How to prevent and treat cannabis heat stress

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Cannabis heat stress
March 25, 2021
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Cannabis heat stress occurs for a number of reasons. In this article, we will cover all possible causes of cannabis heat stress and we will look at different heat plant symptoms. After learning this, we will look at how to treat heat stress symptoms and prevent it from happening in the future.

An Overview of Cannabis Heat Stress

Though most strains can tolerate heat well, cannabis plants become stressed and irritated when they get too hot. What’s more? Even the varieties that look like they are surviving heat stress may end up producing lower yields that are of low quality. If you find that your cannabis plants are looking pathetic in spite of having adequate water, nutrients, and light, you should consider looking into heat stress.

Join us as we discuss the symptoms of heat stress and what we can do to stop it from destroying our cannabis crop.

Homegrown PotCast on Cannabis Heat Stress

This episode of our Grow Weed at Home PotCast discusses how to stop cannabis heat stress and also light stress.

What is Cannabis Heat Stress?

Cannabis heat stress occurs when cannabis plants experience intense temperatures above and below, what they can tolerate. The ideal grow temperatures for cannabis plants in a grow room is between 50-60 ˚F during dark periods and 70-7 ˚F during light periods.

Heat stress can cause these negative effects on your weed grow:

  • Wilting and destruction of your weed plants
  • Reduced weed harvests
  • Bad quality pot bud
  • Destruction of the cannabinoid and terpene profile of your cannabis

Natural degradation of stressed weed

The natural degradation process usually occurs towards the end of the cannabis growth cycle, where THC-A turns to THC. Visibly, it presents as trichomes changing from clear to milky, then to amber. When cannabis plants get heat stressed, this process is accelerated causing them to appear mature before they really are, causing heat stress.

What Causes Marijuana Plant Heat Stress?

Marijuana plant heat stress in an indoor grow may happen due to a fluctuation of environmental conditions. Most of these factors can be regulated or manipulated to achieve the desired result. These factors are: high temperatures, low humidity, and high light intensity.

Let’s examine each of these heat stress factors and learn what solutions can be applied in each situation.    

Temperature and Humidity gage
Temperature and Humidity gage reading 47% – 71p

Low Humidity   

Low levels of moisture can make it hard for your plants to remain cool, so by increasing the temperature they get thirsty! Though low cannabis humidity by itself may not cause any heat stress symptoms, when combined with the other factors it leads to compromised plant growth and low yields.

Use a Hygrometer

You can use a hygrometer to monitor the humidity in your grow room. If it is too high, you can use a dehumidifier to reduce it or place a bucket filled with water in the tent to increase humidity. Water your plants throughout the day to maintain cool plant roots.

High Temperature

As we mentioned above, the ideal temperature for cannabis strains is 50-60 ˚F for dark periods and 70-7 ˚F for light periods. When the temperature exceeds this range, your plants may experience heat stress. You can tell when your plants are experiencing heat stress, because their leaves start to fold inwards.

During the flowering stage, plants are more likely to experience heat stress. This may result in damaged leaves and damaged buds which causes a diminished potency. The abnormal foxtail buds occur when the plant tries to abandon the heat-damaged buds and develop new ones. As mentioned, some of the heat stressed weed signs might be precocious bud maturation which manifests as white pistils. 

High light Intensity can cause heat stress   

Light stress, also called light burn, mostly occurs with indoor plants. It can happen during the transferring process of the plants moving across from a lower grow light to a strong grow light source and it can happen when the grow light is too close to the plants.

High-intensity grow lights
High-Intensity grow lights

Most grow lights come with height recommendations and these should be adhered to so as to avoid light burn.

What does Heat Stress Look Like on Weed?

Symptoms of cannabis heat stress present themselves differently depending on the stage of the cannabis plant and the cause of the heat stress.

The symptoms of heat stress on cannabis plants that would appear during the flowering stage vary from those in vegetation, as does the risk at the different stages. What stresses a cannabis plant in the flowering stage, might not necessarily stress it in the vegetative stage, therefore it it important to understand what heat stress looks like, so you can ‘nip it in the bud’ before it ‘does a number’ on your plants.

The Key Symptoms of Cannabis Heat Stress

Example of Heat Stress Cannabis
Evidence of plant heat stress as leaves are burned at the tips

Some key symptoms which will help you in identifying cannabis heat stress are:

  • The leaves wilting and drooping
  • The entire plant wilting and drooping
  • Drying up and curling upwards of the leaves (also known as “taco” -ing)
  • Leaves at top of plant bleaching
  • Brown spotting on leaves
  • Yellowing in patches, or the whole leaf except the veins and stems, but without physical damage.
  • Formation of foxtails – The heat stress may cause new buds to grow on top of mature buds.
  • Leaves appearing burned at the tips.

It’s important to distinguish between, heat stress symptoms and other similar growing issues, such as light intensity, nutrient defects or excesses and other plant growing plains.

