Understanding The Impact Of Heat Stress On Cannabis Plants: A Crucial Challenge For Growers

As a cannabis grower, you will likely face challenges such as pests, diseases, and unpredictable environmental conditions. However, one particular obstacle that stands out is the issue of heat stress, which can be a nightmare for growers.

Heat stress can significantly affect your plants during their flowering stage. For those involved in cultivating cannabis on a large scale, it becomes more crucial to avoid it completely as it can quickly reduce your profits.

In this article, we will explore everything you should know about heat stress – its causes, symptoms, and the necessary steps to address and resolve this problem.

As the cultivation of cannabis continues to expand and becomes more regulated, it becomes crucial for you to understand the impact that heat stress can have on your plants and learn how to manage its effects.

What is Heat Stress in Cannabis Plants?

It’s easy to grow cannabis even for beginners, but it’s sensitive to environmental factors, which makes it susceptible to heat stress. This condition occurs when the plants are exposed to temperatures that exceed their preferred range.

Typically, cannabis thrives between temperatures of 70°F to 85°F (21°C to 29°C). However, if the temperatures consistently exceed 90°F (32°C), your plants might face the effects of heat stress.

How can Heat Stress Affect Cannabis Plants?

So how does heat stress affect cannabis plants? Well, when cannabis plants are exposed to temperatures beyond the range specified above, they can experience negative impacts.

Here’s how heat stress can impact cannabis plants:

1. Inhibition of Photosynthesis

Firstly, high temperatures can interfere with the process of photosynthesis which is vital for plants to generate energy. Photosynthesis is what allows cannabis to convert sunlight into sugars and oxygen. When a plant encounters difficulties in this process, you can bet that there will be trouble on the horizon.

2. Reduced Growth and Yield

When the temperature rises to uncomfortable levels, it disrupts the process of photosynthesis, which in turn hampers their ability to produce energy for growth and fruiting. As a result, the plants may not grow much and also produce less yields. 

3. Water Loss

Plants release water vapor through openings called stomata on their leaves. This process is called transpiration and is crucial for plants to survive. When exposed to scorching temperatures, the process of transpiration speeds up, causing the plants to lose water at a faster rate than they can absorb from the soil. This excessive water loss leads to wilting, loss of firmness, and dehydration, in the plants. Eventually, they start curling and experiencing cell death (necrosis).

4. Nutrient Uptake Issues

Excessive heat can affect the roots as well and decrease their ability to take in nutrients from the soil. Consequently, this can result in deficiencies, causing the plants to exhibit yellowing leaves or stunted growth.

5. Flowering Issues

Elevated temperatures during the flowering stage can disrupt the development of flowers. If you don’t pay attention and correct the issue, the plant will produce fewer buds, which means that the yields will be affected. Moreover, the plant will struggle to produce terpenes that are responsible for the flavor and aroma of the plant. 

Suffice it to say that the buds will not be as fragrant as healthy cannabis buds. They may also taste bland, so you probably won’t have a pleasant smoking experience. Unfortunately, even the cannabinoids will be affected. And, since cannabinoids like THC or CBD are the compounds that determine the effects, you won’t feel as many effects as you expect from the buds. 

6. Stress-Induced Hermaphroditism

Extreme heat stress can force the plants to turn into hermaphrodites, which means you’ll see both male and female structures in the plant. Since the plant will focus on producing seeds, you won’t get to enjoy sensimilla or seedless buds. 

Now that you know how cannabis plants react to heat stress, let’s get to the symptoms they show. 

Symptoms of Heat Stress in Cannabis Plants

Heat stress shows up in numerous ways in your cannabis plants. These symptoms can be just starting off or be very severe. Also, keep in mind that these symptoms can occur due to other issues including pest infestations and nutrient deficiencies, so observe the plants carefully to be certain that it’s due to heat stress. For instance, plants can show some of these symptoms if there’s a pH imbalance. Thus, check for other problems first before you worry about heat stress. 

