Too much of anything is never good, especially when it comes to heat. Cannabis plants are generally easy to grow and manage; however, continuous exposure to high temperatures can significantly affect the plant’s health and its ability to produce an impressive, bountiful yield. If you have seen plants with scalded leaves, stunted growth, or curled/folded leaves, the culprit was most likely heat stress. One thing is certain; it is not a pretty sight. So, what exactly is heat stress, and what can it do to your prized cannabis plants?

Heat stress refers to the unpleasant experience a cannabis plant undergoes when exposed to higher than optimum temperatures (30°C/85°F and above) for a prolonged duration. The elevated temperatures interfere with normal metabolic processes, slowing down normal plant functioning and growth. If the situation is not salvaged early enough, the plant eventually dies, leading to a huge loss for the cultivators.

In this piece, you will learn everything you need to know about cannabis heat stress, including the causes, signs of heat stress in marijuana plants, how to cure heat stress, and useful tips on preventing heat stress in the future. 

Types of heat stress and their causes

Cannabis heat stress can affect different parts of the plant: the top canopy, the roots, or the plant as a whole. These cases are caused by different factors as highlighted below;

Heat stress on the canopy (top leaves)

Heat stress affecting the top leaves is caused by high-intensity grow lights, or when grow lights are placed too near. When the lights emit heat, it creates hotspots around the leaves, causing heat stress. This is more likely to happen if the grow room has less than optimum ventilation. The hotspots increase the temperature around the canopy, leading to a less healthy plant.

Cannabis light burn is the main culprit of heat stress in cannabis plants. It is imperative to ensure that you check the instructions from the grow light manufacturer on the ideal distance between the plans and the lights. Once you have fixed the lights, monitor the plant and make necessary adjustments to find the ideal distance.

Heats stress on the whole plant

When heat waves strike, not only the animals and humans suffer. Cannabis plants are hugely affected by the sudden increase in temperature, and if the grower does not remedy the situation in time, numerous plants end up dying. A heatwave is characterized by temperatures above 90 degrees Fahrenheit occurring for two or more days.

The elevated temperature affects the entire plant, leading to a fast deterioration of health. If growing indoors, the grow room becomes an oven that gradually kills the plant if measures to salvage the situation are not taken promptly. As we cannot control nature, investing in a good air conditioning system will help save your plant and guarantee a good harvest (more of this later).

Heat stress on the roots

When the roots of a cannabis plant are compromised, it is impossible to recover. Excess heat (31°C or 88°F and above) can mess with THC potency and cause stunted growth. If the situation goes for an extended period(more than 5 days), the roots become incapable of absorbing water or nutrients, which are essential to the plant’s survival. 

The ideal soil temperature should range from 68°F/20°C to 78°F/26°C for the optimum growth of the plants. Factors that may lead to an increase in temperature in the soil include solar radiation (heat from the sun) and season and atmospheric conditions (summer and when heat waves occur).

Signs of heat stress in cannabis plants

Heat stress presents in several ways. However, it would help if you were very careful as some of these signs may be due to other issues, such as dehydration, nutrient burn, or other issues. Here are the top signs you should look out for during the different growth stages.

Seedling and vegetative stage

During the seedling stage, the plant is very vulnerable, meaning a small issue may be detrimental to the plant’s overall health. When heat stress occurs during this stage, you will notice slow/stunted growth, pale and withered new leaves, and some plants may start dying.

If cannabis heat stress affects the plant during in the vegetative phase, the first sign will be the curling up of the pointed leaves. The folded-up leaves will take on an appearance similar to tacos. Soon afterward, the leaves lose the green pigmentation to become yellow or brown, giving them a scaled/burnt look. The leaves will begin dropping within no time as the plant’s appearance and health significantly diminish.

Flowering stage

The flowering stage is certainly the worst phase for your cannabis plants to experience heat stress. Besides it being the critical stage where your plant makes buds, no leaves are growing. This means that the destruction of leaves will reduce the plant’s ability to make the energy required to support the budding stage. Additionally, heat stress affects the plant’s ability to absorb water and nutrients necessary for survival. 

heat stressed weed

When heat stress happens, you will likely harvest less potent flowers as excessive heat interferes with THC production. The heat degrades terpenes, leading to buds with little flavor and aroma. All the hard work poured into the germination, seedling, and vegetative stage will be in vain as you will likely harvest lower and substandard yields. Some of the signs of heat stress during this stage include;

  • The plant may begin to produce foxtails. These are new buds that grow on top of the old non-potent and less dense ones. 
  • Curled up leaves
  • Leaves turn yellow or bleached.
  • Pistils may prematurely turn from white to brown prematurely
  • Dry leaf edges

Factors that may worsen the heat stress

As highlighted above, the main culprits of heat stress are growing lights, heat waves, and high temperatures during summer. Think again if you thought that heat stress was the worst it could get! Unfortunately, some other factors or issues can exacerbate the situation, including low humidity levels, equipment failure, nutrient burn, and poor ventilation.

Low humidity levels

For a marijuana plant to thrive, it needs to be subjected to humidity levels between 40% and 60%. A grow room with less or more than the recommended levels will negatively impact the plant’s overall health. When the plants are exposed to high temperatures and low humidity levels, the transpiration rate increases, leaving the plant dehydrated. Consequently, the plants will begin to wilt/drop. This deadly combination attracts pests and diseases, such as powdery mildew and spider mites, which thrive in hot and less humid environments. 

Nutrient burn or deficiency 

When heat stress affects an already vulnerable plant, the consequences can be serious. A plant experiencing nutrient burn or deficiency is already busy trying to overcome the issues. If heat stress is added into the mix of problems, the plant is unlikely to survive. Always ensure that the plant is getting the right amount of nutrients and supplements at any given time. This will protect it and provide it with the energy it requires to fight off or recover from issues like heat stress.

Poor ventilation system

A lack of proper air circulation in the grow room leads to multiple hot spots. If the temperature is already high and the ventilation system cannot clear out the air, the heat stress worsens, and the situation becomes irreversible. As a cannabis grower, having an effective air extraction system is vital as it might be the element that saves your plants when heat stress strikes.

Equipment failure

In a grow room, you will find multiple equipment and tools, an environmental monitoring system being among the most essential. The system allows you to monitor the growing environment in real-time, no matter where you are. If one or more pieces of equipment (fans, humidifiers, lights, and others) are not working, you can fix it immediately. When the plants are experiencing heat stress and one of the equipment fails to work, the situation might worsen, compromising your yield in the long run. Inspect your tools regularly and have spares around just in case.

How to cure cannabis heat stress

Will plants recover from heat stress? The answer will depend on when the heat stress was detected and the damage caused. If detected early, adjusting the temperature and increasing airflow will reverse the damage. Below are some of the ways you can lower the temperatures;

  • Adjust the distance between the plants and the grow lights. You can consult the manufacturing company to determine the ideal distance to maintain. After moving the lights, observe the plant’s response and make minor adjustments until you find the distance that works best with your marijuana plants. 
  • Invest in an extractor fan to help clear the warm air and replace it with fresh and cool air every 3 to 5 minutes. If the heat is excess, find a fan that can replace the air every minute. An oscillating fan is essential in blowing the hot air (hot spots) from the top canopy. 
  • Use grow lights that emit minimum heat, such as LED grow lights. This reduces the chance of hot spots or a heated grow room resulting from high heat emitting grow lights like HPS bulbs. LED light burn is very rare as the lights are designed to meet the needs of delicate plants.
  • If you are cultivating cannabis in areas where the temperatures get really high, avoid growing the plant during summer. Additionally, if the plants are stressed by excessive heat, turn the grow lights off during the day and turn them back on during the night. This ensures that the plants are not subjected to extra heat produced by the grow lights. 
  • Lower the soil temperature by watering the plants early in the morning and evening with cool water (should not be too cold or too warm). Water the plants in moderation to avoid clogging and inhibiting nutrient absorption due to lack of oxygen. 
  • Silica, humic acid, and seaweed kelp are known to help with various abiotic factors, such as high temperatures. In particular, Seaweed Kelp helps boost germination, rooting, and yields. It also helps the plant recover from heat stress and prevents the situation from happening again.

Unfortunately, sometimes heat stress becomes too severe to reverse. In such cases, you will have to discard your marijuana plants and start again if the heat stress happens during the seedling or vegetative phase. If it occurs in the flowering stage, harvest whatever you can and use the tips provided below to avoid such an occurrence next time.

Measures to prevent heat stress

Indoors

  1. Constantly monitor your grow room’s temperature, humidity, and airflow efficiency and make necessary changes in case of any issue. Inspect the instruments to ensure they are working at optimum levels. If possible, invest in an environmental monitoring system to make your work easier. 
  2. Avoid exposing the plants to grow lights during the day during hot seasons.
  3. Maintain optimum distance between plants and grow lights
  4. Equip your grow room with an oscillating fan
  5. Invest in low-heat emitting grow lights like LEDs
  6. Have an air conditioning system in the grow room if the area is prone to high temperatures and heatwaves.
  7. Use an extractor fan to bring in the fresh air every 3 to 5 minutes or less, depending on the severity of the temperatures.
cannabis heat stress

Outdoors

Unfortunately, outdoor cannabis cultivation gives you very limited options when it comes to dealing with issues such as heat stress. The best option is to put measures into place to ensure the plants are protected from such a menace. Here is what you can do;

  • Start by planting high heat resistance strains, especially if you live in areas with heightened temperatures. These strains include;
  1. Bruce banner
  2. Gelato 
  3. Gorilla Glue #4
  4. Green Crack 
  5. Northern lights 
  6. Sour Diesel
  • If you are planting your seeds on the ground, apply mulch on top of the soil to insulate it. This keeps the root system protected from heat. For container-grown marijuana, avoid black containers as they easily absorb and retain heat. If you have black ones, cover them with a white cloth to reflect the solar rays away. 
  • Regularly water your plants with cool water to maintain the temperatures, but ensure you don’t overdo it.
  • Keep up with the local weather reports to ensure you are prepared before anything happens. In the case of impending high temperatures, you can build your plants a temporary shade. This will help regulate the temperatures until the weather changes. 
  • If your plants are in a container, move them to a place with shade during the afternoon and return them to their growing space in the evening.

Take away

Heat stress can destroy your cannabis plants in an instant. It is imperative that you take the necessary steps to ensure the plants are in a conducive environment and that the temperature stays under control. 

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