September 24, 2022

Leaves turning yellow during flowering: what to do about yellowing leaves on your cannabis plants

Cannabis leaves turning yellow is always a concern for growers. This occurrence can either be a completely normal part of the plant’s lifecycle, or a symptom for a big problem with your plants. Read on to find out how to tell the difference.

Yellow leaves aren’t always bad, but they can be

Leaves turning yellow during flowering isn’t always a bad thing. When a weed plant begins flowering, it naturally diverts energy away from stem, leaf, and root production. This is because cannabis buds take a lot of energy to grow, and at that point the plant’s priority is reproduction.

When this happens, your plant’s leaves will naturally begin to die off. This is especially true towards the end of the flowering phase.

However, there are many causes for yellowing leaves, many of which are harmful to the plant. Left untreated and undiagnosed, these issues can reduce your harvest yields or even kill your plant. The good news is that most of these problems are treatable if the issue is caught early.

Common causes of leaves turning yellow during flowering

Incorrect root Ph

Incorrect root Ph is the most common cause of cannabis leaves turning yellow. Cannabis plant roots need the soil to be the right Ph for them to properly absorb nutrients. When marijuana plants are unable to absorb minerals from the ground, their leaves begin to turn yellow as the plant develops nutrient deficiencies.

Ph issues can be problematic because growers often mistake the issue for lack of minerals in the soil. If growers then overfertilize the soil to compensate, the fertilizer can build up to high levels until the Ph is restored. If this happens, the plant can then go into shock or get nutrient burn.

Cause and symptoms

The Ph levels in soil or hydroponic growing mediums naturally fluctuate, and can get out of balance if they are not monitored. Your plants might have the wrong root Ph if they get the following symptoms:

  • Yellow leaves or other odd coloring
  • “Burnt” looking leaf edges
  • Spots, stripes, or patches

Treatment

To treat this issue, you will need to test your soil and water for their Ph levels. Optimal levels are the following:

  • Hydroponic: 5.5-6.0
  • Soil: 6.0-6.5

If a growing medium has the incorrect Ph, soil growers should flush the soil with neutral water. Both hydroponic and soil growers should then begin using water that has the correct Ph levels.

Improper watering

Improper watering is a surprisingly common cause of yellow leaves, especially for newer growers. While growers should both avoid over watering or under watering their plants, overwatering is much more common.

Weed plants need periods of slight dryness for their roots to properly absorb oxygen and stay healthy. If your plants are continually over watered, you may start to notice a yellowing of leaves. Young plants that have not developed their root systems are especially vulnerable to over watering.

Cause and symptoms

This issue is caused when growers don’t maintain a correct watering schedule, or when they provide too much or too little water at each feeding.

Poor drainage in the growing area can also cause your plant to get too much water. If excess water has no place to go, it can build up at the bottom of the plant container and harm your plant.

Finally, it’s possible that your soil is drying out too quickly if you’re using hot grow lights. Excess heat from HID grow lights can dramatically increase your water needs.

Take note if you notice any of the following signs:

  • Your cannabis plant becomes overly droopy. Some droopiness is normal, but too much can be a sign that something’s wrong.
  • “Swollen” looking leaves mean that they could be getting too much water, especially if they swell up right after watering.
  • If the leaves seem to be dry and/or papery, they could need more water

Treatment

If you think that your marijuana plants aren’t getting enough water, first fix the problem by adjusting their watering schedule.

Make sure that you are using a good soil or growing medium that allows drainage. Also make sure that your grow container is the right size for you plant and is allowing water to drain as well.

When watering your plants, only water until you see a small amount of liquid draining out from the pot. Then it is time to stop watering.

To see check if your plant needs watering, stick your finger or another utensil an inch or two into the soil. If the soil is dry an inch or two down, it’s time to water your plant again.

Light burn

Light burn occurs when the grow light being used for cannabis cultivation is too strong for the plants. Grow lights can cause yellow cannabis leaves if they are either too close to the marijuana plants or their wattage is too strong.

Cause and symptoms

Light burn happens because plant leaves are absorbing too much light, not necessarily because they are getting too hot. It can happen a high wattage/high heat HID light, or even cooler LED lights that are still high powered.

Light burn is most common in the flowering stage because the plant stops making new leaves, so any leaves currently there will have more time to accrue damage and turn yellow.

These are the most commons signs for light burn:

  • The leaves that turn yellow are closest to the light
  • The yellow leaves are hard to remove even after they have died

Treatment

This situation can be remedied by moving the grow light further away from the affected plants. If the light has a dimmer switch, this can also be used to reduce the light intensity.

Since cannabis plants generally don’t replace leaves during the flowering stage, you may be stuck with yellow leaves. However, correcting this problem will ensure the issue does not progress.

See our LED grow light article on properly setting up grow lights.

Nutrient deficiency

If you see that your cannabis plants are developing yellow leaves, there’s a good chance that they might have a nutrient deficiency. While there are many different nutrient deficiencies, the three most common are nitrogen deficiency, magnesium deficiency, and iron deficiency.