Here are a few symptoms indicating heat stress in your plants:

• Leaf Curling: One of the first and most obvious signs the plant displays is leaf curling. This is because the plant puts an effort into minimizing the leaf surface area that’s exposed to intense sunlight to conserve moisture.
• Leaf Wilting: Like curling, wilting is also another common sign of heat stress. This often occurs when there’s excessive water loss due to all that transpiration, causing the leaves to lose their turgidity and become droopy. 
• Leaf Yellowing: Cannabis plants can show yellow leaves even when there’s a nitrogen deficiency or an imbalance in pH. Thus, check if the pH is correct for the plants to absorb them effectively. Heat-stressed plants may also have yellowing leaves and cause other nutrient imbalances as the plants struggle to uptake nutrients. 
• Browning or Burning Edges: The leaf edges and tips may turn brown. They often appear scorched when the plants are exposed to extremely high temperatures and intense light. Known as leaf burn, it can occur when you expose your plants to uncomfortable temperatures. 
• Leaf Cupping: Some plants exhibit symptoms where the leaves begin curling downwards, but it’s also possible that they curl upwards. Known as leaf cupping, it occurs when the plant struggles to decrease the area of the leaves that are exposed to harsh light. 
• Reduced Growth: Heat-stressed plants may experience slowed growth or stunted development, as high temperatures can inhibit photosynthesis and overall metabolic activity.
• Flower and Bud Abnormalities: High temperatures during the flowering stage can lead to irregularities in bud development, resulting in elongated and spiky buds (known as foxtailing) or buds that are dense and more airy in structure.
• Reduced Trichome Production: Heat stress can cause a decrease in the production of trichomes that contain cannabinoids and terpenes. This reduction may affect the quality and potency of the harvest.
• Leaf Damage: Prolonged exposure to temperatures can result in damage to leaves including necrosis or tissue death. When plant leaves are damaged they can turn brown and crispy before falling. 

What are the Causes of Heat Stress in Cannabis Plants?

Heat stress in cannabis plants can be caused by various environmental factors and human actions. But, once you understand these causes, it becomes easier to prevent them. Here are some of the primary causes and the solutions you could employ to mitigate the problems:

1. Scorching Temperatures

Cannabis plants, like most other plants, prefer specific temperatures to grow healthy and produce good yields. Typically, they grow well between 70°F to 85°F (21°C to 29°C), which is simply perfect for various physiological processes such as absorbing nutrients and photosynthesis.

• Impact on photosynthesis

Also, as mentioned earlier, scorching temps can impact photosynthesis, which is crucial for cannabis plants to produce energy. When the temperatures exceed a specific range, the enzymes that carry out photosynthesis become less efficient, which reduces the plant’s ability to convert energy. Ultimately, the plant struggles to grow and produce yields. 

• Transpiration

Heat stress can increase the rate of transpiration, which is nothing but the loss of water vapor from plant leaves. While transpiration is essential for nutrient uptake and temperature regulation, excessive transpiration due to high temperatures can lead to rapid water loss from the plant. If the rate of transpiration exceeds the rate of water uptake from the soil, the plant can become dehydrated, resulting in wilted leaves and overall reduced turgidity.

Solution

To mitigate the adverse effects of high temperatures and prevent heat stress, you can provide some shade to your plants during the hottest parts of the day if you’re growing outdoors. This way, you’ll ensure that the soil doesn’t dry out easily, thereby maintaining soil moisture at all times. 

If growing indoors, you can use air conditioners or air coolers to maintain the temperatures. You can also go for heat-resistant cannabis strains that can better withstand hot temperatures.

2. Heatwaves

Expanding on the previous point, remember that you have to be aware of heat waves if you’re growing outdoors. 

• Prolonged High Temperatures: A heatwave is a prolonged period of excessively high temperatures, that may often last several days or even weeks. They occur due to specific weather patterns, such as high-pressure systems or shifts in atmospheric circulation, which trap hot air masses in a particular region.
  
