. 

It is important to note that if these yellow leaves are less soft and more stiff or rigid, or if they don’t fall off after only a light tugging on them, then the problem is more than likely not a nitrogen deficiency.

How to Fix Nitrogen Deficiency

If you haven’t been feeding your plants nutrients regularly, simply start feeding them the recommended amount right away.

If you are correctly feeding your soil-based plants, you may want to transplant them into a new pot with brand new soil. You could also avoid transplanting them by simply adding nutrients into the water, but a move is in order if this doesn’t help them.

It is important to do something about it as soon as you identify the problem as a nitrogen deficiency because this could eventually lead to plant death if left untreated.

Even if your plant doesn’t die, it could cause a setback in its growth, which will make for a less impressive harvest later.

Magnesium Deficiency

What Causes Magnesium Deficiency

In most cases, a pH imbalance is the root cause of a magnesium deficiency. The pH down at the root level, within whatever medium you’re using (soil for many growers but also coco or the water in hydro setups), needs to be closely monitored to avoid this.

A magnesium deficiency also happens when the grower is watering their plants with highly purified water. This water is too soft.

Magnesium deficiency
Magnesium deficiency

The color change first happens on the bottom leaves towards the plant’s base, so keep a close eye on those leaves at all times.

If they start to yellow and do so between the veins, then you most likely have a magnesium deficiency on your hands.

How to Fix Magnesium Deficiency Problems

Fix a magnesium deficiency only after first checking the pH levels. The pH levels at the roots level should be between 6.0 and 7.0 for soil and between 5.5 and 6.5 for hydro, coco, or other mediums.

Fixing a nutrient deficiency before your plant starts to flower greatly improves the quality of your weed. Learn how to harvest like a pro in my Harvest Guide.


If you establish that pH levels are not to blame, then supplementing your soil might be a good idea. You will need to add calcium and magnesium at the same time for best absorption.

Try Bergman’s Plant Booster or another option like Epsom salts, worm castings, dolomite lime, magnesium sulfate, or garden lime.

Iron Deficiency

What Causes Iron Deficiency Deficiency

An iron deficiency, like a magnesium deficiency, is more than likely caused by a pH imbalance. Marijuana plants, as a rule, don’t need much iron to survive, so that is why it is probably a sign of a greater issue that needs addressing.

In some cases, especially for growers using purified water, the pH level might not be to blame.

This is why it is important to check the pH before deciding where the central problem lies.

Iron Deficiency
Iron Deficiency

Over time, the leaves will become greener from the outside – in, until they are mostly green and less yellow overall.

Sometimes it resembles a magnesium deficiency in coloring; however, if you remember that the difference is how it changes colors, you should not get the two confused.

How to Fix Iron Deficiency Deficiency Problems

Because the main cause of this deficiency is most likely pH level issues, check that first. Always check the pH when you notice signs of an iron deficiency. If the pH levels are off, you should address that problem, as it will help with iron absorption.

It is most likely that the actual amount of iron in your plant’s medium is not the cause of the deficiency; the absorption is.

However, if you do need to feed the plant more iron, be sure to do it alongside calcium and magnesium at the same time. Too much iron can cause other problems.

pH Levels are Too High/Low

As mentioned above, many nutrient issues are actually root pH levels that are too high or too low. In most cases, this is an easy fix once you have identified the problem.

What Causes a pH imbalance

A pH imbalance is a fairly common problem when growers do not regularly check their plant’s pH levels. It can also happen when pH levels are not adjusted for different types of ‘soil.’ Different mediums need different pH ranges.

As a general rule of thumb, soil growers should keep their pH levels between 6 and 7, while coco or hydro growers (or any other type of medium) should keep the levels between 5.5 and 6.5. 

pH Too Low
pH Too Low

PH levels being too low will cause your marijuana plants to suffer. The environment will be too acidic, which will lead to hindered absorption of nutrients like phosphorus, calcium, magnesium, and potassium. You may see symptoms of all these issues when the pH levels are too low.

How to Fix a pH imbalance

Because a pH imbalance is the most likely reason your leaves are starting to yellow, it is essential to know how to fix the problem. Start by flushing the medium (whether that means soil, coco, hydro, or something else) with pH-neutral water. 

This process is crucial to help “reset” the medium and your plants’ roots’ pH levels. After that, be sure to test the water you are feeding your plants to ensure that it has the right pH level.

Pest

Different types of pests could also be the reason for your marijuana leaves turning yellow. Pests are easy to spot when you know what to look for, but they are hard to get rid of.

