What are the causes of cannabis leaves curling up or down?

Cannabis plants are delicate and can be easily stressed. One common sign of this is when the leaves begin to curl up or down. When this happens it indicates one of a number of issues including, nutrient deficiencies, over or under watering, high temperatures, or other environmental stressors. Leaf curling can happen at any stage of growth, but it can easily be fixed if you act fast. In this article, we will explore some of the common reasons why cannabis leaves curl down, how to diagnose the problem, and what steps you can take to reverse any damage.

Leaves curling up or down can be for lots of different reasons from stress, nutrient problems, root issues or environmental stressors. To fix the problem, you must first identify what you think is the main cause of the problem. Have a read through the main causes to curling leaves and choose the most likely cause for your situation.

Leaves curling due to a cold climate

Cause: Temperature below 65°F

Cannabis plants are sensitive to temperature changes, and colder climates can cause the leaves to curl down. When the temperature drops below the optimal range of 68-77°F (during the vegetative stage), it can stress the plant causing it to reduce its metabolic activity. As a result, the leaves begin to curl down and become brittle.

The signs are pretty obvious when this happens so providing you are keeping a close eye on your crop you should be able to spot them. Check for curling leaves or leaves that begin to turn yellow or brown. The leaves may also feel dry or crispy to the touch. This could be caused for a number of reasons so before you do anything, check the temperature in your grow room.

How to cure

To prevent the leaves from curling down in colder climates, it’s crucial to maintain a consistent temperature and humidity level within the growing area. If the temperature has dipped below about 65°F try to increase it so that your grow room falls between 68-77°F and keep an eye both temperature and humidity levels. Consider using a heater to regulate the temperature and a dehumidifier to maintain proper humidity levels.

Leaves curling due to temperature or light stress

Cause: Lights too hot or plants to close to lights

One of the most critical factors to growing healthy cannabis plants is the temperature. Excessively warm grow rooms can cause the leaves of a cannabis plant to curl up or down.

When you are growing indoors, especially when using HID lights that give off a lot of heat, the temperature can easily get out of control if you aren’t keeping tabs on it. When the temperature exceeds the optimal range for cannabis of 68-78 degrees Fahrenheit, it can stress the plant, causing the leaves to curl downwards. 

The signs of heat stress include wilting, yellowing or browning of leaves, and a general slowdown in growth. Leaves can also become brittle and break easily. To fix the problem its simple, you need to lower the temperature in the grow room. This can be achieved through ventilation, air conditioning, or the use of fans.

How to cure

To prevent high temperatures from causing leaf curl, it’s important to monitor the temperature in the grow room regularly. 

  • Place a thermometer or in your grow room and check it daily. 
  • Try growing with less energy intensive lights such as LED’s of CFL’s. LED’s provide just as good light for the plant and use much less energy thus keeping the grow room cooler. 
  • If you are growing in summer, consider putting the lights on at night, and having the dark period during the daytime. Night times are much cooler and so having your lights on during this period will reduce the heat by a few degrees without you needing to do anything. 
  • Invest in a good rotating tower fan for your grow room.

Light stress 

Exposure to high levels of PPFD (photosynthetic photon flux density), which refers to the intensity of light, can lead to light stress. This causes leaves to turn a pale or yellowish color, and  to curl inwards from the edges. This happens because either the light is too powerful or it is too close to the plant. To fix this problem, adjust the distance between the light source and the canopy or reduce the power setting on the light.

Leaves curling due to over feeding

Cause: Feeding too many nutrients

Cannabis plants require a careful balance of nutrients and can easily be damaged or stressed if you get the measurements wrong. Overfeeding, or providing too many nutrients, can lead to a range of problems, including nutrient burn, curled leaves and stunted growth.

When the soil becomes oversaturated with nutrients, roots of the plant are unable to absorb them all. This leads to nutrient burn and which can cause the leaves of the plant to curl down. 

The early signs over feeding cannabis plants are easy to spot. Look for leaves that are yellowing or have brown tips, as well as leaves that are curling or twisting. The plant may also have slowed or stunted growth.

How to cure

To fix the problem, flush the soil with plain, pH-balanced water to remove excess nutrients. This involves thoroughly watering the plant until water runs out the drainage holes, then repeating the process a few times. This will remove any built up nutrients and left in the soil

To prevent overfeeding in the future, use a nutrient schedule and measure nutrients carefully, ensuring that the plant is receiving the right balance of macronutrients and micronutrients. Also keep a close eye on the pH of the soil, remember that without the right pH the plant will not be able to absorb the nutrients as effectively. 

More often than not nutrient burn occurs when you are using synthetic nutrients and miscalculate the dosage. Consider using an organic soil mixture that is naturally rich in nutrients with your next grow.

