Though most strains can tolerate heat well, cannabis heat stress is always a risk if the plants get too hot, too hungry or too dry. Even the varieties that look like they are surviving heat stress may end up producing lower yields and low quality buds.
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.
In this article, we will cover all possible causes of cannabis heat stress and 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.
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What is cannabis heat stress?
In layman’s terms, cannabis heat stress is your plant’s call for help before it passes out from high temps in your grow room. But this answer won’t get you any passes on the science exam, will it?
So technically, heat stress implies your cannabis plants are losing their ability to self-regulate internal homeostasis – read “balance” – processes that help them cool down.
In other words, 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
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?
Picture this. You’re on vacation to Dubai. The sun is scorching hot – so hot you’re sweating buckets. It sure would be nice to have some air conditioning, wouldn’t it? But there is none! How about some water to quench your raging thirst? None either.
The sun here represents your light source in the grow room. Air conditioning could represent your fans, heat extraction, and ventilation equipment. And as for water, there’s no substitute for that, is there?
Before we discuss tips for plant heat stress recovery, let’s take each of these one-by-one and learn how they contribute to plant heat stress symptoms.
When you have a very dry growing area, your plants quickly lose moisture.
Leaves have pores, called stomata, which release the water stored in your plants as relative humidity in your grow room drops below recommended 50% – 60% levels.
(Fun fact: stomata also release moisture ordinarily, as a by-product of photosynthesis.)
Potting soils also lose moisture through evaporation when the climate’s dry.
The double-whammy of water loss from your soil coupled with water loss from your plants leads to signs of heat stress in weed that ultimately culminate in wilting.
And why is wilting bad, you ask?
First, you should know that wilting is a natural adaptation to heat stress.
You can observe the wilting process turn your leaves brown due to cannabis heat stress; this is because they have no chlorophyll left.
Chlorophyll is the green pigment in leaves, and without it, affected leaves cannot photosynthesize. If wilted leaves cannot take part in photosynthesis, they also can’t lose water via the same route (remember our fun fact earlier?)
Unfortunately, this reduction of photosynthesis also stunts growth and impacts yield. Heat-stressed buds may become foxtailed and lack potency if your plants are severely affected.
Moisture always evaporates from plant leaves and potting soil into hotter surroundings.
Lighting sources like HIDs can often contribute to heat buildups that cause cannabis heat stress. However, since you cannot grow cannabis without lights, your real focus should be on making sure your cooling methods, air circulation, and heat extraction are reliable 24/7.
Faulty fans, air conditioners, and heat extractors allow hot hair to stagnate in your grow room. Making sure your temperature control equipment has enough cooling capacity for your room size is also essential.
You can learn more about ideal temperatures for growing weed here.
Did you know that light can burn?
Yes, hold a magnifying over a piece of paper in the dead heat of summer, and you might astound our flint-reliant stone-age ancestors if they were alive today.
Even though light rays hitting your plants’ leaves aren’t going through a magnifier per se, they can still burn and cause signs of heat stress in weed if:
- The light’s intensity is too strong for your leaves.
- Passive heat emissions from your light source get too hot for comfort.
LEDs are generally preferred over HIDs for this reason because they produce less heat and are easily dimmable, so consider switching if you need to reduce light intensity to avoid direct light burn.
How to identify cannabis heat stress?
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
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 weed 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: weed 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.
Resolving cannabis heat stress primarily involves increasing the air circulation in your grow space. This can be done using an elaborate system, or by oscillating fans blowing above your plants.
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 cannabis 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 heat stress indoors
Indoors, finding a quick solution for heat stress can be either challenging or easy to remedy, depending upon myriad factors.
On the one hand, factors that contribute to signs of heat-stressed weed are in your control. On the flip side, you must take charge swiftly and decisively. Here’s what to do.
Seedlings should never be under the impact of intense direct light that emits a lot of heat. Install dimmers for your grow lights to manage their intensity.
Another great tactic is to increase the distance between your seedlings and the light source if you don’t have dimming technology. This not only lowers the light intensity but also prevents heat produced by grow lights from directly beaming onto your young plants.
Cannabis seedlings also need frequent watering, so they don’t dehydrate. In the seedling stage, water your seedlings once to twice daily, taking care not to overwater. This will significantly reduce the chances of cannabis heat stress occurring.
During veg, cannabis plants have lots of leaves which can all photosynthesize. Leaves that participate in photosynthesis can lose water, and when the relative humidity drops, they run dry.
To prevent humidity drops that cause this, place a bucket of water inside your grow room. When your room’s humidity gets too low, some of the water will naturally evaporate to balance the dry air with some moisture.
Next up, manage the heat produced by your lighting. It’s quite easy to install heat extraction hoods that route the hot air produced by grow lights out your room.
