Preventing Plant Heat Stress

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Too much light or elevated temperature could stress your plants, always be on the lookout for these signs.

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.

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.

Symptoms:   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.

Solutions:   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.

Symptoms:   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.

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.

Solutions:   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.

Symptoms:   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.

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

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 is worthless as it will lose potency and it’s smell will be gone.

Solution:   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.

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