By Luke Sumpter


Key Points

  • Cannabis leaves curl up or down when faced with specific environmental stressors.
  • Pests, diseases, temperature, humidity, and other factors can cause leaf curling.
  • Learn to prevent leaf curling by properly managing your growing environment and plant feeding schedule.
  • Discover how to treat the condition using techniques such as predatory insects and proper watering methods.

Leaves can tell you a lot about the health of your cannabis plants. Waxy, dark green, and turgid leaves are a sign of good health. Curling leaves, however, can signal a range of different problems.

Leaves that curl upwards often indicate issues with light, heat, or humidity, while leaves that curl downwards signal overwatering or nitrogen toxicity.

If you’ve come across curling leaves—don’t fret. There are several steps you can take to remedy them and get your crop back on the right path. Discover them below.

A close-up of a cannabis plant leaf showing signs of stress, including browning and curling. Possible causes include nutrient deficiencies, overwatering, or pest infestation.

What Causes Curling in Cannabis Leaves?

Curling leaves are a stress response. Weed plants become stressed whenever they’re faced with detrimental environmental conditions.

Biotic stressors (living organisms), such as pests and diseases, are a significant source of plant stress. By destroying tissues and sucking out sap, they take a severe toll that sometimes results in curling leaves, among other textbook symptoms.

Abiotic stressors (non-living factors) also impact cannabis plant growth and productivity. This long list of environmental variables includes temperature, humidity, light intensity, overwatering and underwatering, pH, and nutrient levels.

Learning to “read” leaves is an important skill for any cannabis grower. Once you get to grips with this foliar language, you’ll be able to quickly diagnose a range of botanical issues, from nutrient deficiencies and toxicity to different pests and diseases.

  • Why Do Cannabis Leaves Curl Up?

Cannabis leaves curling up occurs as a reaction to specific biotic and abiotic stressors. These include:

  • Heat stress: High temperatures cause marijuana leaves to curl upwards as plants attempt to reduce their surface area to limit exposure to strong light and water loss through transpiration.
  • Low humidity: Weed leaves also curl upwards to minimise water loss through transpiration when exposed to dry air.
  • Light burn: Excessively strong light, which causes light stress, can cause cannabis leaves to curl up.
  • Nutrient imbalance: Both potassium and calcium deficiency contribute to the upward curling of leaves because of their role in water uptake and structural integrity.
Green plant leaf with curled up edges and brown spots, listing causes: heat stress, low humidity, light burn, nutrient imbalance.
  • Why Do Cannabis Leaves Turn Down?

Cannabis leaves curling down is typically the result of the following issues:

  • Nitrogen toxicity: Nitrogen plays a fundamental role in healthy growth, but too much is toxic to plants and causes leaves to become abnormally dark green and to curl downwards.
  • Overwatering: Excessive watering can suffocate roots, and reduced oxygen can cause leaves to droop.
  • Cold temperatures: Chilly temps also cause cannabis leaves to curl down, owing to slowed metabolism.
  • Salt build-up: Too much salt in the soil can cause osmotic stress, resulting in the downward curling of weed leaves.
  • Root rot: Certain fungal infections, such as Pythium, target cannabis roots. Subsequently, impaired nutrient and water uptake can cause leaves to turn downwards.
  • Diseases: Various diseases that target foliage can also cause general wilting and downward curling.
Stressed plant with curling down, discolored leaves. Possible causes: overwatering, nutrient deficiencies, or cold temperatures.

Common Causes of Cannabis Leaf Curling and Their Solutions

So far, we’ve given you a general overview of the factors that cause cannabis leaves to curl up and down. Below, we’re going to take a deeper dive into each of them, including their symptoms and how to fix them.

  • Overwatering

Many novice growers make the mistake of overwatering. Cannabis plants certainly need water to survive and thrive, but applying too much does more harm than good.

