A guide to reading your cannabis leaves and treating your plant’s health issues.

Contents:

Plant diseases, nutrient deficiencies, pest infestations, and many other growing problems display clear symptoms in the leaves of the cannabis plant. In this guide, we cover the various plant issues that can manifest in cannabis leaves. We’ll let you know how to read the signs so you can quickly identify and fix the most common cannabis growing problems.

FUNCTIONS OF THE WEED LEAF

As it is with many other plants, leaves are key components of a cannabis plant’s life support system. The green pigment chlorophyll allows leaves to act as solar panels. This sunlight-gathering role, as you probably know, is essential to photosynthesis.

The underside of the leaves are covered in tiny stomata, microscopic holes that open and close like a door. Carbon dioxide goes in, oxygen and water go out. The leaves can also absorb nutrients to feed the cannabis plant in a process known as foliar feeding.

TYPES OF CANNABIS LEAVES

There are three main types of cannabis. These are often officially lumped together under the name Cannabis sativa L.; for practical purposes, though, it helps to make distinctions between sativa, indica, and ruderalis plants.

That being said, most cannabis you encounter these days is a hybrid of two or three of these types. Thus, what you will typically see in your grow room are weed leaves that express a mix of traits. There can be 3, 5, 6, 9, or 11-point leaves, and they come in shapes ranging from thin and slender to wide and round.

Sativa Cannabis LeafSATIVA

Sativa leaves are long and slender-fingered, with some developing as many as 13 fingers. Usually, sativa plants will have a lighter, lime green shade, indicating a relatively low amount of chlorophyll. It is believed that reduced chlorophyll is partly responsible for the longer flowering period of sativa strains.

Indica Cannabis LeafINDICA

Indica leaves are short and wide, typically with 7–9 fat fingers. These leaves are even larger when they belong to the heavier indicas of Afghan origin. Healthy indica leaves are marked by their darker, deeper shade of green. This is a sign of the leaves containing more chlorophyll, which is believed to accelerate the bloom cycle of indica varieties.

Ruderalis Cannabis LeafRUDERALIS


Ruderalis leaves are quite thin and only develop 3–5 slender fingers. Most growers describe them as comparable to the leaves of young sativa plants, both in shape and colour. These plants are special, though, as they have evolved to flower independent of the hours of light they receive.

FAN AND SUGAR LEAVES

Aside from the leaf types corresponding to sativa, indica, or ruderalis cannabis, we can also differentiate leaves depending on where they’re found on the plant. The largest leaves on the cannabis plant, with the typical fingered shape, are called fan leaves. The other type of leaves, which are small and nestled within flowers, are called sugar leaves.

FAN LEAVES

The fan leaves, as we mentioned, are the large leaves that develop during the vegetative growth phase. They function like solar panels, absorbing light and converting it into energy for the plant to grow. These leaves can also serve as emergency storage for certain nutrients like nitrogen. If the plant can’t get them from the soil, it can draw stored nutrients from the leaves. When this happens, as we’ll explain in detail soon, the leaves will start to turn yellow.

The fan leaves contain only trace amounts of THC, CBD, and other cannabinoids.

Fan leaves

SUGAR LEAVES

The sugar leaves can be found nestled within and extending from the buds. Their surface is covered by a “frosting” of white trichomes, which is the inspiration for their name. Essentially, the function of sugar leaves is to give structure to the buds so they stay together.

Sugar leaves are rich in cannabinoid-loaded trichomes, but can introduce a harsh taste when smoked. As a result, cultivators normally trim them from the buds. Although less optimal for smoking, sugar leaves are ideal for making hash or cannabutter.

Sugar leaves

READING THE SIGNS OF CANNABIS PLANT HEALTH ISSUES

What sets experienced cultivators apart from beginners is the former’s ability to “read” their cannabis plants. Leaves tend to send strong signals to the cultivator, informing them about the health and well-being of the plant. In order to fix growing troubles quickly, it’s important for the cultivator to know how to read the signs.

