Cannabis Heat Stress: How to Spot, Prevent, and Treat It
Read our guide on diagnosing, avoiding, and treating heat issues in your indoor and outdoor weed garden
Most of you know how important it is to control the temperature in your weed garden. You also understand that when it’s too hot for too long, the plants will begin to suffer and show you exactly what’s wrong with them. The trick is knowing where to look. In this post, we’ll talk about how the symptoms of cannabis heat stress look like so that you don’t mix’em up with other issues and take proper action right away.
- you can notice symptoms of cannabis heat stress on both leaves and buds
- leaves start to cup (curl up) and later become brown and burned
- in flower, buds may start to foxtail
- control the distance from the light and set up proper ventilation
- maintain the right temperature in the root zone
Experienced cannabis growers recognize signs of heat stress right away. Heat stress may affect leaves as well as buds.
Table of Contents
Symptoms of Heat Stress on Leaves
It all starts when those sharp points on the leaf edges begin to curl up. Thereby, the plant tells you that it has become too hot.
Serrated edges on the leaves start to point up. © Growdiaries © misterb
And if you don’t resolve the heat issue soon, the leaves curl up even more and start to look like tacos.
Cupping leaves and stunted growth due to heat. © Growdiaries © misterb
It’s hard to say why it happens. Maybe that’s how the plant tries to reduce the surface that receives light.
In an indoor setup, leaf cupping often happens on the parts of the plant that are closest to the bulb. It’s because they receive too much radiant heat from the light. And farther down the stem, leaves may remain normal.
And sometimes, only SOME of the upper leaves show these symptoms — where the light creates hot spots.
Of course, during a heatwave, your whole grow room can get too hot. Then you may notice signs of heat stress on both upper and lower leaves.
Let the problem persist, and you’ll see the situation get much worse. The leaves may get yellow and then brown. The discoloration starts at the tips and along the edges. Later, the heat burn spreads to the whole leaf, and eventually, the leaf becomes dry and crispy.
Here, the heat-related issues got out of hand. © Growdiaries © sndwich
Don’t Confuse Cannabis Heat Stress and Calcium Deficiency
It’s easy to confuse the signs of cannabis heat burn and the symptoms of calcium deficiency. Calcium-deficient leaves also have brown spots and may become dry and brittle at the tips and along the edges, but they also twist in every direction.
Brow, dead, distorted leaves are signs of calcium deficiency. © Growdiaries © sndwich
Of course, there are situations when plants have both these issues at the same time. Calcium deficiency makes the plants more vulnerable to heat stress, and heat stress makes the calcium-deficient leaves dry out faster.
Droopy leaves aren’t a sign of heat stress. Rather, they indicate that the plant is thirsty. So, if you see that the leaves on your cannabis hang lifeless, maybe it’s time to water your garden. And don’t forget to water more often when the temperature in your grow room rises.
These plants probably get less water than they need for transpiration. © Growdiaries © stumay111
Btw, droopy leaves aren’t a very specific symptom. So, it’s easy for inexperienced growers to confuse underwatered wilting and overwatered “claws”. Besides, leaves may slightly droop every evening (before lights-out), and it’s perfectly normal.
Both light stress and heat burn often stem from the same problem — the light being too close to the plant tops. Though these two may manifest at the same time, they are different issues. With good ventilation, you may have an ideal air temperature in the grow tent (and no heat stress), but your plant tops will still suffer from too much light.
Light stress starts with the yellowing of the leaves. Please note that usually, it’s not the youngest leaves on the very top that start to yellow, but the ones just below them. It’s because they have been exposed to light a day or two longer and so get affected earlier.
Light stress is a very common issue in seedlings. Growers often think it’s potassium deficiency and try to treat it as such, but all they need to do is raise the light.
The scorching Californian sun isn’t kind to these seedlings. © Growdiaries © misterb
Symptoms of Heat Stress on Flowering Buds
Extreme temperature can damage flowering weed plants in several ways. It can interfere with the production of THC and terpenes, making your smoke less potent, aromatic, and flavorful. It can also lead to lower yields because heat prevents buds from filling out and they remain airy and spongy. In a worst-case scenario, buds start foxtailing, especially if you grow one of the strains with a tendency to foxtail.