Symptoms of Overwatering

The symptoms of overwatering include:

Drooping leaves Leaves curling downwards Yellowing Wilting Slow growth Lesions Pest problems Root rot

How to Fix Overwatering

To fix overwatering, most growers find success by adhering to a simple rule; use your finger to probe the soil, and only water your plants once the top 3 cm has completely dried out.

  • Light Stress

Much like with water, cannabis plants depend on light to grow. However, light sources that are too powerful or positioned too close to the canopy can stress plants out.

Symptoms of Light Stress

The symptoms of light stress include:

Leaves curling

upwards

Bleaching

Yellowing upper

leaves and buds

Stunted bud

growth

Wilting

Fixing Light Stress

To reduce light stress, keep your light source around 35 cm from the top of the canopy at all times. If your light has a dimmer feature, adjust the intensity until you find the sweet spot. Improving airflow and ventilation by installing fans can also help to take the edge off once the previous steps have been taken.

Two potted plants with stress symptoms: the left plant has yellowing leaves from overwatering, while the right plant has small, deformed leaves due to light stress
  • Temperature Extremes

Cannabis plants appreciate the heat and thrive at temperatures of around 21–26°C. Extreme highs and lows in temperature can slow plant growth, damage tissues, and even kill off plants entirely.

Heat Stress in Cannabis Plants

Cannabis plants can handle excessively high temperatures for short periods of time. However, temps of 30°C and above for long periods can cause the following symptoms of heat stress:

Upward-curling leaves Wilting Yellowing or browning leaves Burnt or crispy leaves Stunted growth Reduced flower production Discoloured buds

To avoid heat stress in your weed plants, use a thermo-hygrometer to measure the temperature and humidity of your growing space, and use fans and air conditioners as necessary. Outdoors, shade cloth can help to reduce sun exposure during heatwaves.

  • Cold Stress in Cannabis Plants

Cannabis plants have a good tolerance to cold spells, but extended exposure to temperatures of 15°C and below can cause the following symptoms of cold stress:

Downward-curling

leaves

Slow growth

Yellowing and

browning leaves

Brittle leaves

Poor bud

development

Two potted plants with environmental stress symptoms. Left plant shows discolored leaves from cold exposure, while the right plant has yellowed leaves due to heat stress.
  • Humidity Issues

Humidity—the amount of moisture in the air—is another important variable when growing cannabis. The sweet spot for humidity varies during each stage of the growing cycle:

Symptoms of Low Humidity

When humidity levels drop too low, you’ll encounter these issues:

Leaves curling inwards,

forming a “canoe” shape

Dry, crisp leaves Stunted growth Dry buds Browning leaves

Symptoms of High Humidity

High humidity also has a negative impact on weed plants during different stages of growth. The signs include:

Downward-curling leaves

Collapsing seedlings

(damping off)

Bud rot and mildew Slow bud development

Fixing Humidity Problems

To fix humidity problems, you’ll first need to place a hygrometer in your growing space to get an accurate read of moisture levels. To tackle low humidity, use humidifiers and irrigation. Use fans and dehumidifiers to correct high humidity.

Two indoor plants at different growth stages: a mature plant with a temperature monitor on the left and a younger plant in a humid environment with a humidifier on the right.
  • Pests and Diseases

Both indoor and outdoor cannabis plants are vulnerable to a host of pests and diseases. These organisms cause all kinds of symptoms, including leaf curling.

  • Common Pests That Cause Curling

Some of the most common cannabis pests that cause leaf curling are:

  • Spider mites: These sap-sucking insects leave cobweb-like threads all over buds and cause leaves to curl downwards.
  • Aphids: Also sap-suckers, aphids’ feeding mechanism deprives plants of essential nutrients, causing leaves to curl downwards or inwards.
  • Thrips: In scraping off the outward layer of foliage when feeding, thrips cause cannabis leaves to curl upwards.
  • Whiteflies: These tiny moth-like insects cause leaves to turn downwards and yellow.
  • Fungus gnats: By damaging plant roots, fungus gnats disrupt water and nutrient uptake and cause leaves to curl downwards.
  • How to Treat Pests

