Heat Stress

Heat Stress

Problem: All plant require heat to live and grow, your cannabis plants, although it grows like a weed, can only take so much.  Too much heat often coupled with other variables will have your plants stressed out and if it is prolonged stress will start to negatively impact your yield and your quality.  But how much is too little or too much?


 When Cannabis is too cold growth can slow or stop completely, often when dark cycle temperatures drop below 10 degrees Fahrenheit compared to light on temperatures growth is stalled until temperatures climb within the proper range for  plant processes to start again essentially remaining dormant;  example lights on 85 degrees and lights off 75 degrees. If you are too hot during your lights on stage you may see signs of having heat stress: If you start to notice curling leaf tips or the entire leaf starting to curl up or taco without the noticeable burnt tips often associated with nutrient burn you may be experiencing heat stress.

Fox tailing is often a symptom of too much heat. Fox tailing can occur during the flowering stage when new pistils or flower clusters start to form out of existing established buds often in the flower buds closest to the hot lights, these are often airy semi buds with no real size and structure.  If you are experiencing fox tailing due to heat stress you will most likely also notice that the buds farther away from the lights are not showing the same signs which is a tell tale sign that you are suffering from heat stress, I say this because some strains are genetically predisposed to fox tailing despite the environment.

Heat stress can definitely affect final yields and quality as terpenes and other cannabinoids are highly volatile and can off gas in high heat situations leaving you with lower potency and aroma.


Temperature is a critical component of any growing environment and ties in directly with humidity.  If temperatures are too high and humidity is low then signs can quickly develop of heat stress whereas high temperatures and high humidity can lead to disease and mold including bud rot.  Knowing your temperature and humidity is key, having quality monitors displaying high and lows is essential.  Some things to consider when growing cannabis indoors under lights is proper grow room environments. 

Firstly, you need good air movement within your growing environment, there should be a gentle breeze moving each and every leaf within the room breaking up the stratified stale air moving the hot humid air away from the leaf surface replaced with cooler dryer air enriched with CO2.  Important to note is that you want a gentle breeze not a windstorm causing wind burn.  On the topic of humidity consult a VPD chart or vapor pressure deficit chart that will illustrate the ideal temperature to humidity ratio according to each stage of plant growth and development.

Secondly, ventilation by way of fresh air or by air conditioning will bring your temps into the proper range.  In the event that you can’t cool your space with using fresh air make sure to run your lights as much as you can during the nighttime when outside temperatures are naturally cooler.  

Thirdly, choose your lights carefully, if you are using LED lights you can run slightly warmer temperature because the lights don’t generate as much radiant heat on the leaf surface whereas HID lights such as double-ended HPS lamps need to run at slightly cooler temperatures as they create more radiant heat of the leaf surface.

If you run CO2 your can run a bit warmer than without.   If you are struggling with heat and all of your other factors are ideal make sure you have enough carbon dioxide to allow you to run a bit warmer.  If you are running double- ended HID lamps that generate more radiant heat of the leaf surface run your temperature at around 80 degrees and no closer that 36 inches from the top of the plant canopy, If running LED lights temperatures can be raised slightly to 85 degrees as they offer less radiant heat on the leaf surface.  LED fixtures can be placed closer to the plants as well at 24 inches.  Heat plays a huge role in your growing decisions and your light choice mirrors that importance.  The above temperature suggestions are with CO2 if you don’t have high levels back off by 5 degrees on each light source.

Fourthly, keep your plants well watered-dry roots and high temperatures equals death.  Keep plants well watered with slightly weaker nutrients to help them overcome the stress.   If growing in above ground containers covering then with reflective material or paint will help reflect some of the radiant heat from the lights or sun as long as it doesn’t impair drainage.  If in hydroponics or recirculating systems try and maintain reservoir temperatures of 67-70 degrees.  


Keep plants in the desired temperature zone-lights on HID 80 degrees lights off 70 degrees no less that 36 inches above the plant canopy-LED lights on 85 degrees lights off  75 degrees 24 inches above the plant canopy.  Make sure you have good air circulation moving every leaf slightly breaking up any hot spots.  Use anti stress products such as Ocean Magic, a natural kelp solution high in vitamins and cytokinins shown to reduce stress or Vitathrive which is a solution of vitamins for reducing stress.  Another great product to consider is potassium silicate which helps protect against heat stress and drought.  If you are plagued with high heat a potassium silicate product such as Plant Guard may definitely help.  Lastly, if you just can’t get your temps down especially if you are enduring a heat wave, raise your lights and perhaps turn some them off to reduce the heat load and the light stress.  The weakest link theory applies with all factors being ideal and if one factor is off such as high heat then it reduces the effectiveness of all other factors for example, if the temps are too high the plant can’t utilize the ideal light levels causing additional stress meaning that you would have to dial back the amount of light you are offering until the temperatures are corrected, same thing is true if you have ideal temperature and light levels but low carbon dioxide, the plant can not use the light and the temperature effectively since it is suffocating, on and on and on.

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