Autoflowering Plant Heat Stress Symptoms

13 January 2023
Too much light or elevated temperature could stress your plants, always be on the lookout for these signs.
13 January 2023
9 min read
Autoflowering Plant Heat Stress Symptoms

Read more
  • 1. What is heat stress?
  • 2. What causes heat stress?
  • 2. a. Low humidity
  • 2. b. High temperatures
  • 2. c. High light intensity
  • 3. In conclusion

Your plants can only withstand a certain amount of heat and light. After a certain point, your autoflower will start showing marijuana heat stress symptoms on the bud or leaves. Your leaves can get yellow or brown, appear burnt or bleached, it's also common for the leaves to fold in a taco shape all of those symptoms and more we’ll explain here. 

As most experienced cannabis growers know, cannabis loves a bit of heat. The correct temperatures enable cannabis plants to thrive. An optimal environment drives plant growth in a much shorter period of time. However, it’s very much a balancing act. When passing a certain threshold, too much heat can have detrimental effects on plant growth and performance. Heat stress falls into the category of abiotic (non-living) stress, alongside drought and other adverse weather events. So, how can you prevent heat stress from affecting your plants? There are a variety of strategies available, including proper light placement, fans, and air conditioners. Find out everything you need to know about heat stress below.  

1. What is Heat Stress?

Heat stress can happen indoors or outdoors, it occurs when your autoflower is exposed to high temperatures for a long period of time. This can cause a wide variety of problems such as stunting growth, affecting yield, and ultimately killing your plant.


Autoflowering plant heat stress symptoms: symptoms of heat stress

Symptoms of heat stress on a cannabis plant.

This is why you should be able to control the growing conditions when using high-intensity lights. If you fail to do so, your plant may not be able to transpire properly and can result in a variety of cannabis heat stress symptoms.

Heat stress causes numerous problems for cannabis plants. Among the most obvious, excessively high temperatures over a prolonged period of time increase the rate of evaporation, meaning that your plants will require more water than usual. This means growers have to put in more work to simply keep their plants alive. Plants depend on water for many important physiological functions, including photosynthesis. Extreme heat and a loss of water can also cause leaves to wilt and dry out. If they become too damaged, they’ll lose their ability to conduct photosynthesis and your plant won’t be able to produce as much energy as it once could. 

2. What Causes Heat Stress?

Heat stress indoors is a result of not adjusting your climate for optimal growing. Outdoors it’s harder to control but in both situations, heat stress is caused by the same factors: low humidity, high temperatures, and high light intensity. Let's examine heat stress factors and learn what solutions can be applied to each situation.     

Low Humidity   

The ideal humidity for an autoflower is 60-70% in the vegetative stage and around 45-55% in the flowering stage. Very low humidity can make plants more likely to get heat stressed. Sometimes you’ll get symptoms that look like heat stress even if it’s not that hot, they can be worse because the plant is being affected by very low humidity. 


Low humidity won’t stress your plant, it can affect growth and yield but there won’t be any apparent symptoms unless it is combined with high light intensity and/or high temperatures.


If you’re growing indoors, there are a couple of solutions. You can use a humidifier for a long-term solution or you can place buckets filled with water inside or around your grow tent, always checking the hygrometer to keep humidity at an optimal level.


Autoflowering plant heat stress symptoms: humidity control

Ideal humidity for an autoflower is 60%. You can control humidity with a humidifier or simply place buckets filled with water in your grow space.

Outdoors there’s not much you can do, try to place your plant in the shadow for a few hours a day, we also recommend watering more times with less water throughout the day, this should keep the roots cooled down.

High Temperatures 

The optimal temperature to grow autoflowers is around 25 Celsius (77f). Heat stress is primarily caused by high temperatures. Your plant will always show you when she’s not happy, it’s essential to keep an optimal climate for your autoflowers, keeping in mind that flowering plants are even more susceptible to heat stress.


Plants in the vegetative stage usually start to fold leaves inward in a taco or cup shape and can start to damage leaves even if the temperature is not that high. Combined with low humidity it can wreak havoc. As said before, plants in flowering are even more susceptible and if a lot of leaves are damaged they will respond to overheating by growing buds with less potency and eventually with abnormal growth of buds.

As result, they will look like what is known as foxtail. It does not always look like that, sometimes it’s just an abnormal growth of bud. What happens is the plant is trying to abandon the heat-damaged bud and start a new one. At this stage, you should see a lot of white pistils growing.


Solving this primarily revolves around increasing air circulation in your grow space. This could be as simple as placing an oscillating fan blowing on top of your plants, or it may need some more drastic measures. Indoor cannabis cultivators have a bunch more control over the ambient environmental ranges than outdoor growers which can be used to your and the crop’s advantage. 

