Leaves Turning Yellow During Flowering (Troubleshooting Guide)

You might have a serious problem.

If your cannabis plants have leaves turning yellow during flowering, they could be sending you a warning.

The good news is: most likely it is nothing to worry about, though.

If the leaves are turning yellow during the final weeks of flowering, this is perfectly natural. Your plants are simply diverting resources to bud growth and letting the leaves die off. This is a good thing.

But if the yellowing occurs early in flowering (or even during vegging), it is probably something serious.

And it could be a number of vastly different problems. That is why it is important to figure out exactly what the issue is, before you can fix it.

This guide will help. We’ll begin by giving you the most common causes of yellowing marijuana leaves. Then we’ll go through a step by step troubleshooting guide to help you rule out one possible cause after another. Let’s get started!

 

Leaves Turning Yellow During Flowering

Leaves turn yellow due to a loss of chlorophyll, which is known as chlorosis. As mentioned, if chlorosis happens early in flowering, or even during vegging, it could indicate a serious problem. Read on to find out what might be causing the issue and how to fix it.

However, if the leaves are turning yellow toward the end of flowering, this is called senescence. Senescence refers to the natural yellowing and dying off of plants.

Toward the end of its life cycle, the marijuana plant diverts all of its resources toward bud growth to maximize yield (how convenient for us!), letting its leaves die off.

If you flush the soil or other growing medium to get rid on any residual nutrients before harvesting, this will further cause yellowing. Many growers also withhold water before harvest. Read When To Stop Watering Before Harvest for more.

This type of yellowing is not an issue. If this is what you have, you can safely stop reading at this point and get ready for a great harvest

 

Possible Causes Of Chlorosis

As mentioned, there are quite a few possible causes of chlorosis. Let’s take a look at the most common ones, before going through a troubleshooting guide to help you figure out what might be causing your weed plants to turn yellow.

 

Light Burn

Yellowing leaves can be caused by light burn. This simply means that the grow lights are too powerful and are burning the leaves, causing them to turn yellow.

Obviously, this will effect the leaves closest to the lights the most, so you can easily tell when the yellowing is most likely due to light burn. The leaves are also difficult to pull off the plant, even if they are already dead.

To alleviate the situation, simply hang the lights higher, or dim them, if you have dimmable grow lights.

 

Insufficient Light

Conversely, not enough light can also cause yellowing of the leaves. I know, it’s confusing, right? The good news is that leaves will only be yellow from not getting enough light early in a plant’s life. If they change color during flowering, insufficient light is not the problem, so you can safely cross this one off your list.

 

Over-Watering

Over-watering is a common problem. And it can actually mean several things. It can mean that you are giving too much water at one time, or that you are giving the right amount, but watering to frequently.

You could also be doing everything right, but your pot does not have good drainage (due to poor soil or a pot with inadequate drainage), causing moisture to remain in the pot.

The symptoms from over-watering can resemble those from a nutrient deficiency, but there are some things to look out for, to help you tell the difference. For one, the plants might look a little droopy and the leaves can appear swollen, since they are full of water.

Do the plants droop (or droop even more) immediately after watering? Then they are almost certainly getting too much water.

Fixing this issue is relatively straightforward: simply give your plants less water. But you need to make sure the issue is not due to poor drainage, and you also need to know if you are giving too much water at one time, or watering too frequently.

Make sure you use good soil or coco coir. If you just use random soil you got from outside somewhere, it may not drain well (it may also be infested with pests, so just buy good potting soil). If water takes forever to come out the bottom of the pot when watering, it is likely a drainage issue.

When watering, you want about 10 to 20% of the water you give to run out of the bottom of the pot. That is when you’ve provided enough. Remove this water after watering. Do not simply let it sit in the tray.

To know how often to water, stick a finger in the top of the soil, about a knuckle deep. If it feels dry, it is time to water. For coco coir, water every 1 to 2 days.

Smart pots are a great way to prevent over-watering. But no matter the pot, using one that is the right size helps a lot too. A pot that is too small makes it easy to over-water. A pot that is too large, can lead to the opposite problem.