YELLOW CANNABIS LEAVES

Leaves turning yellow is usually a sure sign that something is wrong with your cannabis plant. This can happen for various reasons, ranging from diseases and pests to problems with nutrients. Note, however, that yellow leaves in the late weeks of flowering are normal. This is the only time you don’t need to worry about it.


OVER - FERTILISATION

The introduction of too much fertiliser will often cause cannabis leaves to turn yellow. Fortunately, this issue is relatively easy to spot: The leaves will first turn yellow or brown at the tip only, a typical sign of a nutrient burn. Only in the later stages of over-fertilisation will the whole leaf turn yellow.

How to prevent:

• Stick to appropriate nutrient levels. Seedlings and young plants require very little fertiliser.
• Be careful when using pre-made potting mixes. These often contain nutrients for 3–4 weeks. Only start feeding when the nutrients in the soil are depleted.
• Feed less than manufacturer-recommended amounts. Start with ½ or ¼ and work your way up as necessary.

How to fix:

• Reduce feeding amount and frequency.
• “Flush” your cannabis to wash out excessive nutrients that may have built up in the soil.
• After flushing, feed appropriate nutrient levels.


OVERWATERING

When cannabis is overwatered, it can lead to all kinds of growing problems. The roots suffocate and can’t access oxygen that the plant needs, and mould, fungus, and pests are more likely to appear. Many times, the leaves will turn yellow during the later stages of continued overwatering.

How to prevent:

• Only water when necessary, not using too much.
• Water according to the size of your plants. Young plants with a small root system don’t drink as much as large plants.

How to fix:

• Stop watering and allow the soil to dry out.
• Adjust your watering regimen, watering less frequently.


NUTRIENT DEFICIENCIES

Nutrient deficiencies, nitrogen deficiency in particular, are one of the most common reasons leaves turn yellow. When lacking in the substrate, the plant draws nutrients from the leaves, turning them yellow in the process.

Know that a nutrient deficiency doesn’t necessarily mean you aren’t feeding your plants enough. Many times, a nutrient deficiency is rooted in incorrect water pH levels, or even from overfeeding. When the plant cannot access nutrients, even if they are present, this is known as nutrient lockout.

How to prevent:

• Make sure to give adequate nutrient levels based on stage of growth.
• Ensure correct pH levels in your water/nutrient solution (soil: 6.0–7.0, hydro: 5.5–6.5).

How to fix:

• Administer required nutrients if deficiency is due to underfeeding.
• Provide nutrients via foliar feeding for faster remedy.
• If deficiency is from incorrect pH or nutrient lockout from overfeeding, you need to address these first. In that situation, flush your plant with plain pH-balanced water, then give appropriate nutrients.


PESTS

Common pests, such as fungus gnats or mites, can lead to cannabis leaves turning yellow. Make sure you know which is on your plants, though, as they cause different issues and require different solutions.

How to prevent:

• Keep an eye on your cannabis to spot a pest infestation early. Use a loupe or a microscope to check on your leaves.
• Use neem oil or slug traps to reduce the risk of pest infestations
• Sterilise your soil if you’re getting it from unknown sources.
• Use beneficial insects (predatory mites, ladybugs, etc.) to keep pests away.

How to fix:

• Use foliar spraying, insecticidal soap, and neem oil to get rid of common cannabis pests naturally.
• Fungus gnats are a sign of high humidity and overwatering. Water less and allow the soil to dry out. Then, cover the soil with sand or perlite to prevent the fungus gnats from coming again.


ROOT ROT

Root rot, as you can tell from the name, is a serious condition that affects the foundation of your cannabis plant. Most of the time, it is caused by a lack of oxygen in the root zone due to overwatering. High humidity and temperatures in the grow room could also be causes for root rot, along with contamination of the growing area (or water reservoir) by harmful bacteria or fungus.

When leaves turn yellow from root rot, it will usually take some time to manifest fully. It may present itself as large, irregular yellow patches, or it may only affect the edges. Leaves will also crumble and wilt at the same time.