  
• Sudden Temperature Spikes: Heatwaves can lead to rapid and significant increases in temperature, pushing conditions far beyond the optimal range for cannabis growth. These abrupt temperature spikes can catch you off guard, whether you’re growing indoors or outdoors. 

• Impact on Cannabis Plants: Heatwaves pose a severe threat to cannabis plants due to the prolonged exposure to extreme heat. The plants can experience a range of problems including heat stress, dehydration, severe yield reduction, and hermaphroditism. 

Solution

To mitigate the adverse effects of heatwaves, you can implement the following strategies:

• Monitoring Weather Forecasts: Keep a close eye on weather forecasts and heatwave warnings so you can prepare and take protective measures in advance.
• Shading and Cooling: Provide some shade to the plants during the hottest parts of the day to reduce the temps. You can use shade cloth and reflective materials outdoors and cooling equipment like fans and coolers indoors. 
• Irrigation Management: Make sure the soil is moist, especially during heat waves. Also, water the plants well to offset the increased transpiration rates and prevent dehydration.
• Cultivar Selection: Some cannabis strains are more heat-tolerant than others. Choose heat-resistant cultivars for a better chance of surviving the heat wave. 
• Timing and Scheduling: In regions prone to heatwaves, opt for specific cultivation schedules to avoid exposing plants to extreme heat during critical growth stages. For example, you can plant earlier or later in the season. Even better — go for autoflowers. 

3. Inadequate Ventilation

When the air in the grow room or greenhouse becomes stagnant, it can trap heat and humidity around the plants. Stagnant air can result from various factors, including the design of the growing space and the lack of proper ventilation equipment.

What happens when there’s insufficient ventilation?

• Heat Accumulation: One of the most significant consequences of inadequate ventilation is the accumulation of heat within the growing environment. Cannabis plants also release heat, but when this heat is not effectively removed through proper ventilation, the temperature in the growing area can rise to levels that induce heat stress.
• Humidity Buildup: Stagnant air can also contribute to the buildup of humidity in the growing space. High humidity, combined with elevated temperatures, creates a stressful environment for cannabis plants. Excess humidity can increase the risk of mold and mildew development and compromise your plant’s health further. 
• CO2 Depletion: Inadequate ventilation can lead to a depletion of carbon dioxide (CO2) levels within the grow room. CO2 is essential for photosynthesis, and reduced levels can hinder the plant's ability to produce energy, slowing down growth and yield.

Solution

• Proper Ventilation Systems: First and foremost, invest in good ventilation systems, including exhaust fans, intake fans, and circulation fans to maintain consistent airflow in the grow tent.
• Air Exchange Rates: If you’re making your own grow tent or even purchasing one, calculate the air exchange rate for your space so that hot air is continuously replaced with fresh, cooler air from outside the grow tent. 
• Regular Maintenance: Clean the carbon filters, fans, and ductwork regularly to prevent clogs or malfunctions that could hinder airflow.

4. Insufficient Cooling

If you live in a hot climate and the room isn’t cooled sufficiently, then the plants will experience heat stress. As temperatures rise, the risk of heat stress in cannabis plants increases significantly and negatively impacts the development of cannabis flowers and buds. Some plants may even turn into hermies if you don’t address the issue. 

Solution

• Sometimes, the room tends to heat up even if you’re using cooling equipment. This is probably because of a malfunction. Over time, cooling systems may develop issues, such as compressor failures in air conditioners or clogged water lines in coolers, leading to inadequate temperature control.
• Therefore, do not neglect cleaning and servicing your cooling equipment as it can lead to reduced efficiency and compromised performance. Dirty filters, evaporator coils, or condenser units can obstruct airflow and heat exchange, making cooling less effective. 
• Also, ensure that the cooling systems you’re using are of the proper size for the growing room. If you have a large growing space, make sure you evaluate the energy efficiency of the cooling systems to reduce operational costs. 