Learn how to avoid pests in your garden by reading my free Marijuana Grow Bible.


What Causes Pests Infestation

If you have discovered that your yellow weed leaves were caused by an infestation of some kind of pest, then you’re probably wondering how on earth you got to this point.

More than likely, your grow environment has less-than-optimal hygienic conditions. Exposure to an external, unclean environment, even from your clothing, can bring in pests.

Other mistakes made by the grower, such as overwatering or poor ventilation, can also lead to an infestation.

Sometimes you started growing an infested clone, to begin with, but you couldn’t tell yet. Then again, sometimes it is just plain bad luck.

Pests on marijuana leaves
Pests on marijuana leaves

Turn over the leaves and look for any signs of moving bugs, eggs, tiny flies, or other insect activity. Sometimes you can find bite marks. It is vital to figure out what kind of pest it is to properly treat it.

How to Take Care of an Infestation

Of course, the best way to avoid a pest problem is prevention; never visit other growers’ gardens, and never take cuttings or clones from fellow growers unless you’re sure they’re clean.

If your plants receive outside air, make sure there is a screen around them so bugs can’t get access. If it’s too late and you already have the problem, identify the pests as soon as possible. Different insects require different treatment, so you have to act fast.

Poor Lighting

Sometimes lighting problems can make your plants’ leaves turn yellow. This is because your plants are not getting enough light or possibly too much of it. Let’s look at the causes and ways to fix them.

What Causes Poor Lighting

Poor lighting can either take the form of too little light or too much (or too close) light. Too much light, or light burn, can be caused by too-close lamps. Light burn is even possible without excessive heat.  It happens most often when you switch the bulbs in your lamps since the new bulbs will be stronger than the old ones.

Not getting enough light is another possible cause. This happens when the lamps are too far away from the plants or are too weak.

Poor lighting
Poor lighting

A lack of light usually causes yellowing leaves if your plants are in their seeding stage. Your young plants’ leaves will not only start yellow, but they will remain yellow if they are not getting enough light. 

How to Fix Low Poor Lighting

Luckily, lighting issues are usually easy to fix. For light burn, simply move the lamp away from the plant, or move the plant away from the lamp. If you’re unsure, check online or with the manufacturer for the best-recommended distance.

If your plants are not getting enough light, the opposite is the best method: simply move the lamp(s) closer to the plant. 

Heat Stress / Cold Shock 

Environmental factors like temperature are also a common cause of yellow leaves. Either heat stress or cold shock can cause this.

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What Causes Heat Stress / Cold Shock

Heat stress or cold shock is caused by climate. It applies to both indoor and outdoor growers, but outdoor ones are particularly at risk.

If you are growing outdoors in areas that experience unexpected frost, your plants may experience a cold shock. Some plants die after one short bout of freezing temperatures. Sometimes, the grow room is the problem.

Heat stress
Heat stress

Temperatures below 50 degrees Fahrenheit will cause your plant’s leaves to start turning yellow. Potted plants are at risk as well. If the ground your pots are sitting on is freezing, that cold will seep through to your plant’s roots as well.

How to Fix Heat Stress / Cold Shock Problems

The best way to prevent heat stress is to improve things like ventilation. However, you could also use air conditioners for extra help. With lighting, keeping your space cool can be a significant challenge.  

For cold, keeping the roots warm helps the rest of the plant avoid cold shock. Keep grow room temperatures above 60 degrees Fahrenheit, even during the cooler parts of the night. If you need to use a heater to accomplish this, do it.

FAQs About Yellow Leaves on Cannabis Plants

Can general stress cause my marijuana plants to yellow?

Your plants can definitely turn yellow from general stress. This stress could come from one cause or multiple causes. In any case, address it as soon as you see yellowing leaves.

Why are my seedlings yellow?

If your seedlings are yellow already, there is no reason to panic! Seedlings may start yellow — the tiny plant curled up inside the seed is yellow, to begin with, and it takes some time to eventually become green. If mature plants are yellowing, then you have a problem.

Are yellow weed leaves always a bad thing?

Sometimes yellow leaves are not an indication of anything bad and are totally normal and healthy. The oldest leaves will eventually turn yellow and die off, which is a normal, healthy process. Also, mature (ready-to-harvest) plants have yellow leaves. 

Robert Bergman

Robert Bergman is an Amsterdam-based marijuana grow expert who has years of experience from small grows to massive operations. His passion for growing lead him to develop his own Gold Leaf strain. Now, Robert is dedicated to sharing his knowledge with the world.... [Read full bio]

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