• Post last modified:February 10, 2022
  • Reading time:11 mins read
  • Post comments:0 Comments

What is cannabis heat stress?

Just like humans, cannabis plants have a limit to the amount of heat they can handle. Excessive heat can destroy plants and ultimately mess up your yield. Cannabis heat stress refers to a condition where your plants are exposed to high heat levels from grow lights or during a heatwave. Water evaporates from the plants’ stems, leaves, and flowers, dehydrating them. As a result, the plant photosynthetic capability is affected, leading to limited root development. This ultimately interferes with the plant’s ability to absorb nutrients. The plant begins to lose its healthy look within a short period, indicating its deteriorating condition. So, how do you know if your cannabis plant is experiencing heat stress?

Signs of a heat stressed weed plant

cannabis light burn
Curling and yellowing of leaves are the first signs of heat stress

Heat stress can affect a cannabis plant at any of its three stages of growth (seedling, vegetative, and flowering). However, you need to understand that other issues (nutrient and environmental) can cause similar symptoms in plants. As such, you need to be very careful not to make the wrong diagnosis and further damage the plants with the wrong treatments/solutions. Below are signs of heat stress weed from the seedling to the flowering stage. 

Cannabis heat stress during seedling stage

Seedlings are fragile and can easily be damaged by conditions or diseases that an otherwise grown plant would overcome. If you notice a lack of development in growth, pale and withered newly formed leaves, and sometimes the death of a seedling during your daily inspections, there is a strong possibility the heat is in excess. 

Cannabis heat stress during vegetative stage

This is the stage where cannabis plants experience accelerated growth. When heat stress strikes, you will notice the following:

  1. Dry, brown spots on the leaves
  2. Leaves curl up, and the leaf edges appear withered
  3. Leaf tips appear curled and burnt
cannabis heat stress during flowering stage

Cannabis heat stress during flowering stage

By this stage, the plant has completed its growth sprout and is solely focused on making buds/flowers. At this point, heatwave causes;

  1. Yellow and bleached leaves
  2. Otherwise would be flat leaves curl upwards
  3. Hairy buds with no matter that appear on top of other buds. These are referred to as foxtail buds
  4. More white pistils than orange ones.

Factors contributing to heat stress

Heat stress can be caused by several factors, most of which can be solved. Identifying the problem early is essential in returning the plant to its former glory for a bountiful harvest. Below is a list of these factors…

Cannabis light burn

Grow lights produce heat. When placed near the plants for an extended period, it can lead to heat stress and eventually cannabis light burn, especially if the temperature in the grow room is already high. Additionally, too much light encourages the leaves to absorb extra nutrients, leading to bleached lead tips. If the issue is the distance between the plants and the grow lights, check with the manufacturer to determine the ideal space to maintain. If you have high-intensity grow lights with PPFD higher than 1300, consider utilizing CO2 supplementation. Maintain the following PPFD for cannabis plants.

Growth stageIdeal PPFD
Seedling 100 – 300 μmol/m2/s
Vegetative 400 – 600 μmol/m2/s 
Flowering 800 – 1,000 μmol/m2/s

Low humidity levels

The recommended humidity level for growing is anywhere between 40% and 50%. It should never fall below 30% or go above 60%, beyond this. Low humidity accelerates the transpiration rate of plants, leading to cannabis heat stress, and ultimately, wilting. This explains why the initial sign of heat stress is wilting of leaves. The wilting, in turn, changes the plants into a brown hue due to a lack of chlorophyll. The chlorophyll must be present for photosynthesis (food-making process) to occur. Without it, the plant experiences stunted growth and low-quality, less-potent yields. 

Heatwave

Cannabis plants thrive when exposed to temperatures of 70-79˚F during light hours and 50-60 ˚F during dark hours. During a heatwave, cannabis plants, especially outdoor-grown ones, are exposed to high temperatures for a prolonged duration. The increased temperature leads to plants exhibiting signs of cannabis heat stress, such as wilting, cupping leaves, and dry leaf edges. Unfortunately, heatwaves are part of nature, and there is not much you can do to change it. What you can do is take good care of your plants to ensure they get through the episode with minimal damage (more on how to deal with this later).

Inadequate airflow in the grow room

Unequal distribution of heat in the grow room may lead to heat stress in certain plants. Investing in an effective ventilation system will discourage the formation of hotspots under grow lights.  It also encourages the free flow of air around the grow room. 

Malfunctioning or damaged equipment

Your environmental monitoring equipment is essential in maintaining optimum growing conditions. When a gadget like a humidity scale is not working, it compromises the quality and quantity of your yield. Ensure that all the equipment is in good condition by conducting regular inspections.

How to help your plant recover from heat stress

The big question is, will plants recover from heat stress? Well, your heat stress weed can gradually recover over several weeks if the situation is discovered early enough. Here are a few tips you can use to help your plants recover;

Growing cannabis indoors

  • Reduce the light exposure to nighttime only. At sunset, turn the lights on and leave them on until sunrise. This will minimize the heat experienced during the hottest hours of the day.
  • Move your grow light considerably far from your cannabis plant. This reduces the amount of heat and prevents light burn cannabis.
  • Invest in oscillating fans to maintain constant air movement within the grow room.
  • It is rare to hear of an LED light burn as the lights tend to produce less heat than HIDs, HPS, and other grow light variations. Use LED lights in the grow room. They may be relatively costly, but the services they provide are worth it.
  • Air conditioning can help create more conducive conditions for your plants in areas where the temperatures are constantly high. 
  • Invest in a powerful extraction fan capable of introducing fresh air into the grow room every five or so minutes.
  • Monitor the temperature and humidity levels of the grow room and ensure it falls between the recommended scale. 

Growing cannabis outdoors

When growing outdoors, the options you have of altering the conditions are limited. However, there is much you can do, such as;

  • If you live in areas where heatwave occurs frequently, you need to plan ahead and plant cannabis strains with high heat resistance, such as Bruce Banner #3 and Amnesia Haze. Alternatively, grow your strains in an area where shade is readily available during the day.
  • Water your plants in the morning and evening to help maintain a constant water supply. 
  • Use large containers and avoid ones with a black hue. Black absorbs heat fast and will only worsen the situation. If you already have black containers, try and cover them with clothes with more reflective colors, such as white. 
  • Use organic seaweed and kelp fertilizers to help restore the plants’ health. 
  • Build your plants a temporary or permanent shade, depending on your budget and time in hand. 

When do you know the situation cannot be salvaged?

If the plants experience heat stress during the vegetative stage, there is a high chance of recovery. They can gradually develop other leaves over several weeks before transitioning to the flowering stage. You may end up with a decent quantity of high-quality yields. However, if the heat stress was severe and occurred during the flowering stage, it may be impossible to salvage the situation. What you can do is harvest the available buds and use the tips given in this piece to avoid similar future occurrences.  

Take away 

Cannabis heat stress can have detrimental effects on your plants. Sometimes it can be dealt with, and sometimes, it can’t. The best approach is to plan and equip yourself with the necessary skills to avoid it. If it does happen, identifying the signs early enough and dealing with them will allow you to have a bountiful harvest. 

Leave a Reply