5. Lighting Systems

Bright lights like High Pressure Sodium (HPS) or Metal Halides (MH) lamps are commonly used in cannabis cultivation. Make no mistake — they give you a high return for your investment in terms of yields, but they also generate a significant amount of heat.

And, placing your plants close to these lights can be a recipe for disaster. Not only will that reduce the yields as the plants fail to produce thick and dense buds, but they will also bleach the buds. The plants may also start wilting and curling the leaves. Eventually, these lights have the potential to significantly raise the temperature inside the grow room. If you don’t take action, it can push the temperature beyond the range for cannabis growth.

Solution

• First off, maintain a good distance between the lighting fixtures and the canopy. Follow the manufacturer's recommendations for hanging height and observe your plants to make any changes. 
• If you’re using high-intensity lights, it becomes imperative to use proper cooling equipment. Don’t forget to use exhaust fans and air circulation fans to dissipate the heat generated by lights. This will ensure that warm air is distributed away from the plants, resulting in cooler and consistent temps. 
• Also, you can adjust the lighting schedules. For example, if you’re running the lights on an 18/6 basis, let the lights switch on at 6 p.m. and switch off at 12 p.m. the next day so the room can cool down during the hottest parts of the day. 

6. Overcrowding

Whether indoors or outdoors, placing the plants too close to each other can restrict airflow within the canopy and create air pockets. Although this doesn’t seem significant, it can hamper the airflow in the room and cause problems eventually. Consequently, stagnant air becomes trapped and amplifies the risk of heat stress by retaining heat and humidity.

Overcrowding may also result in hot spots in certain areas of the grow room. These hot spots will affect the plants that are close to them. Remember, a packed canopy will also reduce light penetration, which means that the lower leaves and branches don't get light for photosynthesis, reducing plant growth.

Some growers employ training techniques like Sea of Green (SOG) to improve the yields. This technique involves growing numerous plants in close proximity. Although this technique is effective, it increases the likelihood of heat stress due to overcrowding. 

Solution

• If you’re using the SOG technique, make sure you have a lot of fans and other cooling equipment to reduce the heat. Or, if you're growing several plants simply because you want to get more yields, consider growing fewer plants in the next cycle because you can get optimum yields even with fewer plants if you take good care of them. This will ensure sufficient light distribution for all plants. 
• Apart from that, you can use techniques like pruning and LST to increase airflow and light penetration. Defoliate the leaves regularly to improve light penetration as well. However, don’t overdo this as it can stress the plants. 

7. Insufficient Watering

Overwatering can kill your plants, but so can underwatering. You must ensure adequate and timely watering if you want your plants to thrive. The same applies even to inconsistent watering. Even if you forget to water them for just a few days when the weather is hot, your plants may face heat stress. Or, if you allow the medium to dry excessively between waterings, your plants will suffer again. 

Cannabis plants respond to water stress by closing their stomata. This helps them conserve water; however, it also limits their ability to absorb carbon dioxide potentially affecting photosynthesis and growth.

In addition, when plants don't receive water to maintain their turgidity, they can become dehydrated. Not only will the growth be stunted, but you’ll also see wilted leaves. And, when there’s too much heat, the plant's ability to cool itself through transpiration is affected, making it less efficient in getting rid of heat.

Solution

• Try to set up a regular watering routine based on the specific requirements of your cannabis plants. Keep an eye on the soil moisture levels to ensure that the growing medium remains sufficiently hydrated. Most importantly, make sure the water you use is free from any contaminants. If you’re uncomfortable using tap water, use filtered water.
• Next, check your container’s draining abilities, as it’s crucial to grow in containers that drain the water well to avoid waterlogging or suffocating the roots. Make sure there are enough drainage holes so that excess water can drain out.
• On the other hand, if the soil is drying pretty quickly and lacks moisture, you can apply mulch on top of the soil to reduce evaporation. It also helps regulate the temperature of the soil. You can also use drip irrigation systems to ensure there’s consistent irrigation. 
• Remember that it’s important to maintain a balance in the soil. It shouldn’t be too dry or too wet. Water the containers only when the top soil is dry. Or, lift up the containers and water only when they are light. 