How to prevent:

• Don’t overwater your cannabis plants.
• Ensure adequate drainage in your planting pots.
• Keep the temperature in your grow room cool and humidity in check.
• Keep your grow area clean and sterile.

How to fix:

• When growing in soil, repot into planters with fresh soil.
• Provide oxygen to the roots. H₂O₂ added to water might help.
• Add beneficial root bacteria to your water.
• If growing in hydro, use a more powerful pump that can supply more oxygen.


PH FLUCTUATIONS

Sometimes, as we mentioned before, yellow leaves can be a result of pH fluctuations around the roots of your cannabis plant. This will stress out the plant and cause spots to appear. Most of the time, the yellowish, brownish part will be in the middle of lower, older leaves.

How to prevent:

• Ensure your water/nutrient solution has the correct pH. For soil, the optimal pH is 6.0–7.0. For hydro, ensure pH is 5.5–6.5.

How to fix:

• Flush your plant with plain, pH-balanced water.
• After flushing, make sure your water/nutrient solution always has the correct pH level when feeding.


LEAF SEPTORIA

Leaf septoria, also known as septoria leaf spot or yellow leaf spot, shows as yellow-brown spots on the upper and lower sides of the leaves. It is a fungal disease brought on by spores, and is particularly prevalent in wet and humid conditions.

How to prevent:

• Control humidity levels and temperatures in your grow room.
• Ensure good air circulation.
• Keep your grow room clean. Remove dead foliage and weeds, as these are breeding grounds for fungi.
• Prune your plants to improve air circulation.
• Don’t reuse old substrate, as this can contain fungus and other pathogens.

How to fix:

• Remove infected foliage.
• If possible, separate infected plants from others.
• Improve air circulation in your grow room. Have fans blow a gentle breeze across your plants.

CURLY LEAVES

The leaves of your cannabis can curl for a variety of reasons. They may bend downwards or upwards, roll up on the edges, or curl and crumble in irregular ways. These are the most common causes of curly leaves:


HEAT STRESS

Heat can dry out the leaves and make them curl. In a typical indoor grow setting, heat stress is due to plants being too close to the grow lights.

How to prevent:

• Keep your plants at a safe distance from your grow lights. 
• Keep plants away from heaters and other electrical equipment in the grow room.
• If growing outdoors in a hot environment, ensure your plants are shaded from the sun during the hottest time of the day.

How to fix:

• Increase distance between your lights and plants.
• Use cooler-running grow lights, such as LED or CFL.


OVERWATERING

When leaves curl due to overwatering, your weed plant will take on a “heavy” appearance, as if something is bogging it down. The leaves will bend downward in an arc and make an “eagle claw”.

How to prevent:

• Water only as much as necessary.
• Allow the soil to dry out between waterings.
• Lift the pot and check its weight. Water only when the pot is light.
• Ensure good drainage in the pot.

How to fix:

• Cut down your water usage.
• Adjust your watering schedule (read: do it less often).


OVER-FERTILISATION

The first sign of overfeeding, or nutrient burn, is a tiny crispy tip on the leaves. If overfeeding continues, the entire leaf will turn crispy and brown in turn.

How to prevent:

• Ensure you’re only feeding the plant the required amount of nutrients. 
• Do not feed seedlings and very young plants like they’re fully grown.
• Give less than the recommended dose. Less is often better! Start with ½ or less of what the manufacturer says, especially when growing autoflowering strains and other small varieties.

How to fix:

• Flush your plants with plain, pH-balanced water. 
• After flushing, only feed the plant the required amount of nutrients.


COLD

Sometimes cannabis leaves can curl due to cold temperatures.

How to prevent:

• Ensure proper temperatures in your grow room (about 21°C is best).
• Stick with cold-resistant cannabis strains.

How to fix:

• Use a powerful grow light to increase temperatures in your growing area.
• If possible, move your plants to a spot where they are better protected from wind and weather.
• Instal a heating system in your growing area.

