cannabis heat stress

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.

Recognising Heat Stress In Cannabis Plants

Published : Sep 27, 2017
Categories : Cannabis cultivation

While heat is a necessary component of healthy cannabis plant growth, overheating can cause adverse reactions that will detract from proper development and ultimately, the quality of your yield. Find out how to recognise and prevent heat stress from damaging your crop.

All plants require heat to grow and develop properly. Too much heat, however can be destructive. Below, we attempt to find a balance below the upper threshold of how much heat your plants can handle.

Temperature plays a huge role in a plant’s life. When it becomes too cold, plants cannot grow or even germinate from their seeds. A stable temperature will help young plants develop root systems faster. By keeping track of night temperatures, you can make sure that above-ground growth, that means leaves and stems, will also be abundant.

Essentially, when your marijuana plants get too cold, their growth is stunted, slowed down, or in some cases, halted completely. And while heat prevents this from occurring, there is also such a thing as too much heat for your cannabis plants.


There are several signs that indicate your plants are suffering from heat stress. First, the ends of leaves will start to curl up – a phenomenon easily and often confused with overfeeding. The main difference between heat stress and overfeeding is that with the former, the entire sides of the leaf will curl up, not just the ends. Also unlike overfeeding, leaf ends will not become “burned” by nutrients. Therefore, be on the lookout for curling leaves unaccompanied by other overfeeding symptoms.

If there is very low humidity in your grow room, or your local climate is very arid, cannabis plants are more prone to developing “cupped” leaves from the heat. Even though the heat might not appear to be very harsh, when combined with low-humidity, the two can wreak havoc on your plants. Especially when you want to keep your plants in low humidity to combat top rot, keep a very close eye on your leaves’ edges.

Flowering plants are even more susceptible to heat stress. If too many leaves become damaged by overheating, the cannabis plant will respond by growing buds with lower potential. This abnormal formation of buds can manifest in what is known as “foxtailing,” which usually occurs in buds closest to your grow light.

The buds may also display new growth at a much higher rate toward the top of the bud than on the bottom. These newly formed pistils are usually white and give the appearance that the bud is not ready for harvest. If this happens, it is a telltale sign of overheated cannabis plants. Upon deciding when to harvest your plants, pay attention to the lower end of the buds as this will give you the best indication of their actual maturity.


There are several ways to help your plants withstand higher temperatures or recover from past heat damage. The availability of solutions, however mostly depends on the environment that you are growing in. The most universal solution is to make sure you have adequate temperature monitors in place. These help to detect if your grow lights are too close or if the sun might be too intense for your plants on certain days.

Outdoor growing may leave you with fewer options concerning temperature regulation, but there are still tactics that can be used to help your plants survive blistering temperatures. The root system appears to be very helpful in preventing heat damage to the upper regions of your cannabis plants. A frequent and thorough watering schedule, if possible adjusted to the current weather forecast, is a good way of keeping your plant’s root system cool. Another option is to dig a hole in the ground (about as deep as the height of your pots) and put the plant, including the pot, in there. The cooler temperature from the ground should take some of the heat off of your plants.

Using pots that insulate can also help a great deal in preventing heat stress or damage. Planting pots made out of ceramic help a lot with insulation and keep the heat out. If insulation is still not enough of a measure to keep your plants safe from the heat, consider moving the potted plants to a shaded area. Putting up an old sheet or tarp takes away most of the direct heat.


Heat stress solutions for indoor growing mostly revolve around increasing the air circulation inside your grow space. Consider increasing the distance between your plants and the light source. If this is not possible, an oscillating fan that blows over the top of your plants might be a good alternative. The fan ensures that the heat is dispersed more evenly across the grow space.

Another option is to vent out the hot air by using an exhaust system. This might release aromas of marijuana into the rest of the growing area, so adding a carbon scrubber to remove all traces of scent might be a good idea if you are engaging in a stealth operation.

This last tip also applies to outdoor growers, seeing as it helps with situations in which the damage has already been done and you want to help your plants recover: seaweed kelp extract. It comes as a liquid fertiliser and in addition to containing nutrients and minerals, it contains cytokinins, which are highly beneficial in reducing stress. This extract helps your plants recover from heat stress and even has the power to help protect them more effectively in the event of future cases.

A Guide To Watering Your Cannabis Crop, Indoors And Outside

Come and see how you can make sure that heat works to your marijuana plant's advantage and does not cause irrevocable stress or damage to precious buds.