Posted on by Jason G

REDUX: Heat Stressed Plants

This week's tutorial is a redux of a December 2017 tutorial

Growing indoors in the hotter months can be quite tricky for growers not reliant on climate control systems have to control this aspect of the grow themselves, using a variety of methods and tricks to reduce the rising temp.

So, let’s jump right into topic which is heat stress and environmental temperature.

Heat stress can cause irreversible damage to plant tissue, light is not the issue here as plants can never have enough light, the problem is the accumulated heat energy that causes the surface temperature of plant tissue to rise so much that the plant are unable to photosynthesise light causing a systematic metabolic shut down.

Heat stress can cause many things to go wrong, in both phases of the plants lifecycle there are different issues associated with this.


Heat stress in vegetation

Leaves can curl upwards and create a canoe type of look, high temps coupled with low humidity can make this symptom even worse.

FIX – Raising the light and reducing the wattage output of your lighting system if it is adjustable, go for par over power in this instance as it’s not the light causing the burn it’s the excess heat energy from the light, also lowering the amount of food you give them is highly recommended as water consumption is a preference on particularly hot days, also as lights are due to go off, a weak foliar spray will help clean the stomata of any dust and will help hydrate locally much faster.


Excess heat to plant canopy can drastically alter the plants leaf surface temperature, which can severely impede stomata function on the underside of the leaf as the stomata on the top are overworking by “sweating” water into the atmosphere to increase humidity and cool itself down, which in turn reduces the amount of carbon dioxide the plant can absorb. 

FIX – Same as above, however if dimming the lighting is not an option and the PPFD readings of your light is exceeding 1000 then CO supplementation will be needed to circumvent any potential air restrictions.


The root zone is also affected by the temperature increase, ideally most root zones should be maintained between 18.5c – 22c max! The higher the temp of the medium (substrate) the higher the likelihood of root bound diseases and pathogens. 

FIX – Short of the obvious - Frequent replenishment of nutrient solution, installing a water chiller, making and immersion wort chiller etc. There is a compound (Microbial) that can be added to your nutrient solution beforehand which leaves behind a protective barrier making the roots more resilient to the major temperature swings thus reducing the chance for a fungus or pathogenic attack.


(Don't be this guy, plants don't like scorching hellfire) 

Heat Stress

Heat stress in Flowering

“It’s not the end, even though it looks like it!”

Heat stress in flowering can be detrimental to the outcome and quality of your harvest, it is one of the worst things to happen, follow the next few suggestions and you should be able to salvage some of the better aspects of the plants.

Ensure that you do not overwater! Heat stress is not a drought! Do Not spray plants with anything whilst your lights are running. A general rule is never spray in flower unless you have an unexpected mite infestation, in which case you would still spray with the lights off, excess moisture on the plant whilst lights are on and temps are climbing will almost guarantee a scorched crop.

When flowers become heat stressed they result in “fox tailing” which is a stress related type growth, where secondary pistillate flowers actually start forming vertically from the top of the existing flowers. (Note: Some of the newer poly-hybrids can naturally foxtail as an expression from one of the parents.)

FIX: Dimming the lights and raising slightly will not prevent this (fox tailing) from happening but it will lower the amount of heat energy being exposed to the plant canopy allowing the plant to re-direct energy to primary flower growth. Fox tailing occurs much closer to the light and is exposed to the rising heat within the room, install air moving fans to stratify the surrounding air and if the exhaust system you are running is variable in speed I would suggest increasing the volume by an additional 10-25%.


Problems with heat stress impact everything in our environment, if heat stress is present then this stress will affect any variable like hydroponic systems, growing substrates, brain tanks etc..

Heat stress in nutrient tank reservoir’s directly impact EC concentration, EC will rise due the evaporation of water, always pay attention to your brain tanks or reservoirs on particularly hot days, I suggest lowering the EC by 0.5-0.8 and closely monitoring the pH and EC until the conditions have subsided. Understanding that as water evaporates from the nutrient solution, the EC will rise and pH will drop.

Other important factors to pay attention to is vapour pressure deficit (VPD) so, if the humidity is very low (20-30%) and the temperature is very high (29-32c) VPD is high, heat stress symptoms can occur due to low humidity therefore in an effort to cool itself down and increase the ambient humidity the plant will transpire.

If the environment is too high in temperature and too high in humidity, VPD is too low, so you will need to acquire a de-humidifier to lower the humidity and ideally some form of cooling apparatus like a portable air-conditioner, when this happens in the grow room the vapour pressure deficit of the room (VPD) is so low that the plants cannot adequately cool themselves down resulting in wilt and poor stomata operation.

TIP: A Hygrometer will give an accurate temperature and humidity reading, to raise the humidity most indoor gardeners opt for humidifiers/foggers to increase the humidity but they can be quite expensive, fortunately innovation has allowed others to emerge with products that work just as well, for example Integra released the Cargo dry strip which can be hung in your flowering or drying space and will maintain 50-55% humidity for up to 50 days.


So, there you have it lets hope some of these suggestions can help you dial it all in!

Heat stress isn’t any fun (at all), the learning curve is a great one though!