8. Low Humidity

Humidity levels refer to the amount of moisture in the air. Meaning, when humidity levels are low, the air is relatively dry and contains water vapor.

There are various reasons as to why your grow room may have low humidity. For instance, certain locations naturally have low humidity levels, especially in dry and semi-arid climates. If you're growing cannabis outdoors in such locations, remember that humidity levels will be low. 

Inadequate ventilation and airflow can make the problem of humidity worse. This occurs because stagnant air remains in the cultivation space instead of being circulated. 

And, the temperature and humidity levels are interconnected. When the temperatures rise too much, the humidity levels decrease. This is because warm air has the capacity to hold moisture, so when temperatures rise it contributes to higher humidity levels. All this contributes to a build of heat, and the plants may start wilting again. 

One major problem with low humidity is that cannabis plants tend to transpire at a faster rate. As the air becomes drier, plants lose water vapor through their leaves quickly, leading to dehydration.

So, if you live in areas where the air lacks moisture, the plants face challenges in getting rid of heat. This can make them more vulnerable to heat stress when the temps are high. 

Solution

• If you’re growing indoors or in greenhouses, you can use humidification systems to add moisture to the air and maintain a stable humidity level for cannabis plants. In addition, you must ensure proper airflow and prevent the accumulation of dry stagnant air to regulate the humidity levels. 
• You can increase humidity levels temporarily by misting plants or the air around them. However, don’t overdo it because excessive moisture can promote the growth of mold and mildew. To avoid this, use hygrometers as they help you monitor the humidity levels. 
• If you’re growing outdoors, you also use shade cloth or screens to reduce sunlight exposure and slow down the evaporation rate to retain moisture in the soil. 

Summary: Understanding the Impact of Heat Stress, on Cannabis Plants: A Crucial Challenge for Cultivators

As you can understand by now, heat stress can be detrimental to cannabis plants. Typically, they occur when the temperatures exceed normal levels. Like most other plants, cannabis plants also grow well when they enjoy normal temps. 

They thrive when the temps are between 70°F to 85°F (21°C to 29°C). Although it’s okay if the temperatures exceed this range sometimes, the plants will suffer if it occurs frequently. Simply put, cannabis plants will begin struggling if the temps drop below 50°F (10°C) or go above 90°F (32°C). 

There are various reasons as to why the temps can increase in the grow room. Whether you grow indoors or outdoors, heat stress can be a problem. Although you can control it to an extent indoors, it’s not possible to do the same outdoors. 

Heat stress can affect plants in many ways, from disrupting photosynthesis to destroying growth. It can eventually lead to reduced yields. As heat stress also causes excessive water loss due to transpiration, the plants also suffer from dehydration and exhibit signs of curling, wilting, burned edges, yellowing, cupping, bud abnormalities, and leaf damage. In addition, the buds may lose their aroma and lose their flavor as well. 

Heat stress can also cause hermaphroditism in extreme cases, and the plants may develop both female and male organs. Unfortunately, heat stress can make the plants vulnerable to pests and diseases and the plants find it rather challenging to defend themselves. 

The primary causes include extremely high temperatures, underwatering, inadequate cooling and ventilation, and more. Your plants may also be stressed if you position the grow lights too close to them. 

You can protect your plants from heat stress by using fans and other cooling equipment indoors. Outdoors, you will have to plan ahead and plant the seeds when the weather isn’t too hot. If you’re worried about growing plants that take a long time to flower, you can opt for autoflowers instead as they also do well outdoors. You can also provide shade to your plants during the hottest parts of the day. 

That said, do your best to prevent heat stress rather than finding a cure for it.