Solutions: Plant Heat Stress Recovery

Whether your cannabis plants can recover from heat stress, and to what extent, is dependent upon a couple of factors such as the stage of development of the plants, and the extent of the damage.

Cannabis Heat Stress Recovery
Evidence of cannabis plant heat stress

Resolving cannabis heat stress primarily involves increasing the air circulation in your grow space, which can be done using an elaborate system, or by oscillating fans blowing above your plants you’re your plants experience heat stress while growing outdoors, cooling your roots down by increased watering could go a long way in reducing the heat they are experiencing.

Either way, one of the many luxuries of growing indoors is the control you have over the grow climate.

Let us look at how to deal with cannabis plant heat stress indoors:

Dealing with Cannabis Plant Heat Stress Indoors

Indoor cannabis growing involves an efficient system that maintains factors such as light and temperature at levels that are comfortable for the plant. Installing proper exhausts and conditioning, and also constantly monitor the temperature for growing weed to keep the correct degree/ Fahrenheit.

The HID lights that are often used for indoor cannabis growing are ideal for providing ideal light, but they could end up overheating the room, especially in areas that are hotter.

Quick fixes for minor cannabis heat stress

  • Increasing the distance between the canopy and grow lights
  • Spread oscillating fans around the grow room to circulate the air around the grow canopy
  • Consider using LEDs instead of HIDs as they produce minimal heat
  • Install air conditioning for cases where temperatures go beyond what is recommended
  • Upgrade exhaust fans to bring fresh air into your grow room at least every 5 minutes
  • Maintain grow room temperatures at below 25°C

If you live in a cool area, it might not be necessary to install air conditioning. However, you could still invest in a dehumidifier. Doing this ensures the air in your grow room does not get too humid and consequently, too hot. Constantly measure the temperature and humidity in your grow room.

Decreasing temperature in your grow room and improving air circulation are great ways to reverse heat stress weed indoors. Placing a small fan above your plants will ensure they do not develop heat spots. As it stops them from being positioned directly below growing lights.

Many indoor setups require you to set up a method of grow tent ventilation. The aim is to remove hot air using a fan or exhaust system. Removing the hot air, and introducing fresh new air is likely to go a long way in preventing heat stress. An oscillating fan placed strategically in the room will also provide a gentle breeze that is ideal for rejuvenating them.

Also moving the grow light further away from the plants will keep them cooler.

In order to prevent and reverse heat stress, ensure that the temperature is kept at a comfortable low throughout. A good way to test this is that if you feeling too hot, the plants most probably are too.

Keep plant roots cool

Another way to minimize heat stress weed is to always keep the roots of your plants cool. This helps the plant deal with any heat issues better. Constantly watering your flowers with small amounts of water will go a long way in ensuring that they stay cool.

Seaweed kelp extracts have also been lauded as a great way to help cannabis plants recover from heat stress. They’re often available conveniently as liquid fertilizer. This extract helps a lot in recovery and might even protect the plants from future heat stress.

Indoor Seedlings

In the seedling stage, heat stress might cause wilting and the paling of leaves. Seedlings are very sensitive, and heat stress can cause stunted growth, and even death very quickly. If you notice your seedlings experiencing heat stress, immediately adjust the conditions in the grow room and ensure that they are properly hydrated.

Vegetative plants

Cannabis Heat Stress in Veg
Evidence of cannabis heat stress at vegetation stage

This is the stage at which cannabis plants have the best chance of full recovery from heat stress. Why? Because they are rapidly growing and they have enzymes that could kick in for recovery when conditions are corrected.

Luckily, while cannabis plants are in the vegetation stage, heat-stress cannabis plants can recover quickly. If you catch the early stages of heat stress during the vegetative stage, make sure you solve the problem ASAP. Doing this will prevent the problem from going into flower.

Flowering

As we have seen, the damage caused by heat stress during the flowering stage is usually impactful, often irreversible. Consider using seaweed kelp fertilizers if the damage is already done to spur them into recovery. These seaweeds contain cytokines that reduce the stress being experienced by the plants, allowing them to recover.

A Final Word on Marijuana Plant Heat Stress

Heat stress can have very dire consequences but it is totally avoidable. Simply by thinking ahead and creating an optimal environment, the plants in the grow room, or these growing outdoors can be prevented from experiencing heat stress and its adverse effects.

We hope we have equipped you to avoid, and even reverse heat stress.

Cannabis Heat Stress FAQs

  • What is heat stress?

Heat stress is the negative effects experienced by marijuana plants when exposed to too much, or too little heat over a considerable period of time

  • What are the symptoms of heat stress?

Plant heat stress symptoms include turned up leaves. The plant may also droop, experience stunted growth, browning leaves, malformation of heat stress buds, and early maturation.

  • How do you prevent heat stress?

By preparing your space beforehand to ensure heat does not build up.

  • How can plants recover from heat stress?

By conditions being adjusted to optimal, the application of kelp and cooling roots.

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