DRY AND CRISPY

In certain cases, if your plant goes without adjustments after the leaves start to curl, they could start to get dry and crispy. As many owners of house plants know, this is a sign that immediate treatment is needed.

HEAT STRESS

As we mentioned before, this usually just means your plants are too close to your grow lights. However, heat stress can also occur if you’re growing outdoors, or in a greenhouse in hot temperatures. The leaves, as we’ll discuss in a bit, can get dry and crispy in the late stages of heat stress.

How to prevent:

• Use LEDs or CFLs instead of HID lighting.
• If growing outdoors, use shading to protect your plants.
• Ensure adequate air circulation. If you’re inside, instal fans.

How to fix:

• Move your plants further from your grow lights.
• Lower temperatures in your growing area. 
• If needed, instal air conditioning in your grow room.


LIGHT BURN

Light burn is often preceded by bleaching, during which the leaves turn white. Sometimes, however, light burn has already occurred by the time bleaching starts. If light and heat issues aren’t addressed, the leaves will become crispy and die. This one is rather easy to spot, as light burn typically appears on the leaves closest to the light source.

How to prevent:

• Keep plants at a safe distance from your grow lights. 
• Consult manufacturer for the recommended hanging distance.

How to fix:

• Reduce light intensity (dimmer).
• Increase distance from your plants to your grow lights.


OVER-FERTILISATION

Starting with dried-out leaf tips, prolonged nutrient burn will manifest as whole leaves becoming dry and crispy. This is the last stage of over-fertilisation. The leaves, and therefore your cannabis plant, are dying. That doesn’t mean you can’t save them, though. 

How to prevent:

• Use less nutrients, especially if you’re growing autoflowering plants. Use ½ or ¼ of the recommended dose.
• Do not feed seedlings. Only start feeding once your plant has 4–5 real cannabis leaves, or when your plant has reached a height of 15–20cm.
• If you’re using pre-fertilised soil, don’t feed in the first 3–4 weeks. After that time, start feeding a low dose of nutrients.

How to fix:

• Flush your plants with plain pH-balanced water.
• Give adequate nutrient levels according to your plant’s size and age. Keep feeding at the low end and increase only if necessary.

WILTING LEAVES

Wilting leaves, just like discoloured and spotted leaves, can be caused by many things. Here are the most common issues cultivators should be aware of:


UNDERWATERING

The main sign of underwatering is a drooping plant with wilting leaves and branches. If the plant doesn’t get water after that point, the leaves will start to dry out.

How to prevent:

• Make sure your plants get adequate amounts of water.
• Water at the first sign of plants wilting.

How to fix:

• Give sufficient water so the entire substrate is saturated. Ideally, wait until you see about 20–25% runoff.


WIND BURN

This is more of an issue if you’re growing outdoors with your plant exposed to the elements. However, it can also happen indoors when your plants are too close to a powerful fan.

How to prevent:

• Ensure an adequate distance between your plants and the fans in your growing area.
• Don’t run fans at full blast. A gentle breeze is optimal.
• When growing outdoors, choose an area where your plant isn’t exposed to the elements. 
• Plant among larger companion plants for natural protection.

How to fix:

• Decrease fan speed or move fans further away from your plants. 
• When growing outdoors, erect wind shading to protect your plants.


VERTICILLIUM

Verticillium wilt is a serious condition caused by fungus that can reside in soil. In short, it attacks the roots of your cannabis, leaving them near death with no possible cure. It presents as a yellowing and shrivelling of the lower leaves, followed by parts of the plant suddenly wilting. The best thing you can do is start with preventative measures; that way, it doesn’t have a chance to devastate your crop.

How to prevent:

• Use only fresh and sterile soil.
• Avoid hot and humid conditions, as they promote fungus growth.
• Ensure good drainage when growing in soil.
• Consider growing soilless instead (coco, hydroponics…)

How to fix:

• Remove infected plants and soil.
• Separate healthy plants and hope the fungus hasn’t spread.