Every single indoor grow room, no matter how big or small, should have some form of temperature and humidity gauge installed. Without one of these (or multiple depending on the size of the room), you are essentially growing blind. Air exchange in and out of the grow space, and throughout the canopy, is of the absolute highest importance. This is not just important for heat control, but also for oxygenation levels. You want to set up the air exhaust system so the intake (which can just be a passive opening) is located at the lower half of the room, with the exhaust being located near the top of the room. This is due to the fact that warm air rises, so having the intake lower than the exhaust will ensure a supply of cool, fresh air is always being sucked into the room or tent. 


Autoflowering plant heat stress symptoms: heat stress from high temperatures - solution

Solutions for heat stress from high temperature for indoors and outdoors.

If the damage is already done, use seaweed kelp fertilizers to help your plant recover, they contain cytokinins which help reduce stress.

If you are using HID lighting then it is critical that you have a way of moving the highly heated air away from the lights and plants and out of the room. A cool tube light reflector that is hooked up to the exhaust system is perfect for this job. By running the exhaust through the cool tube, the heated air is instantly pushed away from the plants and out of the room. 

You may also need to need multiple fans placed towards the canopy, especially if you have a large grow-op and as the plants reach the flowering stage and start to fill out. It is not uncommon for pockets of warmer, humid air to become trapped in the canopy which can lead to not only heat stress, but also other critical issues like mold or fungi development. 

If you’re growing outdoors you should try to cool down your roots. Instead of watering your plant once a day, you should water it multiple times with less amount of water. This way you keep the medium moist and it can help to cool down the roots.

High Light Intensity   

Light burn or light stress only occurs indoors, it can happen when transferring a plant from a weak to a strong light or if the light source is too close to your autoflower. It also can happen to older leaves that have been exposed for a long time but that’s not common. Usually grow lights come with height recommendations for both stages (vegetative and flowering), you should always experiment and test what works better but you should never start with placing the lights too close.


The most common symptoms are yellowing, burned leaves, and bleached buds (when the bud starts to turn white). The first sign a plant is getting too much light is when the leaves start pointing up (sometimes you don’t even see any symptoms until the yellowing starts). Often the leaves start turning yellow but the veins stay green and may appear pale. If it goes for a long time, leaves will start to taco, the tips start to turn brown and crispy, and eventually start breaking to touch.


Autoflowering plant heat stress symptoms: cannabis bud bleaching

One of the most common symptoms of heat stress - bleached buds.

Bud bleach is most common with LEDs, basically happens when the buds get too much light and start turning white at the top, the affected part will lose potency and its smell.

Light burn should not be confused with a nitrogen deficiency, we recommend paying attention to the small differences. Nitrogen deficiency starts from the bottom and moves up, nitrogen-deficient leaves will fall on their own. Light-burned leaves are hard to pluck off and in most cases, the yellowing will occur on top of the plant.


If your plants are getting too much light, try moving your lights further away, removing some of the lights, or look into a dimmer to control the intensity of your grow light. To prevent bleaching you should look into low-stress training to prevent your plant from stretching too much. Unfortunately, there is no way to bring your plants back to normal other than let them grow it out or harvesting before they die in the worst cases. The best way to prevent this is by taking all precautions before it happens.

You can also consider other lighting solutions. While HID lighting was the absolute gold standard for decades, these days it has been overtaken by LED panels. LEDs are still slightly restrictive in terms of pricing, but they do offer a few key advantages over HID setups, with heat being high on that list. LED panels produce almost zero heat, meaning you can place them very close (or even hang them vertically within) the canopy. 

You should always aim to keep your light at the correct distance from the top of your plant canopy. Instead of leaving this all up to guesswork (which can increase the risk of harming your plants), you can use a couple of tools to make sure you do things properly. Using a lux metre will enable you to measure the amount of lumens falling onto a certain area. While these devices only measure the light perceivable to the human eye, they’re still a useful tool if you have no other choice. Ideally, you’ll use a PAR meter to measure the amount of photosynthetically activated radiation falling onto your plants during different stages of the growing cycle to get your light intensity within a sweet spot range.

3. In Conclusion

When making changes to your plant’s environment it’s best to make changes as slow as possible. To keep a good climate for your autoflowers you need to prevent a sudden change of humidity, temperature, and light. Keep in mind these 3 factors as they are tied together when talking about climate. A thermo hygrometer is an instrument that measures temperature and humidity, always use one when growing indoors. It costs around 15 bucks and it can help save your harvest if you’re having the problems we discussed above. You don't necessarily need expensive equipment to control your growing conditions. By using a cheap oscillating fan and leaving your window open you might be able to grow beautiful flowers without spending too much. So remember, it's a matter of spending what you can and making the most out of what you have!


If you have any tips and tricks to help fellow growers ideal with high temperatures, feel free to leave a comment in the comment section below!