This Critical Purple Auto grown on a rooftop in NorCal displays both the foxtailing and the taco-ing of the leaves. © Growdiaries © misterb
If the heat is extreme—even for a short period of time—the pistils may change their color from white to brown prematurely. Pistils are the female hairs on cannabis buds. It’s normal for them to become amber or brown at the end of the life cycle when the buds are already big and dense. But if the flowers have only begun to form, they should have white pistils.
How Much Heat is Too Much?
Just like humans, marijuana plants can tolerate very high temperatures for a short time. The problems begin when the temperatures are consistently above optimal levels. You may ask: what are those optimal levels?
Well, we’ve covered that in another article where we cite available research on the topic. In a nutshell, aim for 77-86°F (25-30°C) during the day and 10°F (8°C) lower at night. When the temp levels creep above the allowed maximum, photosynthesis slows down and you begin to notice stunted growth and heat stress symptoms.
And there’s one more thing we didn’t mention earlier: cannabis pests LOVE hot and humid environments and multiply in great numbers.
A lot of tiny spots on the leaves point to some pest infestation. © Growdiaries © sndwich
How to Take Care of Heat Indoors
First of all, don’t plan any indoor growing for the summer months. Otherwise, your grow tent will turn into an oven when a heatwave comes. Second, try to use lights that generate less heat. This means LED quantum boards instead of HPS bulbs.
A very clever trick is to run your lights during night hours—when it’s generally cooler—and turn them off for the day. The only issue could be light leaks. They don’t matter if you grow autoflowers, but with photoperiod strains, make sure there are no light leaks at the seams. Otherwise, they can disrupt flowering.
You can also try and raise the levels of CO2 to help cannabis deal with heat stress. For example, you can place CO2 bags in your grow tent. Please bear in mind that this method is controversial. Available research in other crops draws a very complicated picture. Sometimes, more CO2 is helpful in dealing with high temps, and sometimes it hurts plants even more.
Clearly, the most important thing is ventilation. Your extractor fan should refresh the air inside your grow space at least once every 3-5 minutes. And if you think problems with heat are possible, make it every minute. The same goes for small grow tents. The smaller your tent is, the easier it is to overheat.
So, calculate the volume of the grow tent in cubic feet, write down this number, and buy a fan with the same CFM (the cubic-feet-per-minute capacity).
Also, install an oscillating fan that will blow just above the canopy level. It will help protect your plant tops from heat burn.
The Temperature in the Root Zone
Hydroponic growers know very well the importance of keeping the roots nice and cool. If the temperature of the nutrient solution gets way above the recommended 65°F (18°C), problems begin. The levels of dissolved oxygen go down, and the pathogens thrive.
The temperature in other mediums, such as soil or coco, is just as important, but soil growers often forget about it and learn how important that is the hard way.
So, when growing weed under powerful bulbs indoors, make sure to shade your pots from the light. Especially if the pots are black and thus absorb the radiant heat easily. For the same reason, it’s safer to use terracotta containers with thick massive walls than thin plastic ones. And when watering your plants, be sure to use water that is not too warm.
Important! Even if the temperature of the air is way off the mark but the temperature in the root zone is within the ideal range, the plants will be perfectly healthy and show no signs of cannabis heat stress.
Measures Against Cannabis Heat Stress Outdoors
Outdoors, you can help your weed garden deal with heat in three ways:
- Keep the roots cool.
- Shade the plants from the sun during the hottest part of the day.
- Help them survive a heatwave with regular watering.
If you grow cannabis outdoors in pots (rather than on beds), make sure the containers themselves don’t receive any direct light. And when the plants grow in the ground, use a lot of mulch to cover the ground and thus insulate it from heat.
In a hot and sunny climate, plant your cannabis in a spot where trees or walls shade them for a couple of hours in the afternoon. However, a more efficient way is to plant your garden in an open spot and spread shade cloth over it when necessary. And if you grow weed in pots, you can move them around, chasing either the sun or the shade as needed.
Watering your garden generously each morning will help the plants deal with the heat. And if the water is cool, it will not only provide moisture but also lower the temperature in the root zone.