Rather than spraying plants with toxic pesticides, there are a range of organic and natural ways to treat pests on weed plants. Check out some of the most effective methods:

  • Neem oil: Containing azadirachtin, neem oil repels insects and inhibits their ability to feed on plants.
  • Insecticidal soaps: These products kill insects by damaging their cell membranes.
  • Predatory insects: Beneficial insects are an effective way of dealing with pest issues; ladybug larvae are capable of eating hundreds of aphids each day.
  • Diseases That Cause Curling Leaves

As well as pests, cannabis plants are susceptible to bacterial, fungal, and viral diseases that cause leaf curling. Some of the most common culprits include:

  • Bacterial blight: Leaves develop necrosis and curl downward.
  • Verticillium wilt: This disease affects stem vasculature and causes leaves to turn downward.
  • Botrytis: Also known as bud rot, this disease causes leaves to curl downward.
  • Tobacco mosaic virus: TMV manifests as patchy discolouration and causes leaves to curl either upwards or downwards.
  • Genetics

Environmental factors are the primary driver of leaf curling. There are no well-documented correlations between genetics and leaf curling in cannabis. However, genetic mutations can give rise to unusual leaf shapes and colours. These include tri-leaf seedlings, variegation, and leaves with complex and chaotic leaflet patterns.

Close-up of a damaged plant leaf with holes and discoloration, likely caused by pests such as insects or disease.

Preventing Cannabis Leaf Curling: Best Practices

Leaf curling isn't just an eyesore, but it indicates stressors that have the potential to impact plant health and productivity. To prevent leaf curling, you need to stay on top of key environmental variables.

  • Environmental Monitoring

Thermo-hygrometers are an indispensable tool in preventing leaf curling, providing real-time data on temperature and humidity.

Considering these figures, you can use fans, humidifiers/dehumidifiers, and air conditioners to keep your plants in their sweet spot. Check your thermo-hygrometer at least once per day and make the necessary changes.

To save work, you can automate your grow using a sensor and controller to adjust lighting, temperature, and humidity accordingly.

  • Routine Inspections

Pay close attention to your plants throughout the entire growing cycle. As well as checking for leaf curling, frequently measure light distance, soil pH, and look for signs of pests and diseases.

  • Balanced Nutrient Regimen

Develop a nutrient regimen to meet the needs of your plants throughout each stage of the growing cycle. Make an effort to stick to it to avoid nutrient deficiency or toxicity.

  • Cleaning and Maintenance

Keep your growing space clean and tidy to prevent pests and diseases that cause leaf curling. Be mindful of bringing infected plant material into the area, and use preventative measures such as sticky traps to prevent insect infestations before they get a foothold.

Avoid Curling Cannabis Leaves for Better Results

The aim of preventing and treating cannabis leaves goes far beyond the leaves themselves. Whether leaves curl up or down, they’re telling you something about the health status of your weed plants.

Now, not only are you aware of what they’re trying to say, but you’re equipped with the necessary steps to remedy each cause. Try your best to prevent leaf curling from happening, and quickly deploy the treatments listed above to bring your plants back to full health.

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Privacy Policy - Royal Queen Seeds

To ensure a safe online environment and guarantee adequate data protection, we strictly comply with all legal requirements. In this privacy statement, we provide information about how and for what purpose data is collected, safety measures, storage periods and contact details.


COMPANY NAME: SNORKEL SPAIN, SL (hereinafter ROYAL QUEEN SEEDS)

C/ Vilar d'Abdelà, 5 (nave 1) CP: 08170 de Montornès del Vallès

+34 937 379 846

support@royalqueenseeds.com


The present Privacy Policy sets out the terms on which we will treat personal data at ROYAL QUEEN SEEDS; this includes any personal data collected through our website https://www.royalqueenseeds.com/ as well as any other data we process in the course of our business activities.