 

Under-Watering

Under-watering is much less common than over-watering, but it can still happen, especially if your pot is too large for the plant. Leaves will be thin, since they lack water. If your plant looks droopy, but it perks up right after watering, it is likely not getting enough water.

 

Nutrient Deficiency

A deficiency in several nutrients can cause yellow leaves. Conversely, so can an excess of some nutrients, but this is much less common. We’ll cover the most common deficiencies here, but you can check our chart of nutrient deficiencies in marijuana to see which other ones could cause leaves to lighten.

 

Nitrogen

The most common culprit is nitrogen. With a nitrogen deficiency, you will notice the lower leaves yellowing during flowering (the older ones toward the bottom of the plant). These leaves are easy to pull off and actually fall off on their own most of the time. Plants will also look pale or lime green.

A nitrogen deficiency does not necessarily mean you are not providing the plant enough nitrogen. It could also be a pH imbalance that is blocking nitrogen absorption. The best water for weed plants has a pH within a specific range (see below).

If you are providing the recommended amount of nutrients, a nitrogen deficiency is unlikely. Most soils already contain nitrogen, but once that is used up, you will need to supplement. Coco coir does not contain nitrogen, so you will need to provide it from the beginning.

Too much nitrogen can also cause leaves to turn yellow, but the remaining leaves will look dark green, instead of lime green or pale.

 

Magnesium

A magnesium deficiency causes a different kind of yellowing. Instead of the entire leaf changing color, the veins stay green and only the parts in between the veins turn yellow.

But if you see these symptoms only on lower leaves that are not getting any light (or very little) and that also look limp or droopy, then this is not a deficiency. Plants divert resources away from leaves that don’t get any light, so as long as the leaves that are getting light look fine, your plant is getting enough magnesium.

If it is not getting enough, it is most likely a pH issue, and not a result of you not providing enough magnesium, since most water contains this mineral. But it you are using filtered water, your plant may not be getting enough.

Check the pH first, before adding more magnesium (generally via a CalMag supplement). You want to make sure you do not add a cal mag supplement, unless you are actually facing a calcium or magnesium deficiency. Or both.

 

Iron

If you plants are not getting enough iron, it is the newest leaves that are yellow, not the older leaves. This makes it fairly easy to diagnose this issue. The leaves actually start out yellow, then begin turning green, from the outside edges inward.

It is highly unlikely that your plant is not receiving enough iron. Cannabis needs very little and most water contains enough.

You will only need to supplement iron (also using a CalMag supplement, which all contain iron as well), if you are using completely pure water (reverse osmosis, for example). In most cases, an iron deficiency results from an incorrect pH.

 

pH Imbalance

In the end, an incorrect pH in the root area is the most common reason for yellowing leaves. When the pH is not within the correct range, cannabis roots are not able to absorb certain nutrients, which leads to a deficiency. While a lack of a certain nutrient is the direct cause of the yellow leaves, it is the pH imbalance that is the root cause.

The ideal pH for marijuana plants growing in soil is 6.8. For weed plants growing in water (hydroponics) or other soil-less media, the ideal pH is 5.8. Use a pH meter to test the pH of runoff water (this works better than using a soil meter to test the soil itself).

If the pH is incorrect, you’ll want to flush the root area with neutral water. Then give the plant water with the correct pH. You can adjust it using pH up/down solutions.

 

Incorrect Temperature

We already mentioned the possibility of your grow lights causing light burn if they are too close to the plants, and thus too hot. But even if the lights are not directly the problem, an ambient temperature in the grow room that is too high or too low can also cause leaves to turn yellowish.

You want to make sure the temperature stays between 60° F and 85° F. It will be lower at night, but should never drop below 60°. It should never go above 85° during the day.

Use fans, ventilation ducting, air conditioning units, grow lights and heaters to regulate the temperature. You won’t need all of those things, of course.

In mini grow tents, you often don’t need anything more than your grow light and a regular fan. In big grow tents, you’ll probably need more heating and/or cooling equipment.

 

Pests

An infestation of pests can also cause yellow leaves. Even if you have a completely sealed grow tent, you can track in pests on your shoes or clothing, or there could already be eggs on a clone you bring inside.

