Heat Stress Symptoms of Cannabis Plants

Heat stress does what it says on the tin. Cannabis loves warm temperatures but sometimes it can get too much, causing a number of problems that can seriously affect health if it is not taken control of. Intense heat can be a real issue that can permanently damage or even kill a cannabis plant. This article dives into the symptoms of heat stress and what you can do to avoid it.

If your plants are showing signs of stress, it could be due to overheating. Just like humans can become irritated and stressed when they're too hot, plants can also show their signs of discomfort. Cannabis plants can survive through heat stress, however bud quality and yields may be drastically reduced if left unattended.

Why Do Cannabis Plants Overheat?

Cannabis plants are fairly resistant against many environmental conditions but sometimes temperatures can simply be higher than they can withstand. Indoors, the main difficulty is maintaining equipment to control temperatures so it doesn't get too hot or cold. Outdoors, growers have to face the extremities of the weather, which can bring intense heat waves that put plants at risk.

Overheating doesn't just happen because it's hot. Although this is probably the most likely reason your cannabis plant is stressed, there are some other factors to consider that will help you better diagnose the problem to find the root cause. Normally these issues are associated with high temperatures.

• Low Humidity - Low levels of moisture in the air make it harder for plants to stay cool, increasing temperatures because they get 'thirsty'.
• Nutrient Burn - Excess amounts of minerals can disrupt a cannabis plant's ability to regulate its own temperature.
• Too Much Light - Intense lights are often the cause of too much heat. Light stress is also possible, signalled through bleaching of leaves.
• Poor Ventilation - Plants that do not receive enough fresh air are at risk of overheating. Replenishing air helps to keep temperatures down.
• Equipment Failure - Indoor cannabis plants rely heavily on equipment. Air conditioning units need to be monitored regularly to avoid failure.

What Does Heat Stress Look Like?

When a cannabis plant is stressed from too much heat it will display some pretty unhappy signs. A lot of it depends on the stage of growth, which also determines how problematic the heat problem could be. A plant that develops stress during vegging will have different symptoms and risks to a plant already well into flower.

It is common that when plants are too hot, the canopy will begin to develop symptoms of overheating. Naturally, the top of a plant will be warmer as heat rises. The canopy is also usually exposed to the most light so temperatures will be higher around the top areas of your garden. 

The leaves are where heat stress first becomes apparent and usually curl upwards around their edges to form a canoe like shape. Heat stress looks different to any nutrient deficiency so detecting this type of curling gives you a clear indication that your plants are suffering from a heat problem. It differs slightly from strain to strain, but you can check for the following signs of overheating if you are worried:

Plants that become stressed during flowering can develop foxtail buds as an extra effort to reproduce. These are long buds that emerge from the top of other buds and  usually grow upwards. Calyxes are spaced out and not as dense as normal flowers. The sooner you lower temperatures after noticing heat stress, the less of a risk it will be to plant yield.

Cannabis plants severely affected by heat stress will likely not recover, especially if the problem occurs during the flowering phase.

Tip: Make sure you do not confuse heat stress for other problems, as the symptoms can look similar to some deficiencies.

Heat Stress Indoors

Growing cannabis indoors involves setting up efficient equipment to keep temperatures at a comfortable level. Installing the proper exhausts and air conditioning are the best ways to cool your grow room. By increasing the circulation, we remove heated air from around the plants and allow it to vent out effectively as fresh air enters the space.

Indoor growers commonly use HID lighting which produces a lot of heat. This can be beneficial in colder climates, however HID lights can still cause heat issues if placed too close to plants. Indoor heat stress issues can be prevented or solved by improving your grow space.

• Increasing distance between canopy and lights
• Spread oscillating fans around grow room to move air around canopy
• Consider using LEDs as they produce almost no heat
• Install air conditioning if temperatures reach extreme levels
• Upgrade exhaust fans so fresh air fills grow room at least every 5 minutes
• Keep grow room temperatures below 25°C

Air conditioning might not be necessary unless you live in a particularly hot climate but a temperature control system is recommended nonetheless if you can afford it. Humidifiers are also a good method of adjusting grow room temperatures. As a minimum, have a wall thermometer/hygrometer with a probe so you can monitor temperature and humidity in the canopy at all times.

Heat Stress Outdoors

If you are growing outdoors, controlling heat stress might be a little harder but there is still plenty you can do to help your green ladies along the way. Outdoor cannabis plants may face long hours of strong sun, which comes with intense heat waves that can last days. Whatever your region and weather forecast, you can plan ahead to make sure your plants are protected and ready for extended periods of heat.

Pots - Pots can be moved around to shaded areas if the heat becomes too intense. Large containers or ceramic pots retain more moisture, helping to keep temperatures down.

Kelp Extract - Nutrient rich seaweed supplements aid roots by regulating temperature and moisture, reducing the risk of plants drying out if overheating occurs.

Water In The Morning - Give plants enough water so you can water every morning before the strong sun hits. Avoid watering during the hot hours of the day.

Building Shade - If you are unable to move plants, consider putting a covering over them to provide shade for a couple of hours through the hottest periods.

Choose Heat Resistant Strains - Some genetics deal with heat much better than others, so check if your seeds are bred to grow in warm climates.

Planting directly into the ground means roots stay cooler for longer, although it is more difficult to protect them from the sun if you need to. This can be solved by digging a hole in the ground that you can place the pot into during the day. Using this method is a great way to retain moisture in the soil and to avoid roots drying out.

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Conclusion

Heat stress is easy to avoid if you follow these basic requirements. Setting up the proper space before starting can save you a lot of headache later on so make sure you dedicate time to learning about the perfect environment for your cannabis plants. If you are worried about your grow area getting too hot, plan ahead and you will have no problems.

External References

Characterization of Nutrient Disorders of Cannabis sativa. Applied Sciences. - Cockson, Paul & Landis, Hunter & Smith, Turner & Hicks, Kristin & Whipker, Brian. (2019).

This article was updated September 2020