Can You Revive a Heat-Stressed Plant?
In most cases, all you need to do is adjust the temperature and simply wait for the plant to get better. However, some growers propose more hands-on approaches.
Silica (Silicon Dioxide)
Many nutrient manufacturers offer supplements that contain silica. They claim that silica makes cell walls tougher, and tougher cell walls—among other things—make sure leaves lose less water during extreme heat and drought. There are also other mechanisms of silicon’s effect on heat resistance.
The good news is that research on crops other than cannabis, such as rice and tomatoes, fully supports these claims. Silica really makes higher plants more tolerant of heat.
And this is a list of cannabis growing supplements that contain silica:
- Advanced Nutrients Rhino Skin,
- True Plant Science Silica Gold,
- Botanicare Silica Blast,
- B.A.C. Silica Power,
- NPK Industries RAW Silica,
- Cyco Platinum Series Silica,
- Advanced Hydroponics of Holland Advanced Silica.
There is ample evidence that humic acid enhances heat tolerance in plants. Researchers have observed these effects in Arabidopsis, tomatoes, and oats. The first of these studies even suggested the exact mechanisms of how humic acid makes plants more heat tolerant. Namely, it regulates several relevant genes, including Heat-Shock Protein (HSP).
Cannabis growers widely use humic acid which is a part of the following products:
- Green Buzz Liquids Humic Acid Plus,
- NPK Industries RAW Humic Acid,
- Earth Juice Rich Humic,
- Guanokalong Black,
- Humboldt Nutrients Hum-Bolt and FlavorFul.
Seaweed Kelp Extract
Seaweed contains plant growth regulators, aka plant hormones. As their name suggests, these molecules can regulate many important aspects of plant growth. So, growers use seaweed to boost germination and rooting and to increase yields. Some say seaweed kelp extracts help cannabis recover from environmental stresses, including heat and drought.
However, in her review of the available scientific literature, Dr. Linda Chalker-Scott notes a huge disproportion between aggressive marketing of this “wonder supplement” and its middling benefits. In other words, you’ll end up overpaying for something that in fact doesn’t do much.
With that said, here are a few seaweed kelp products that weed growers use:
- Kelpak Seaweed Extract ~ Vegetable Fertiliser,
- Technaflora Soluble Seaweed Extract,
- Gaia Green Soluble Seaweed Extract,
- Platinium Hydroponics Engrais Seaweed Extract,
- Greenleaf Nutrients Kelp Extract,
- Maxicrop Liquid Seaweed / Liquid Seaweed Plus Iron,
- Plant Magic Catalyst.
Well, this wraps it up. Now you know how to diagnose, prevent, and treat heat stress in cannabis. And if you live in a hot climate or your grow setup has permanent temperature problems, try cannabis strains that are resistant to heat.
How to protect your cannabis plants from heat stress
As beneficial as its end-result may be, cannabis can be quite the finicky plant to take care of. Too much heat, too little heat, too much humidity, or too little humidity, can cause issues to arise during its life cycle. Which is why cannabis heat stress can plague a garden with problems. Heat stress cannabis plants will lose out on quality, and yields – so it’s important to know how to avoid. Whether you’re growing indoors or outdoors, brush up on your cannabis heat stress knowledge now with our ultimate guide.
What is heat stress?
Put simply, heat stress is a condition that occurs when a cannabis plant is exposed to too high of temperatures. Generally, indoors and outdoors, you should keep your growing climate under 80°F for an optimal environment. Any temperature higher than 80°F can cause cannabis heat stress to occur.
Any type of ‘stress’ to plants will affect their overall growth. But, heat stress is particularly concerning for vegetative plants. As they haven’t established a healthy root system yet. In this case, veg plants can develop elongated stems and will slow growth overall. This not only affects yield quality, but yield size, too. In flowering, heat stress will not only slow down growth but can also cause buds to become airy versus thick and dense.
How to identify heat stress?
What’s difficult about identifying heat stress, is the common symptoms often look like nutritional or feed deficiencies. Luckily, there are a few specific signs to look for to confirm its heat stress versus other issues –
- Leaves curling upward – This is the telltale sign of a cannabis heat stress problem versus feed issues.