Keep grow lights 24 to 30 inches away from your cannabis canopy at all times.
Finally, manage cannabis hydration aptly during vegging. Water at least once daily in the morning or evenings while the potting soil is still cool.
Heat stress during flowering is never a good look.
The final height stretch of flowering can often pose a heat stress danger since flowering cannabis stretches towards the light source in the final stages of growth, risking light burn and heat damage.
If you have enough height clearance in your grow room, adjust the distance between your plant canopy and the lights to solve this issue.
Also, consider low stress training your cannabis plants during veg because this prevents them from reaching an unwieldy height at the peak of flowering.
Low humidity and stagnated heat buildups can still cause heat stress during flowering. So take care to water your plants frequently and perform checks to ensure your fans and heat extraction equipment are working fine.
Managing heat stress outdoors
Managing plant heat stress symptoms outdoors isn’t much different from indoors. The exception being that the natural environment is out of your control. Thus, you need to get creative with your mitigation measures.
If you live in a hot climate and already know you’ll grow your cannabis outdoors, it makes sense to choose a cannabis strain with natural heat tolerance. Agree?
Strains like Kaya Gold and Sour Diesel can survive in climates of up to 100°F without exhibiting plant heat stress symptoms.
Furthermore, you should grow your seedling stage plants outdoors in pots.
This way, as soon as you identify signs of heat stress in weed, you can transfer your seedlings to a cooler area with shade. If you’ve got the time and materials, you could also build a shade for your plants.
Never use black pots outside. This will cause your soil to store more heat.
Outdoor plants are also susceptible to faster water loss due to their direct exposure to humidity and heat outside, so remember to water twice daily, morning and evening, to avoid heat-stressed buds.
Vegging cannabis plants have lots of leaves and a higher surface area for moisture loss when the humidity drops too low outside, and the heat gets out of control.
The best way to get on top of humidity outdoors is to work with the weed seasons and plant at just the right time for your veg stage to coincide with agreeable moisture levels outdoors.
Ask experienced cannabis farmers in your area what planting windows work best so you can follow proven tracks and avoid cannabis heat stress.
Heat may be overpowering and cause additional water losses from your potting soil. Place a finger into your soil; if it feels dry up to your first knuckle, your soil water is diminishing, and your cannabis plants are probably thirsty.
Water vegging plants at least once daily. And if possible, it’s a great idea to move your vegging plants into the shade when you first spot signs of heat-stressed weed.
Wondering what heat stress looks like on weed that’s flowering? Not good!
Nothing gives away heat stress during flowering like foxtailed buds. Fox-tailed colas lack potency, weight, and bag appeal and are formed due to your cannabis plant abandoning further flowering at the sites where heat stress occurred.
To mitigate heat stress during flowering outdoors, erect some shade over your plants. Growing weed in a greenhouse can also be a great strategy if you have the budget.
You must water your flowering cannabis plants at least once every two or three days. Or even daily if your conditions are arid.
Watering not only replaces the moisture plants lose. It also creates an evaporation cooling effect where the escaping moisture serves as a coolant to the plant.
Kelp extracts additionally offer a way of cooling cannabis plant roots because of the cytokinins they contain. Besides this cooling effect, kelp has many of the trace minerals your flowering cannabis needs.
Keep it cool, and it’ll be fine
Cannabis heat stress can be devastating if you don’t know how to spot the symptoms or mitigate them.
Heat-stressed weed plants show two main signs:
- The cannabis leaves curl into a taco shape.
- Your leaves wilt and turn brown.
If you spot the first signs early, recovering heat-stressed plants is a cinch when you follow these no-brainer tips:
- Move your plant into the shade and wait for it to cool down before watering.
- Reduce the light intensity or increase the distance between the canopy and your lights.
- Install heat extractor hoods over your lights to route the hot air they produce outside.
- Consider installing LEDs over HIDs; they emit less heat.
- Ensure your fans are functional and circulating air 24/7.
- Place a bucket of water in your grow room to regulate moisture when humidity drops.
- Water your plants as frequently as needed for each stage of growth.
Mitigating and avoiding cannabis heat stress starts with proper planning. Keep these tips within reach at all times, and you’ll know what to do when you encounter signs of heat-stressed weed!
Keep learning with more amazing tips to avoid cannabis heat stress & light stress on our Homegrown PotCast.
Each episode is packed with lessons from our own trials and errors, so you don’t have to repeat them. Discover everything from our best tips for ensuring a high germination rate to proven grow tent ventilation tips that will keep your grow room cool and balanced 24/7.
About the author: Derek LaRose
Also known as Kronic from The Cannabis Kronicles, Derek LaRose is a young ambitious cultivator and a staple educator for indoor cultivation.