Nitrogen deficiency

A nitrogen deficiency is by far the most common nutrient deficiency for a plant to have. Plants need lots of nitrogen during their vegetative stage to grow branches, roots and stems. While nitrogen requirements do go down during the flowering phase, plants can still develop deficiencies.

Cause and symptoms

Nitrogen deficiency is caused when growers use the same soil for too long without adding additional fertilizer, as plants will eventually absorb all the available nitrogen. It can also be caused in hydroponic growing if the grower does not provide adequate nutrient solution.

Your plant might have a nitrogen deficiency if:

  • Your plant turns lime green or pale
  • Yellow leaves appear mainly towards the bottom of the plant
  • These leaves are easily pulled off
Treatment

This issue is treated by giving the plants more fertilizer or nutrient solution. You can also repot the plants in fresh soil.

Magnesium deficiency

Magnesium deficiencies are less common than nitrogen deficiencies. This is because most water has magnesium in it, so most plants will get it by default when they are watered.

Cause and Symptoms

Most of the time a magnesium deficiency is caused by incorrect water Ph or by using purified water. Since this nutrient is readily available in water, there are usually additional circumstances such as these that cause the deficiency.

The yellow cannabis leaves caused by a magnesium deficiency look a bit different the yellow leaves caused by other nutrient deficiencies. These are the symptoms:

  • Cannabis plants with a magnesium deficiency will see their outer leaves turn yellow while the leaf veins stay green
  • This will generally occur on leaves all over the plant. If yellow leaves are just at the bottom of the plant, this may not be a magnesium deficiency. If plant leaves that are at the bottom of the plant and not getting light, the plant may let these leaves die as part of its natural process.
Treatment

Check your water Ph, and either stop using filtered water or start giving your plant a Cal Mag supplement to replenish its magnesium.

Iron deficiency

Iron is another mineral that plants should get by default, due to its availability in the water supply. Additionally, cannabis plants have very low iron requirements. A lack of iron is almost always caused by additional factors.

Cause and symptoms

Like magnesium nutrient deficiencies, a lack of iron is almost always caused by using filtered water or using an incorrect water Ph.

Symptoms of a iron deficiency are:

  • Newer growth rather than older growth turns yellow
  • New leaves turn yellow immediately when they come in
  • Leaves may have yellowing on their inner parts, but the outer edges remain green
Treatment

Fix your water ph or stop filtering the water that you give your marijuana plants.

Nutrient burn

Nutrient burn is the opposite of a nutrient deficiency. This issue occurs when plants get too many nutrients, with the most common cause being too much nitrogen.

When a cannabis plant gets too much nitrogen, it will push out it out to the branches, then the leaves, then the tips of the fan leaves. These nutrients will accumulate at the leaf tips, where they will cause the tips and eventually entire leaf to turn yellow and die.

Cause and Symptoms

The most common cause of nutrient burn is cannabis growers fertilizing their plant too much. Marijuana plants will absorb as many nutrients from the ground as possible, so growers need to be careful not to overfertilize.

Cannabis plants in the flowering phase are very vulnerable to nitrogen overload. This is because when the plants begin to bud, they require far less nitrogen than in their vegetative stage.

Even if a grower reduces their fertilizer use during the flowering phase (or switches to bloom booster fertilizers designed for the flowering phase), excess nutrients already in the soil can cause nutrient burn. Therefore, it can be a good idea to flush the soil with Ph neutral water when a plant begins to flower.

The most common symptoms of nutrient burn are:

  • Fan leaves with edges that turn yellow
  • leaf tips that curl or are bent at 90 degrees
  • Fan leaves inner parts turn darker green even though the tips are yellow
  • Stalks or stems that turn deep purple

Treatment

Treatment for nutrient burn consists of a couple of easy steps:

  1. Trim dead and yellow fan leaves away from your plant. These leaves won’t recover. Trim them away so that the plant does waste nutrients and energy on them.
  2. Flush your plant’s growing medium. This will wash away excess nutrients and minerals so that the plant can begin to recover.
  3. Adjust your nutrient levels. Make sure not to overfertilize your plants in the future. If growing your plant hydroponically, make sure to adjust your nutrient solutions accordingly.
  4. Tend to the cannabis roots. Make sure your roots are in good condition by giving them root-specific nutrient solutions to nurse them back to health.

Not enough light for your cannabis plants

Light provides the energy that your plants need to grow and thrive, and they especially need lots of light while they’re growing buds. In fact, it’s one of the most important factors for growing dense buds in the flowering stage.

Causes and symptoms

While this article is mainly focused on the flowering phase, a lack of light usually causes yellowing leaves during the seedling stage. A lack of light can harm flowering cannabis plants as well, but it’s more likely that the leaves will turn dark green due to less photosynthesis using up nutrients.