COLOURED LEAVES

Cannabis leaves usually come in shades of green, ranging from light lime green to deeper forest hues. Sometimes, though, they can also turn other colours, depending on the strain and certain environmental factors. In most cases, you have no need to worry! These colours add to the bag appeal of a strain and are highly prized by cannabis enthusiasts.

However, there are some cases where leaves turning a different colour may be a sign of a serious problem.


NITROGEN TOXICITY

Overfeeding can cause various changes to the leaves, but nitrogen toxicity specifically will turn the leaves a very dark green. This could happen if you use a fertiliser with ample nitrogen for the vegetative phase and forget to switch it out during flowering.

How to prevent:

• Don’t overfeed your cannabis. Specifically, keep an eye on nitrogen levels.
• Use the correct nutrients depending on the growing stage (growth, flowering) of your plants.

How to fix:

• Decrease the amount of nutrients you feed your plants.
• Use a different cannabis fertiliser with less nitrogen.


PURPLE LEAVES ARE OK

Reddish and purple leaves are usually nothing to worry about. In fact, some strains are bred to display beautiful purple leaves and buds. Some strains may show the usual green colours at first, but can turn red or purple with colder night time temperatures.


WHITE LEAVES

The leaves of some cannabis strains have a sparkling white appearance due to layers of milky trichomes covering them. In those cases, it’s a sign of good-quality bud. However, there are also cases where white leaves are due to something serious.


POWDERY MILDEW

Powdery mildew is a layer of white, powdery mould on the leaves of cannabis. It may appear as white spots or bumps, and it may look like patches of flour on the leaves. If left untreated, powdery mildew can cause serious damage to your plants.

How to prevent:

• Maintain cool temperatures in the grow room.
• Ensure good air circulation.
• Monitor humidity levels to avoid wet and humid conditions.
• Keep the growing area clean.

How to fix:

• Remove infected plants and growing medium.
• Foliar spray with two teaspoons of cider vinegar to 1 litre of water. Alternatively, mix 60% milk and 40% water to spray your plants. Use commercial fungicide as a last resort.

HOLES AND DISCOLOURATION

If you spot holes in the leaves, or notice irregular spots and leaf discolouration, pests may be your problem. A keen eye, along with a jeweller’s loupe, will help you spot them before they can do more damage.


LEAF MINER LARVAE

Leaf miner larvae “tunnel” through the leaves, leaving characteristic yellowish or white marks resembling the shape of a worm.

How to prevent:

• Row covers prevent adult leaf miners from getting to your plants.
• Sticky traps catch them as well.

How to fix:

• Apply foliar spray of neem oil and/or insecticidal soap to affected areas.


CATERPILLARS

Caterpillars, as they do with other plants, will eat large holes in your buds and leaves. 

How to prevent:

• Spray plants with neem oil.
• Use parasitic wasps or praying mantis to keep caterpillars at bay.

How to fix:

• Remove caterpillars by hand.
• Use Bacillus thuringiensis to naturally get rid of caterpillars.

THRIPS

Thrips are small dark or yellowish insects. The larvae, or nymphs, may look like tiny worms. They feed on chlorophyll and cause extensive irregular yellow/white spotting on cannabis leaves.

How to prevent:

• Foliar spray with neem oil.

How to fix:

• Insecticidal soap.


APHIDS

Along with thrips, aphids are the most common cannabis pests. These tiny sap-sucking insects come in various colours, and are some of the most destructive cannabis pests out there. You’ll need to use a jeweller’s loupe to see them! Signs of an infestation include yellowing, spotting, and speckles on the leaves.

How to prevent:

• Foliar spray with neem oil.
• Introduce ladybugs (they feed on the larvae).

How to fix:

• Spray insecticidal soap. Repeat once every 2–3 days until infestation is under control.