ROYAL QUEEN SEEDS collects the following personal data for the purposes listed below:

SECTION 1 – PERSONAL INFORMATION WE COLLECT

1.1 Account purchases

Account purchases can only be made if you are in possession of a personal account. When you create an account or purchase something from our shop, as part of the buying and selling process we collect the following personal information that you provide to us:

This information is required for delivery. In addition, when you browse our shop, we automatically receive the Internet Protocol (IP) address of your computer. Based on this information, we can optimise your online experience and at the same time protect our online environment.

Purpose of data collection

We collect and store account-related data for the following purposes:

(a) to carry out obligations arising from any contracts between you and us, and to provide you with information, products and services that you may request from us;

(b) to set up, manage and communicate with you about your account and your orders;

(c) to conduct market research and analysis;

(d) to confirm your age and identity, and to detect and prevent fraud.

1.2 Newsletters

With your explicit permission, we may send you newsletters about our shop, new products and other updates. We send newsletters based on your explicit consent. In the event that you purchase a product, and in accordance with current regulations, we may send you commercial communications in accordance with the legitimate interest of our company, always about products or services similar to those you have purchased or contracted. In any case, you may exercise your right of opposition through the channels announced in this Privacy Policy. The following information is collected in relation to the newsletter:

We do not need to know the sex of the person in order to send the newsletter (data minimisation: by law we must ask for data that is strictly necessary to provide the service, and in this case knowing the sex is not necessary to send the newsletter).

Purpose of data collection

The data collected is used to:

(a) personalise our emails, including your name and gender;

(b) provide gender-specific content.

You can withdraw your consent at any time by using the link provided in the newsletter or the contact information provided in section 2.

1.3 Customer service and contact form

In order to provide appropriate support, our customer service employees have access to information related to the account. Consequently, their support will be highly effective and friendly. The data provided in our contact form is used by our CRM provider, SuperOffice. We will only use your details to respond to your message.

SECTION 2 – LEGITIMATE INTEREST

If you have purchased any of our products, please note that we may process your personal data for promotional purposes, based on Royal Queen Seeds' legitimate interest only to offer you products or services from our company and about products or services similar to those you have purchased. You may exercise your right to opt out of future messages by the means set out in this Privacy Policy or through any notification you receive.

2.1 How do you withdraw consent?

If you change your mind, you can withdraw your consent for us to contact you for the purpose of collecting, using, or disclosing your data at any time by reaching out to us at: support@royalqueenseeds.com.

SECTION 3 – DISCLOSURE

We may disclose your personal information if we are required to do so by law or if you breach our Terms of Service.

SECTION 4 – HOW LONG DO WE KEEP YOUR DATA?

At Royal Queen Seeds, we will not retain your data for longer than is necessary for the purposes described in this Policy. Different retention periods apply for different types of data; however, the longest period we will normally hold any personal data is 10 years.

4.1 Account information

Data relating to the account remains relevant for as long as the consumer is in possession of an account. Therefore, the data remains documented for as long as the account exists. When our customers delete an account, the associated data will be deleted within a reasonable period of time. Requests regarding the inspection or correction of stored personal data or the deletion of an account can be sent to support@royalqueenseeds.com.

4.2 Newsletters

In the event that you give us your consent to inform you about our products or services, we will keep your data until you express your wish not to receive any further communications from us. However, we regularly (every month) carry out a relevance check. Registered customers (and their personal information) will be deleted whenever customers do not reply to our request. In addition, our newsletter mailing has an opt-out feature. Consumers can withdraw their consent by using this opt-out feature.

SECTION 5 – COOKIES

Cookies are small information files that notify your computer of previous interactions with our website. These cookies are stored on your hard drive, not on our website. Essentially, when you use our website, your computer displays its cookies to us, informing our site that you have visited before. This allows our website to function more quickly and remember aspects related to your previous visits (such as your username), making your experience more convenient. At Royal Queen Seeds, we use two types of cookies: functional and analytical.