Poor conditions in your grow space make your plants an easier target, so ensuring you keep your room clean and you don’t over-water can go a long way to reducing the chance of an infestation taking hold.

To check if an infestation of pests is the cause of your discolored leaves, inspect your plants carefully to see if you detect any bugs crawling around.

If you do, identify the pest and look up how best to get rid of it. This is not always an easy thing to do, unfortunately.

But some, like aphids, are easier to get rid of. This article shows you how to get rid of aphids during flowering.

 

Bud Rot

Bud rot thrives at cooler temperatures of around 60° to 70° F, with a relative humidity over 60%. If you see gray or brown mold growing inside the cola at the base of any yellow leaves, you have bud rot.

The yellow leaves are usually the first sign you’ll see. They will change color quickly, often overnight, and fall out easily. Once bud rot has taken hold, there is not much you can do. So do your best to prevent it.

The same goes for powdery mildew, which is basically a type of bud rot. Once your plants are affected, it is almost impossible to remove successfully, no matter how much you spray your buds against powdery mildew.

The key is good circulation. Ensure that buds get plenty of fresh air. Remove larger leaves on bushy plants to improve air flow with a pair of trimming shears, especially toward the end of flowering. Try to keep humidity under 50% during flowering and the overnight temperatures above 65° F.

 

Troubleshooting Yellow Cannabis Leaves

The most likely cause of yellowing leaves is a nutrient deficiency or light burn. But when troubleshooting, it does not make sense to begin with a nutrient deficiency, because it is more difficult to rule out than some other potential causes. Instead, we want to begin by ruling out the ones that are easiest to rule out.

 

Step 1: Ensure Correct Temperature

Temperature is definitely not the most likely cause, but it is the easiest to check, so we recommend starting here. Make sure that the temperature in your grow area stays above 60° F at night and below 85° F during the day.

 

Step 2: Ensure Correct Amount Of Water

This is another fairly easy thing to verify. Go back to the section above on over- and under-watering and see if that could be the case for you. If not, move on to the next step.

 

Step 3: Check Lights

Here you just want to make sure that the lights are not burning the plants. Check for the signs of light burn mentioned above in the section on this issue.

You can also do the “hand check”. Simply hold your hand by the top of the canopy, with the back of your hand facing the grow lights. Leave it there for a minute or so.

If the light begins to get too hot on the back of your hand, it is going to be too hot for your plants as well. Hang the lights higher and see if that resolves the yellowing. If not, move on to the next step.

 

Step 4: Check For Pests

This is another simple check. Inspect affected plants carefully to see if there are any pests crawling around on them. If so, they are the likely culprit and you need to take steps to eliminate them. If not, move on to the next step.

 

Step 5: Check pH Levels

With everything else ruled out, the issue is most likely a problem of insufficient nutrients, or perhaps too many nutrients. Most of the time, a nutrient deficiency is actually caused by an incorrect pH that is blocking the absorption of that nutrient, instead of  you not providing enough nutrients.

The best way to check the pH level of the root area is to test the pH of the water runoff as described in the section on pH imbalance above. If the pH is off, adjust it as described in that same section.

If the pH is good, then you are probably not providing your plant enough of a certain nutrient. Or you may be giving it too much.

 

Step 6: Check For Nutrient Deficiency Or Excess

The key is to figure out exactly which nutrient is causing the issue. For yellow leaves, it is usually nitrogen, magnesium or iron.

The section on nutrient deficiencies above will help you diagnose which nutrient is deficient. Once you have that figured out, take the appropriate steps to rectify the problem. Trimming off the lower branches can often help, because it frees up resources for the remaining leaves.

 

Yellowing Leaves During Bloom: Final Thoughts

Yellow leaves are not always a problem. In fact, most of the time, they are nothing to worry about. But other times, they can indicate something serious.

The first step is to determine which of these is the case for your plant. If you have unusual yellowing early in the flowering stage, you probably have a more serious problem that needs to be fixed in order to get a good harvest. Use the troubleshooting steps to figure out the issue and remedy it.