Cannabis plants are highly sensitive to their surroundings and can easily be stressed if the environment doesn’t replicate their natural habitat. A common sign that a plant is not happy is when the leaves begin to curl. Cannabis leaves curling up or down is a sign that something is wrong, the most common causes are underwatering, nutrient deficiencies, pests, disease, or even environmental stress such as temperature.
Identifying the underlying cause and restoring a stable environment for the plant early on is essential to prevent further damage. In this article, we will explore the common reasons why cannabis leaves may curl, how to diagnose the issue, and what steps you can take to address the problem and prevent it from recurring.
What are the causes of cannabis leaves curling?
Cause: Grow room too cold
In the wild, cannabis plants thrive in warm, tropical climates. However, when you grow them indoors, if the temperature and humidity are not close to their natural environment it can stress the plant. One of the common signs of cold stress is that the leaves begin to curl down.
As the temperature drops below the optimal range of 68-77°F, cannabis plants can become stressed. They respond to this by reducing their metabolic activity and diverting their energy towards focusing on survival rather than growth. This causes the leaves to curl down, begin to yellow and become brittle
If you are checking your plants every day the signs are very noticeable. Look for curling leaves that may be turning yellow or brown, and feel for dry or crispy foliage.
How to fix
To repair the damage, it’s essential to provide the plant with a warmer environment, good lighting, make sure the humidity levels are correct. This will allow the plant to recover from the stress, and continue growing.
To prevent this from happening again it is important that you keep checking your grow climate regularly and maintain a consistent temperature and humidity. If you notice any changes to the temperature or humidity use a heater and a dehumidifier to keep it within the ideal range for your plants.
Cause: Excessively hot grow room
Excessively high grow room temperatures can cause heat stress, one of the early signs of this are the leaves curling down. When the temperature in the grow room exceeds 80 degrees Fahrenheit, cannabis plants can become stressed, and the leaves can begin to curl up or down. Other typical signs of cannabis heat stress are the wilting, yellowing or browning of leaves.
How to fix
To reduce the temperature of your grow room open windows, use fans and air conditioning units. Basically use anything you have that will lower the temperature quickly to allow your plants enough time to recover. Once you have the temperature under control use a thermometer to keep on top of the heat in future.
Cause: Light stress
When plants are exposed to high levels of PPFD, which is the intensity of light, they can experience light stress. This is characterized by pale or yellowish-green leaves, and inward curling of leaf edges.
How to fix
To fix this issue, the distance between the light source and the canopy can be adjusted or the power setting on the light can be reduced.
Cause: Overwatering cannabis plants
One of the most common errors made by growers is overwatering. In our constant effort to keep our plants healthy and watered the dangers of overwatering can easily be overlooked. Overwatering cannabis plants can lead to a cascade of other problems, such as root rot, but one of the early signs is drooping or curling leaves.
When soil becomes oversaturated, the roots of cannabis plants struggle to absorb any oxygen or nutrients. As a result, the leaves begin to curl down and wilt.
The signs of overwatered cannabis plants are obvious once you know what you are looking for. Look out for drooping or yellowing leaves, slow growth, and a weak stem. These are all indications that the plant is suffering from too much water.
How to fix
To fix the problem, stop watering the plant and allow the soil to dry out. If the plant is in a pot with very few water holes, gently make some extra holes in the bottom of the pot to improve drainage and allow the soil to dry faster. Be patient and let the soil dry out completely for a couple of days before watering again.
To prevent overwatering in the future, make sure to check the soil regularly and water only when the top inch or two feels dry to the touch. Use a well-draining soil mix and a pot with good drainage holes. Consider using a moisture meter or smart pot to help regulate water levels.
Cause: Underwatered cannabis plants
When a cannabis plant is underwatered, it starts to conserve water by closing its stomata (the tiny pores on leaves that allow for gas exchange) to reduce transpiration, which leads to reduced photosynthesis and growth. As a result, the plant’s leaves begin to curl inward or downward as a response to water stress. This is the plant’s attempt to reduce the surface area of its leaves and minimize water loss through transpiration. Another clear sign of underwatering is that it causes the leaves to become dry, brittle and they begin to turn yellow.
How to fix
To help an underwatered cannabis plant recover, it is important to rehydrate the soil and provide adequate water to the roots of the plant. Here are the steps to follow:
Water the plant: Slowly and evenly water the plant until the soil is moist but not waterlogged. It is important to water the plant deeply, ensuring that the water reaches the root zone so keep watering the plant until the water begins to drain from the bottom of the pot.
Monitor the plant: After watering, monitor the plant closely for the next few days to ensure that it is recovering. Look for signs of new growth, healthy leaves, and a return to normal color.
Adjust watering schedule: Adjust the watering schedule to ensure that the plant receives enough water on a regular basis. This may require more frequent watering or adjusting the amount of water given at each watering. Make sure you keep feeling the surface of the soil, if the top 1-2cm feels dry then you can water it again.
Cause: Over feeding cannabis plants
Overfeeding cannabis plants causes nutrient burn and one of the early signs of this can be curled leaves. When the roots of a plant are unable to absorb the nutrients, they will build up in the soil, causing the plant to become oversaturated. This will first be visible from the tips of the leaves as they turn yellow and the edges become brown and crispy. The leaves may also curl up or down or show signs of twisting.
To fix the problem, flush your cannabis plants with plain, pH-balanced water, this will begin the process of removing all of the excess nutrients left in the soul. You will need to flush them until the water begins to drain out, repeat this process a few times until you begin to see the plant to return to normal health. To prevent this from happening again use a nutrient schedule and measure nutrients carefully to ensure that you are giving your plant the correct dose of macronutrients and micronutrients.
High Nitrogen levels
Leaf curling caused by overfeeding is often because of high nitrogen levels. This can happen if you miscalculate your nutrients, or because there are excessively high nitrogen levels in your soil which sometimes happens with organic fertilizers such as bat guano or compost.
How do you fix high nitrogen levels in soil?
To fix high nitrogen levels you need to add a layer of mulch on top of the soil. Mulch naturally breaks down nitrogen and uses it up so it’s a great simple and effective way of fixing the problem. A layer of sawdust or wood chippings will help to soak up excess nitrogen and reset the soil to its correct nutrient levels.
Cause: Poor root health and dense soils
Good root health is vital if you want to keep your plants healthy and happy throughout your grow. The root system is the foundation of the plant, that delivers nutrients and water to the leaves, branches and flowers. If they become damaged or diseased, this hinders the plant’s ability to absorb nutrients resulting in stunted growth. One of the early signs of root issues could be if the leaves of your plants are curling down.
The difficulty with root problems is unless you are growing with hydroponics it’s difficult to carry out a visual health check on your roots. So, it’s always best to start off with a good soil and make sure that it is airy, not too dense and provides good drainage. If you find your soil is too dense, you can amend it with soil additives such as perlite and vermiculite.
If you are growing with hydroponics then to spot this issue, check the roots of the plant for signs of poor health and look out for discoloration, rotting, or a foul odor.
Is it normal for leaves to curl during the flowering stage?
It is completely natural to see some slight leaf curling in cannabis plants during the flowering stage, this is just the plants response to the changing environmental conditions. During the flowering stage, cannabis plants undergo significant changes to their growth and development, diverting more energy and resources away from their leaves to produce their buds, this can cause the leaves to yellow and curl slightly.
However, it is important to note that excessive or severe leaf curling can be a sign of other issues, such as overwatering, nutrient deficiencies, or cannabis pest infestations.