REVEGETATION

When the flowering period of photoperiod cannabis is interrupted or halted, usually due to light issues, it reverts back to the vegetative stage. This “re-vegging” causes significant plant stress and leads to odd growing patterns with unusual leaf shapes.

Revegetation can happen by accident, but it can also be intended. A technique called monster cropping, for instance, involves re-vegging plants for a second harvest.

How to prevent:

• Ensure an uninterrupted dark period of 12 hours for flowering cannabis. Make sure to prevent stray light from street lamps, etc.

How to fix:

• Correct the light schedule for your flowering cannabis.
• Alternatively, allow the plant to revegetate and have it re-flower like normal at a later time.

WEIRD PATTERNS AND PATCHES

There can be various other reasons why cannabis leaves show unusual patterns instead of an even green colour.


TOBACCO MOSAIC VIRUS

Chief among these reasons, according to some, is the tobacco mosaic virus. It’s a disease sometimes seen in tomatoes and other vegetable crops, but opinions differ on whether TMV can infect cannabis.

How to prevent:

• Keep the growing area clean.
• If you’re smoking tobacco, wash your hands before entering your grow room. Preferably, change your shoes and clothes as well.

How to fix:

• Discard infected plants and seeds.
• Sanitise your entire growing area, along with all equipment (including pots).
• Start a new grow in a clean and disinfected environment.


MUTATIONS

Cannabis, like other plants, can display mutations on the leaves. Some of these mutations are initiated by breeders to better camouflage the plant or make it less identifiable as marijuana; other times, mutations are a minor defect inherent to some strains.

Apart from the common, gnarly-looking sets of leaves that appear on young plants, widespread leaf mutations, like even numbers of fingers or ugly deformities, are very rare. Excessive mutations are indicative of bunk genetics and poor breeding practices.


ALBINO

Albinism is an extremely rare mutation in cannabis. With white leaves and white buds, these plants are mostly a curious novelty. Rather than peculiar appearances, growers are usually more concerned with bud quality, taste, and potency.


LEAF BUDS

This is a rare mutation where the buds don’t emerge at the nodes where the stalks originate. Instead, they grow directly on the centre of the leaves. As these “leaf buds” are tiny, most cultivators remove them before they take up too much nutrients.

How to prevent:

• Avoid poor genetics. Get your cannabis seeds from a reliable source.
• Avoid stressing out plants (excess heat and light, re-vegging, pests, harsh conditions).
• Grow in a controlled environment to limit plant stress.

FOLIAR SPRAYING

Some cannabis issues, as we’ve mentioned here and there, can be fixed with foliar spraying. The spray makes direct contact with the leaves themselves, making it the quickest way to provide nutrients to cannabis plants.

WHAT CAN BE DONE WITH CANNABIS FAN LEAVES AND SUGAR LEAVES?

So, what use do the leaves have beyond indicating disease?

Well, although the fan leaves contain only trace amounts of THC or CBD, there are several ways they can be effectively repurposed. To start, cannabis fan leaves can be juiced alongside other healthy ingredients to take advantage of a host of nutrients and cannabinoid acids. While the fan leaves won’t get you high, they will contribute to a healthy diet. Alternatively, for a super-light buzz, you can make fan leaf tea. Add a little coconut oil to allow the cannabinoids to bind to the fat, and you’ll have a nice soothing beverage to enjoy.

The fan leaves are also ideal for making cannabis ointments and salves. If you have enough leaves, you can successfully extract the cannabinoids and filter out excess plant matter to create your own homemade skincare line. Finally, fan leaves can simply be added to a compost pile to support your next grow. After all, what’s better than growing cannabis with cannabis leaf compost?

And don’t forget; sugar leaves can be put to good use too! Although they’re widely considered too harsh for smoking, their heavy trichome load makes them perfect for rendering into hash or cannabutter. Unlike fan leaves, sugar leaves are more packed with cannabinoids, so you can achieve quite the buzz off sugar leaf hash if you have enough raw material.

Essentially, all parts of the cannabis plant can be put to good use. No need to waste anything!

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