heat stress weed

Autoflowering Plant Heat Stress Symptoms

Your plants can only withstand a certain amount of heat and light. After a certain point, your autoflower will start showing signs of stress 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.

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, stunting growth, affecting yield and ultimately killing your plant.

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% depending on the stage it’s in. 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.

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 to water 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 it will respond to overheating by growing buds with less potency and eventually with abnormal growth of buds. In 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, if this is not possible an oscillating fan blowing on top of your plants might be a good alternative. 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.

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

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 signs 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.

Tip: 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.

Bud bleach is most common with LEDs, basically happens when the bud gets too much light and starts turning white at the top, the affected part will lose potency and it’s smell.

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.

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.

Your plants can only withstand a certain amount of heat and light. After a certain point, your autoflower will start showing signs of stress on the bud or leave

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.