- Dry and withered appearance – Your plants will appear generally under watered, dry or withered in appearance. The plants may also droop or wilt.
- Irregular or large brown spots, or yellow blotches on leaves – These types of spots will occur primarily on the edge of fan leaves, and on the top of the canopy closest to the lights.
- Fox-tailing in flower – Foxtailing is the act of new buds growing out of top of already developed buds.
Tips for protecting plants from heat stress outdoors
Obviously, it can be harder to protect outdoor plants from heat due to a lack of control over the environment. Which brings us to our first tip – if you’re growing in a region with high heat or humidity, try growing indoors instead. This allows you to implement temperature control tactics that can cool the room when necessary. Otherwise, choosing a strain that’s well suited for high heat or humidity is recommended.
But, if you’re stuck growing outside there are some things you can do to protect your plants from heat stress. Including the following tips –
- Don’t underwater or overwater – As tricky as it may sound, you won’t want to overwater your cannabis plants, or underwater them. Keep a close eye on your plant’s soil to make sure it never dries out. Overwatering is a cause of concern since hotter water, contains less oxygen. When your plant receives less oxygen, it will show those telltale signs like drooping. Adding 30% perlite to your soil mix can also help avoid overwatering issues.
- Time your watering – Speaking of watering , timing it just right helps to avoid cannabis heat stress, too. When its unusually hot out, you’ll want to water your plants early in the AM or in the evening, when temperatures are cooler. Water drops actually magnify the light of the sun, so watering when the sun is strong can cause extra heat you’re trying to avoid.
- Keep roots cool – Keeping the roots of your plant cool, will help lower the temperature of the plant overall. When growing in containers, it’s best to protect the outside of the pot from direct sun or heat. One way is to use two different sized pots, with some type of barrier in between. Another way is planting your container, into the ground directly.
- Move plants – If you’re in an area that experiences high heat, definitely make sure your plants are in movable pots. During the hottest times of the day, you can then move your plants to shaded areas to stay as cool as possible.
- Use shade cloth – When you can’t move your plants, installing a shade cloth cover over your outdoor grow area can help dispel hot direct heat. Many growers build a frame around each plant, so they can add shade cloth over top while still having optimal airflow.
Tips for protecting plants from heat stress indoors
When growing indoors, it’s much easier to avoid heat stress from the start. If you implement a few key pieces of equipment into your grow room, you can keep an optimal environment at all times.
Here’s a few tips for protecting plants from cannabis heat stress, indoors –
- Install A/C and use fans – Fans help dispel hot, stagnant air for a convection like effect that cools your room down. If you’re able to, and if your garden is big enough, you’ll also want to consider installing a single A/C unit to help. Especially if you’re in a hotter climate, or if your indoor area runs hot.
- Adjust your lighting – What adds to heat stress when growing indoors, is the hot lights producing energy for the plants. Keep this in mind, and make sure your lights have enough ceiling space or can be adjusted to avoid direct heat to the canopy. You may also want to consider upgrading to LED lights which output less heat in comparison to HID lights.
- Installan air exhaust system – Being able to remove air, and replace with fresh air is key for keeping a cool, optimal environment indoors. Installing an air exhaust system in your indoor grow area is ideal for this purpose.
Indoors, or outdoors, you can also supplement your feed mix to help deter stress from heat. Many growers use add-ins like seaweed, kelp extracts or silicon to help keep roots and plants cool. These minerals and nutrients help boost the plant’s health to be more heat-resistant, overall.
What you can do if plants do become heat stressed
So now that you know how to identify heat stress, and avoid it you may wonder – what do I do if I’m too late? If you have heat stressed cannabis plants, you’ll need to have some patience. As soon as you’ve implemented cooling techniques, the plants will require some time to recover from the stress. During this time, add seaweed kelp extract to your soil or feed which will help boost the recovery process.
Maximize growth with optimal temperatures
To maximize the growth of your cannabis plants, you’ve got to make sure a number of factors are maintained and monitored. That includes, the temperature of your grow room or outdoor climates. Cannabis heat stress can cause growth to slow down, which affects your overall quality and yields. Ensure you’re maximizing growth with these tips to control temperatures, no matter where you grow.