High nitrogen levels

Another possible cause of leaves curling is high nitrogen levels in the soil. Even with organic soil mixes you can experience excess nitrogen, and if you do the culprit is usually compost, if you are using liquid nutrients then the cause is usually incorrect nutrient calculation. To address this issue, add a layer of mulch on top of the soil. Mulch breaks down nitrogen and helps to regulate nutrient levels in the soil. Applying a layer of sawdust or wood chippings can help absorb the excess nitrogen and restore the soil’s balance.

Leaves curling due to overwatering 

Cause: To much water or poor drainage – cannabis leaves begin to droop rather than curl

Overwatering cannabis is probably the most common mistakes that growers make and is one of the most likely reason that the leaves of your plant might be curling down. If a plant is overwatered, the cells of the plant swell and the leaves droop down. If over watering is not rectified quickly this can soon lead to water logged soil which leads poor root health and a much more difficult problem to solve. When there is too much water in the soil, the plant may try to compensate by reducing its water uptake, leading to the leaves curling down.

How to cure

Fixing overwatered cannabis depends on the severity of the situation, but generally involves improving drainage and allowing the soil to dry out. Here are some steps to follow:

  • Stop watering the plant immediately.
  • Place the plant in a dry, warm area with good air circulation to allow the soil to dry out. Avoid direct sunlight.
  • Allow at least 2-3 days for the soil to fully dry out and for oxygen to get to the roots. 
  • Once the soil is dry, resume watering the plant, but be sure to water only when the top inch or two of soil feels dry to the touch.
  • If the plant shows signs of stress, such as yellowing or wilting leaves, consider adding a diluted solution of fertilizer to help the plant recover.

How to prevent overwatering

To prevent over watering in the future, read our guide on how to correctly water your cannabis plants. There are some hints and tips that could save you from making the same mistake twice, such as using smart pots. Smart pots, are plant pots made of breathable fabric and are designed to prevent overwatering by allowing for better drainage and aeration. Unlike traditional plastic pots, smart pots are made from a porous fabric that allows water to drain straight through them. This makes is almost impossible to over water your plants and prevents water from pooling at the bottom of the pot and leading to root rot. Smart pots also help to regulate soil temperature, preventing the roots from getting too hot or too cold. I would highly recommend them to any new growers as they make keeping your plants healthy a whole lot easier. 

Leaves curling due to root issues

Root problems can also cause of cannabis leaves curling up or down. The roots of a cannabis plant are like its veins and are responsible for absorbing nutrients and water from the soil and delivering them to the body of the plant. When there is a problem with the roots the result is usually nutrient deficiencies which leads to stunted growth along with a whole host of other issues.

If the root system of the plant is damaged or diseased, it can prevent the plant from absorbing water and nutrients. This causes stress to the plant which is visible from the way that the leaves begin curl down. This can also happen if the plant is overwatered or underwatered, so don’t immediately jump to the conclusion that it is the roots. 

How to prevent:

To prevent root-related issues from causing cannabis leaves to curl down, it’s essential to maintain a healthy growing environment for the plant. This includes providing proper drainage for excess water, avoiding over-watering or under-watering, and ensuring adequate nutrition levels. Using a soil mix with good drainage and providing adequate oxygen to the roots through air pruning or other techniques can also help to prevent root rot and other issues.

Cannabis leaves curling down during flowering

Yes, it is normal for cannabis leaves to curl down slightly during the flowering stage. This is because the plant diverts a lot of its energy towards producing buds, which causes the leaves to curl down or droop.

However, cannabis leaves can also start to curl down, due to over or under-watering, nutrient deficiencies, heat stress, and root problems. If you notice that your cannabis leaves are curling down excessively, you may want to investigate these other potential causes and make adjustments to keep your plants healthy.

What are other signs of poor health to watch out for?

There are several signs of poor health that may be visible on cannabis leaves, including:

Discoloration – leaves may be yellowing, browning, or showing other abnormal colors.

Spots – leaves may have spots or lesions on them.

Wilting – leaves may be drooping or wilting, even when well-watered.

Deformities – leaves may be misshapen or have unusual growth patterns.

Stunted growth – plants may be smaller than expected for their age, or they may be growing slowly.

Burned or damaged edges – leaves may have brown, crispy edges, which can be a sign of nutrient burn or heat stress.

If you notice any of these signs on your cannabis leaves, it’s important to investigate the cause and address it promptly to prevent further damage to your plants. Common causes of poor health in cannabis plants include over or under watering, nutrient deficiencies or imbalances, pest or disease problems, pH imbalances, heat or cold stress, and light burn.

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