heat stress on cannabis plants

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.


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).

In this article we discuss heat stress symptoms in cannabis plants. Learn how high temperatures can negatively affect your garden.

How To Protect Your Cannabis Plants From Heat Stress

Growing cannabis can be a tricky proposition. However, there are some tried and true techniques to preventing damage to the plants from overheating.


Are your plants looking a little worse for wear? Despite following an accurate feeding, watering, and light schedule, you are frustrated to discover discoloured and deformed leaves on your cannabis plants. Don’t worry, you’re probably doing everything else correctly, it could just be the signs of heat stress.

Cannabis is a hardy and resilient plant, and many original landrace varieties adapted to survive in harsh regions such as the Hindu Kush mountain range and the tundras of Central and Northern Asia. Despite the resilience of this plant species, it can only deal with a certain amount of heat before its physiological systems can’t take it anymore.


One of the telltale signs of heat stress is when the tips of fan leaves begin to curl upward. They’ll also exhibit a generally dry and withered look. This factor indicates that heat is the most likely culprit, removing the possibility of most nutritional deficiencies. As well as looking generally unwell, leaves will develop large brown spots or blotches that manifest in irregular shapes, primarily along the edges of the fingers of fan leaves. These unsightly scars are usually accompanied by yellow patches of discolouration. These symptoms mainly affect leaves located near the top of the canopy and around the perimeter of the plant—areas in the direct line of fire.

If you detect this set of symptoms in the vegetative phase, you can be sure that heat stress is at the bottom of it. Indoors, this could be caused by allowing the tops of your plants to grow too close to a powerful light source. Outdoors, it could be the result of a particularly brutal heatwave or an extremely hot and dry climate.

Heat stress manifests differently during the flowering phase. Sometimes, much to the shock of many growers, new buds can start to erupt out of the tops of older ones. This can cause what is known as a foxtail, a pillar of small buds and sugar leaves. This is a survival mechanism initiated by the plant as it attempts to form new buds capable of reproducing and generating seeds.

Below, we’ll cover indoor and outdoor methods to prevent and treat heat stress.


Heat stress can affect an indoor grow in multiple ways. For one, growers who live in hot climates may find it hard to regulate indoor temps during the dog days of summer.

Regardless of climate, grow tents can become very warm if certain measures aren’t put in place. The heat generated from different light sources can pound down upon leaves and subject them to significant stress. A lack of adequate fans and an exhaust system prevents convection currents that would otherwise cool down the interior of the tent. Here are some tips to help you avoid heat stress indoors.


This is a simple and cheap solution to start cooling down your growing environment. Fans disrupt still, hot air and will effectively create a convection current that will help to cool things down. The wind generated by fans will also gently stress your plants into developing thicker and stronger stems.


This might seem obvious, but it’s definitely a viable solution. Indoor growers have the advantage of taking complete control over the microclimate of the grow tent, provided they have access to the right equipment. This is a fairly expensive option, but can be a real life-saver if you live in areas such as Spain or the southern United States. Place an air conditioning unit in your grow tent to keep cool air circulating.


If your plants begin to exhibit signs of heat stress, consider how close they are to the light source. If only the top fan leaves are showing symptoms, then readjust the position of your lights so that they hang further away.

If changing the position makes no difference, then you might need to change the type of light you are using. Most lights put out a fair amount of heat, and if you’re growing in a confined space in a warm climate, things are going to heat up quickly. LEDs are a good option for growers dealing with these conditions. They put out a lot less heat than HID light sources; plus, they are often much cheaper to run.


An exhaust system is like a reverse fan. It works by sucking stale and hot air out of the grow space. If you have fans running at the same time, these will serve to replace old air with fresh and cool air. If you’re running a deaerator, it’s advised to fit it with a carbon filter that will remove any suspicious scents from blowing out into the surrounding area.


Some growers find success using supplements to treat the symptoms of heat stress. These may be effective, but will only work long-term if the environment is cooled down using the techniques mentioned above.

Seaweed or kelp extracts work well to remedy some of the effects of heat stress. They are loaded with minerals and nutrients that make plants more resistant to high temperatures. Then there’s silicon, a compound that increases the resilience of cell walls, makes plants more resistant to heat and cold stress, and helps them absorb key nutrients such as zinc, copper, and phosphorous more easily.


Outdoor growing is more difficult to control than the alternative. Crops are subject to the whims of the elements, as well as countless forms of pests and infections that can damage vitality and yield. Heat stress is no exception. Indoor growers have the advantage of full climate control, whereas outdoor growers have to deal with heatwaves, monsoons, and everything in between.

Heat stress is a prominent threat to those growing in warmer regions closer to the equator. Although increased levels of sunlight can be advantageous, too much can lead to heat stress symptoms. Here are some tips on how to protect your outdoor crop.


During a heatwave, try to water plants either in the mornings or evenings when the temperature is less intense. Watering your plants in the middle of the day might seem like a good way to cool them down, but it can actually be harmful. Water droplets can magnify sunlight and cause increased heat to be directed toward plant tissue.


If you’re growing outside in a warm climate, it’s a good idea to grow in large containers or pots as opposed to directly in the ground. This makes your plants portable and gives you the option of physically relocating them to a sheltered spot if the sun gets too intense.


Keeping your plants under a shelter all day long will inhibit photosynthesis and have a detrimental effect overall. However, making a temporary shelter using cloth or a tarpaulin can help to protect your crop during the peak heat of the day. Erect the shelter during the hottest hours, and let your plants bask in sunshine before the heat sets in and after it tapers off.


Seaweed or kelp extract can help increase plant resilience both indoors and outdoors. Use the formula according to the manufacturer’s instructions and give your plants some extra protection using these nutrient-rich substances.


There are thousands of cannabis strains available on the market, and each one has been selectively bred over time to display specific desirable traits. Some breeders have managed to create cultivars that are particularly resilient to high temperatures. You can significantly reduce the risk of heat stress by starting with one of these varieties. Here are three heat-resistant strains we recommend.


Fruit Spirit is a hardcore strain that laughs in the face of high temperatures (within reason). This beast is the offspring of legendary parent strains White Widow and Blueberry. She features a closely balanced genetic makeup of 60% sativa genetics and 40% indica. Fuelled by 18% THC and a medium CBD level, she offers a high that relaxes the body and invigorates the mind. Her name is a tribute to the sugary and fruity tastes of her flowers, which dance across the tongue when smoked.

Fruit Spirit grows well in both indoor and outdoor environments. Indoor plants reach a height of 80–120cm, feature a flowering time of 8–10 weeks, and yield up to 425g/m². A single plant grown outdoors can reach up to 220cm in height and produce a harvest of up to 525g.

Cannabis is a hardy plant, but it can also be damaged by too much heat exposure. That said, there are some easy ways to prevent heat damage.