5.1 Functional Cookies

Functional cookies are used to enhance your online experience. Among other things, these cookies track what is added to your shopping cart. The use of these cookies does not require prior authorization.

5.2 Analytical Cookies

Analytical cookies are used for research and market analysis. The data collected with these analytical cookies is anonymous, making it unusable for third parties. The use of these cookies does not require prior authorization.

SECTION 5 – THIRD-PARTY SERVICES

Third-party services are required to conduct transactions and provide our services. In general, the third-party providers we use will only collect, use and disclose your information to the extent necessary to enable them to perform the services they provide to us.

However, certain third-party service providers, such as payment gateways and other payment transaction processors, have their own privacy policies regarding the information we must provide to them for your transactions.

We encourage you to read the privacy policies of these providers so that you can understand how these providers will handle your personal information.

In particular, certain suppliers may be located or have facilities located in a different jurisdiction than yours or ours. Therefore, if you choose to proceed with a transaction involving the services of a third party, your information may be subject to the laws of the jurisdiction in which that service provider or its facilities are located.

Once you leave our website or are redirected to a third-party website or application, you are no longer governed by this Privacy Policy or the Terms of Service of our website.

Web analytics service (anonymous data)

On this website we have integrated an element of a web analytics service (with anonymisation functionality). Web analytics can be defined as the gathering, processing and analysis of data about the behaviour of visitors to websites. An analytics service collects, among other things, data about which website a person came from (the so-called referrer), which sub-pages they visited or how often and for how long they visited a sub-page. Web analytics is mainly used for website optimisation and for a cost–benefit analysis of internet advertising.

Courier service

To complete deliveries we use a courier service. This courier service carries out the delivery between our company and the consumer's home. To complete these logistics, the company requires access to the consumer's name and address information.

Mailing service

Royal Queen Seeds uses a third-party mail service provider to send its newsletter. This provider has access to limited account information related to opt-in consent (e.g. email address).

Marketing services

Royal Queen Seeds has the support of a company that specialises in marketing and communication activities. Their access to personal information is very limited and mostly anonymous.

Payment services

At Royal Queen Seeds we use external payment services to handle our transactions (e.g. credit card payments).

SECTION 6 – SECURITY

To protect your personal information, we take reasonable precautions and follow industry standard best practices to ensure that it is not inappropriately lost, misused, accessed, disclosed, altered or destroyed.

If you provide us with your credit card information, the information is encrypted using secure socket layer technology (SSL) and stored using AES-256 encryption. Although no method of transmission over the internet or electronic storage is 100% secure, we follow all PCI-DSS requirements and implement additional industry standards that are commonly accepted. Information related to the account is protected with a hashing method. This method transforms the information into a generated hash. As a result, confidential information is protected and invisible, even to us. In addition, our databases are exceptionally protected against unauthorised access. For example, access to the database is only possible and permitted via approved IP addresses (e.g. from Royal Queen Seeds headquarters). Other attempts and addresses are rejected at all times.

Furthermore, data is anonymised as much as possible, so it cannot be directly linked to a specific consumer. With this data, however, we may be able to carry out market research and analysis. In addition, the third parties concerned (e.g. mailing service) are examined prior to our collaboration, comply with the GDPR from the EU and receive a processing agreement. Within Royal Queen Seeds, employees are assigned different access permissions. Specific permission provides access only to information that is strictly required to perform a task. Digital security measures are subject to change and must meet high requirements to ensure the safety of online customers. That is why, at Royal Queen Seeds, we appoint a security officer. Regular verification and improvement of security measures (where necessary) are part of the role.

SECTION 7 – CHANGES TO THIS PRIVACY POLICY

We reserve the right to modify this Privacy Policy at any time, so please check back regularly. Changes and clarifications will be effective immediately upon being published on the Website. If we make substantial changes to this policy, we will notify you here that it has been updated so you will know what information we collect, how we use it and under what circumstances, if any, we use and/or disclose it.

SECTION 8 – YOU HAVE THE RIGHT TO: