Growing cannabis outdoors should be an enjoyable and enriching time for a grower. Cannabis heat stress can set an outdoor grower back, and the damage can often be irreversible.
In this article, I explain what cannabis heat stress is, what causes it, how to prevent it, cannabis heat stress symptoms, signs to look out for and all you need to know to keep your cannabis plants as healthy and happy as possible outdoors in 2025.
What Causes Cannabis Heat Stress?
As an indoor grower, you will have complete control over your environmental settings, from the temperature, humidity, air currents, light intensity and the darkness period. However, growing cannabis outdoors can be a total gamble, especially when living in a hot climate such as Southern Europe.
When the temperature outdoors exceeds 30 degrees Celsius, photoperiod cannabis plants and an auto flower weed plant can begin to experience increased levels of transpiration. This has a knock-on effect on how hot the roots become, how much water and nutrients the plant can utilise, and significantly impacts growth rates, leaf structure, and yields.
Hot winds can also seriously affect cannabis plants and be a significant cause of cannabis heat stress. Growers who live in a mountainous, dry area will experience a growing environment that is beyond uncomfortable for plants during the seedling, vegetative, and flowering stages.
● Temperatures over 30 degrees Celsius can promote cannabis plant heat stress.
● Dry winds, combined with excessively high temperatures, are a primary cause.
● Pots can become very hot, causing the soil and roots to dry out and become depleted.
● Cannabis plants are more susceptible to pest damage.
How to Avoid Cannabis Heat Stress Outdoors
When batting with the outdoor elements, there may not be much a grower can do to avoid an unexpected heat rise. There are several ways to prevent cannabis heat stress with outdoor grown cannabis, and those are covered below.
Shaded Areas
Even though providing cannabis plants with as much PAR and sunlight as possible seems logical, there are times when keeping your plants shaded will be most beneficial. Reducing cannabis heat stress by using a shading net to cover your plants during the hottest parts of the day or placing pots in shaded areas will be beneficial over the long term.
Building Polytunnels
Homemade polytunnels made from polythene film serve as a great way to maintain a high level of light intensity while lowering the temperature. Building your own polytunnel is an excellent way to reduce cannabis heat stress.
Another advantage of using a polytunnel is that you can use a hosepipe to spray the outside and inside of the tunnel, increasing humidity levels during dry periods and cooling the plants down.
Greenhouses
Growing cannabis plants inside a greenhouse is advantageous in reducing hot and harsh winds that may otherwise cause transpiration levels to spike. Greenhouses will have windows and vents on top, allowing better air circulation. Greenhouses are available in different shapes and sizes.
Using Felt Pots
When growing cannabis plants in plastic pots, they tend to get hot quickly and can produce temperatures that cause the roots to become stressed and damaged. Outdoor cannabis plants grown in black pots can become excessively hot. They may encourage the growth of harmful bacteria within the growing medium, ultimately infecting the roots and causing the plants to become unhealthy.
Raising Pots off The Floor
I have found that raising your pots off the ground helps significantly to combat high temperatures and eliminates heat stress in cannabis plants. The easiest way to raise pots on a hot surface is to use wooden pallets, which allow air to blow underneath.
Watering In The Morning and Evening
An excellent way to keep plants well-saturated during the hotter days is to water them in the morning around 10 am and later in the evening around 8 pm. During these times, the temperature will be significantly cooler, and transpiration will be at its lowest point.
Foliar Spraying from 12pm – 4pm
Cooling plants by spraying them with a hosepipe between 12 and 4 pm is a good idea. However, doing so can cause leaves to become burnt and scorched. Avoid spraying your plants during this time. Instead, do so later in the evening, once the sun has set.
Using Mycorrhizal Fungi and Trichoderma
One way to help reduce cannabis heat stress is to inoculate the plant’s root system with mycorrhizal fungi and trichoderma. Cannabis plants can endure drought periods more easily when inoculated with fungi.
Your plants will also have a greater chance of holding onto more water. Symbiotic microorganisms as found in the Atami VGN nutrient line form a relationship with the root zone, increasing root mass and enhancing nutrient uptake and availability.
Cannabis Heat Stress Recovery
In many cases, once a cannabis plant has suffered excessive heat stress, the damage can be irreversible. However, suppose you have managed to save the plants in time. In that case, you can help nurse them back to full health by providing a mild nutrient solution and keeping them in a shaded area away from direct sunlight.
MSNL advises that plant stress comes in a variety of forms, including heat and light stress, nutrient deficiencies or toxicity, root shock, over/under watering, and environmental disturbances, so pay close attention to your plants as there as many triggers of stress.
Outdoor Cannabis Heat Stress Signs
There will be an abundance of cannabis heat stress symptoms to look out for when growing outdoors. Below is a guide to the signs a heat-stressed cannabis plant may display.
| The sides of the leaves curling | If the sides of the leaves are curling upwards, this is a sign that the leaves are trying to reduce the amount of light they can absorb. |
| The tips of the leaves curl upwards | Another sign that the leaves are suffering from heat stress. The ends of the leaves will curl upwards and appear dry and brittle. |
| Wilting leaves | Plants suffering from cannabis heat stress will produce leaves that wilt and appear to curl downward. |
| Dry growing medium | When checking the pot, the growing medium will feel dry and dusty, indicating that there is no water inside the pots, due to hot temperatures. |
| Stunted plants | Cannabis heat burn will cause plants to cease growth, resulting in stunted plants. This means the plant is not growing at the same rate as before and will produce low yields. |
| Burnt leaf tips | The colour of the leaf tips will have a scorched appearance that can be yellow or brown in colour. A plant suffering from cannabis heat stress will display burnt leaf tips that cannot become healthy again. |
| Dry and thin leaves | When touching the leaves of your plants, they will have a paper-thin and weak appearance and structure—often associated with yellowing. |
| Lots of insect damage | Cannabis heat stress will cause outdoor cannabis plants to be more susceptible to insect attacks. Thin and frail leaves will be an easy target for insects that can easily penetrate the leaf tissue. |
| Excessive stretching | One way to determine when an outdoor plant is too hot is by observing excessively long internodal spacing. |
| Small, undeveloped bracts and calyx. | A flowering cannabis suffering from heat stress will produce elongated and small-sized bracts and calyxes. |
Our Conclusion
Cannabis plants are well-adapted to hot and harsh conditions; however, time is of the essence once the temperature exceeds 30 degrees Celsius. Heat-stressed cannabis plants will have difficulty maintaining a comfortable transpiration level, which has a knock-on effect on nutrient uptake, salt buildup, and plant growth and yields.
The roots of a cannabis plant are susceptible to hot weather. They will become severely affected when pots and the growing medium become too hot. Using fabric pots, keeping plants well-shaded during the hottest part of the day, raising pots off the ground, watering and foliar spraying early in the morning and in the evening will be the best form of prevention. Good luck growing an outdoor crop this year and pay close attention to your outdoor cannabis plants daily.