These are the most common symptoms of a lack of light:

  • Cannabis leaves turning yellow (seedling)
  • Dark green leaves (flowering stage)
  • Lots of spacing between buds

Treatment

To fix this issue, try using a stronger grow light or moving your light closer to your cannabis plants if possible.

Heat/Cold shock

Like all plants, cannabis will not grow well if its too hot or too cold. Cannabis typically likes cool to warm weather, and survives well in temperatures ranging from 60-85 degrees Fahrenheit.

Temperatures outside of this range can harm the plants and cause them to develop yellow leaves.

Cause and symptoms

While plants grown outdoors are at the mercy of mother nature, plant grown indoors usually need some kind of climate control. The most common issue with climate control is that a confined space and warm grow light will cause the area to get too hot. Poor air circulation will add to the problem. Of course, a space can also get too cold depending on the outside climate and heat availability.

Symptoms of heat/cold shock:

  • If the issue is too much heat, the leaves closest to the light will turn yellow
  • Fan leaves will start to curl, forming a “taco” shape due to the heat
  • Try holding your hand where the tops of your plants are and keeping it there for a minute. If the growing environment is too uncomfortable for you, it’s probably too uncomfortable for your plants.

Treatment

This issue can be fixed for indoor growers with proper climate control. The first step is making sure that your grow area has proper ventilation so that your plants can get fresh air. If additional climate control is needed, consider investing in an air conditioner or heater.

Bugs or pests

There are many different bugs and pests our there that would like to make a meal of your marijuana plants.

Bugs and pests can damage your plant and that stress can cause leaves to turn yellow.

Cause and symptoms

Bugs and pests are everywhere outdoors, so if you have an outdoor plant, bugs and pests are inevitable.

For indoor growers, plants may attract bugs if the grow area isn’t well sealed. Otherwise, growers themselves may track the bugs in, especially after visiting another grow site. New plants or seedlings can also bring bugs or bug eggs into a new area. An unclean or poorly ventilated area can make it harder to get rid of an infestation.

Symptoms of a bug or pest problem:

  • Oftentimes, you will be able to see bugs or even eggs on your plant, especially on the fan leaves
  • Yellow leaves combined with spots or bite marks are common

Treatment

Treatment will generally depend on the type of infestation that you have. Things like fungus gnats and spider mites are a very common pest for cannabis to have. If you determine that you have an infestation, it’s best to identify the type and then look up how to get rid of it.

Mold problems

How to Prevent Bud Rot & Mold - Wikileaf

Mold issues are very common in cannabis plants. Cannabis mold generally thrives in hot, humid environments with poor ventilation. It also does well when plants are packed tightly into a space.

Bud rot and root rot are the most common forms of mold that can cause yellow leaves on your plant.

Bud rot

This is a very nasty form of mold that can kill your plant buds from the inside out, making it somewhat hard to detect.

Cause and symptoms of bud rot

Bud rot generally occurs in humid areas with poor ventilation, and often attacks strains with denser buds like indica plants.

Your plant may have bud rot if it has the following symptoms:

  • Yellow leaves are mainly around the buds and usually not elsewhere
  • You see any mold or mildew-like growth at the base of those buds
  • The leaves turn yellow overnight
  • The yellow leaves are very easy to pull out
Treatment

To treat bud rot, first trim and remove all affected parts of the plant so that the mold cannot spread further. Reduce the humidity in your grow area and make sure that it is properly ventilated. Spread out plants and defoliate them if they are very dense.

Root rot

Root rot can occur anywhere there is overly damp soil. Too much water will leave the plant roots unable to absorb the oxygen that they need to stay healthy. These weakened root systems are then vulnerable to invasion from different fungi.

Cause and symptoms

Root rot is caused by growers overwatering their plants. It can also happen if the soil or growing pot don’t allow for proper drainage.

Symptoms of root rot include:

  • Wilted, brown, or yellow leaves
  • A plant decline without obvious reason
  • Thinning canopy
  • Any visible roots look brown or slimy
Treatment

Unfortunately, it’s very difficult to save a plant if it has root rot. Try letting the soil drain and dry. You can also try to feed your plant root nutrients.

Steps to take when your plant’s leaves turn yellow

As we’ve seen, there are many issues that can make your plants leaves turn yellow. Trying to determine the exact cause can be a bit of work. If you see your leaves turning yellow, follow this checklist to try and diagnose the issue.

  1. Check your soil and water Ph. This is the most common issue for yellow leaves.
  2. Make sure that you’re watering your plant correctly.
  3. Determine that your plant’s nutrition levels are optimal
  4. Make sure that your grow light is a proper distance from your plants
  5. Check the heat levels of your growing environment
  6. Looks for bugs or pests
  7. Inspect your buds and roots for mold or mildew

Concluding thoughts

Although cannabis leaves turn yellow naturally during the flowering phase, yellowing leaves can be a symptom for a whole other host of issues. Knowing the symptoms and causes for these issues can help you to properly diagnose and treat your problem.

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