Even though weed plants are notoriously known for their love of warmer climates—cannabis heat stress is a real threat.
In moderation—warm climates create the perfect growing environment for weed—but too much of a good thing is never good.
There are various causes for marijuana heat stress. Let's see what they are and how to work.
What is Heat Stress?
Whether you're growing outdoors or in a grow room, seeing your marijuana plants with heat stress can really disappoint you. It usually happens when your crops experience temperatures out of the range they can handle.
Marijuana plants thrive in temperatures between 60–85℉ depending on whether it's day or night.
Problems such as stunted growth and poor yields often accompany marijuana plant heat stress. If left untreated, it ultimately results in the death of your crops.
What Causes Heat Stress in Marijuana Plants?
Preventing cannabis heat stress is much easier indoors. Outdoors is a completely different ballgame as you can't control all the factors.
The same factors are responsible in both situations—luckily, you can regulate and manipulate all of them.
Low humidity, high temperatures, and light intensity cause heat stress in your buds.
Let's check out these factors and how to remedy the situation once a disaster has struck.
Low Humidity
You might notice plants with heat stress symptoms even when the temperatures are in check. In these situations, you'll usually find very low humidity.
Your plants have a hard time keeping cool when there's a lack of moisture.
By itself, low humidity doesn't cause heat stress in weed. Unless it's combined with light intensity or high temperatures, you probably won't see any symptoms. It can, however, stunt growth and negatively impact your yield.
Make sure to use a hygrometer to measure the humidity in your grow room often. A dehumidifier is a great solution to reduce if the level is too high. On the opposite end of the scale, you can place water buckets around the room for some added water.
Keeping the humidity under control isn't all that easy when growing outdoors. A solution, if you’ve planted in pots, is to move them to the shade for a couple of hours each day.
The best way to prevent cannabis heat stress outdoors is by watering more often throughout the day. It should help to keep the roots cool.
High Temperature
High temperatures are the primary cause of cannabis heat stress. It's easy to pick up heat stress in weed. When the leaves start curling inward, you should act quickly.
The ideal temperatures to grow top-notch bud range between 60–80℉—dependent on the stage of its life cycle.
Heat stress damages buds and leaves, which may lead to reduced potency.
Marijuana plants with heat stress symptoms come in various forms. A phenomenon that is known as foxtail can occur as a sign of heat stress in weed. This is an oddly-shaped bud formation, created by a bunch of unfertilized calyces. As a result, the plant tries to replace the heat-damaged nugs with new ones.
Cannabis heat stress is most likely due to high temperatures during the flowering stage, as your crop is more susceptible during this time. Combined with low humidity, your plants are in trouble.
Increasing air circulation in the area you're growing is the best way to solve heat stress during flowering. For outdoor cultivators, cooling down the roots is your best bet. As mentioned earlier—water your plants more often but with less liquid.
Light Intensity
Cannabis heat stress due to light intensity—also known as light burn—only happens indoors. It occurs under the following circumstances:
- When your plant is transferred from a weak to strong light.
- If your plants are too close to the lights.
- Older leaves that have experienced prolonged exposure to lights.
Even though grow lights usually come with recommendations for height in various stages—you should always test what works best for your plants. Don't start by placing your crops too close, though.
It's easily rectified by moving your plants further away from the light source.
How to identify Cannabis Heat Stress?
You'll easily see when your plants are unhappy and suffering from cannabis heat stress.
They'll probably look withered, and the edges of the blades will start to curl up. These will resemble a taco or canoe shape. The changes in the leaves eliminate nute deficiencies—leaving heat as the only culprit.
The signs of heat stress in weed plants differ in the various growth stages. Look out for these signs of cannabis heat stress:
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Stage |
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Seedling |
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Vegetative |
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Flowering |
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As soon as you notice any signs of heat stress in your buds or leaves, it would be best if you reduced the heat or cool the roots. The quicker you take action, the less likely you are to lose your entire yield.
Solutions: Heat Stress Recovery
A great way to treat or ultimately protect your marijuana plant from heat stress is to monitor the temperature. It allows you to pick up any changes and be proactive.
Let's look at some easy ways to deal with heat stress in your weed if the damage is already done.
Dealing with Heat Stress Indoors
Regulating temperatures in grow tents and rooms can be arduous when steps are skipped—especially in the midst of summer.
Grow lights produce a tremendous amount of heat that can be extremely stressful to your plants. You need to create convection currents by using an exhaust system and oscillating fans. Let's jump into some ways to help your plant with heat stress recovery.
- Use oscillating fans - this is a no-fuss, cheap way to cool down the grow room. It creates convection currents that prevent hot air from affecting the plants. It's also a great way to increase resistance in your crops as they don't have access to natural wind.
- Make use of an air conditioner - this is a great method for circulating cool air in your grow room. It's an expensive option, so probably not recommended for newbie growers.
- Install an exhaust system - it goes hand-in-hand with fans by replacing stale air with a fresh breeze. We strongly recommend fitting your deaerator with a carbon filter. It prevents that dank aroma from hitting your unsuspecting neighbors.
- Treat your stressed-out plants with nutes - you can't reverse the damage done to your marijuana plant by heat stress. Many users reported great success with plant heat stress recovery by using supplements.
Supplementation is only effective when the cause has been fixed.
Seaweed kelp fertilizers have loads of cytokinins which can help your stressed plants recover. The nutes in these fertilizers will also aid your plants in heat resistance. Silicon fortifies the cell walls of your marijuana crop, which also aids in the fight against temperature issues.
- Don't be afraid to move or change the light source - if you notice signs of heat stress in your weed, look at how close it is to the lights.
If the damage is mostly around the top of the canopy, move the grow lights further away.
In case readjusting your lights prove ineffective—you'll possibly need to switch to a different source. HID bulbs tend to give off more heat than LEDs. It's the best option for indoor growers in small spaces or warm climates—or both. Not to mention the cost reduction these lights offer.
Seedling
If you notice dry or curled leaves in your seedlings—move your grow lights away. It's easier to start further and move closer. Place your hand above the plant, and if it feels very warm, the lights are still too close.
Vegetative
Most grow lights will come with operating instructions. Ensure to either adjust the intensity or move them further from your plants for the next few weeks.
Flowering
Once again, make sure to adjust your lighting as soon as you hit this stage.
Managing Heat Stress Outdoors
Unlike growing indoors, the alternative has little mercy. Not only do you have to be wary of pests, but the elements are outside your control too.
Heat stress is a bigger threat for those growing in warmer climates—usually closer to the equator. Let's run through some proven tips on protecting your outdoor plants.
- Focus on timing your watering - this is especially important during a heatwave. Whenever possible, water your plants in the evenings or early mornings. Wetting around midday could lead to cannabis plants with heat stress—it magnifies the sunlight, which harms the plant tissue.
- Pots are always a good idea - these mobile planters make it easy to move your crop out of harm's way. This way, you can relocate them when heat waves strike.
- Temporary shelters could be a permanent solution - you can't leave your herb under cover all day—it will hinder photosynthesis. You can use a makeshift covering made from cloth or tarpaulin at the hottest time of day. This way, they can still bask in direct sunlight before and after the extreme heat hits.
- Use supplements - similar to your indoor plants, seaweed kelp fertilizers are a great addition—especially with cannabis plant heat stress. Use it as per the instructions on the packaging, and you should be golden.
Seedling
Many growers prefer to start their plants indoors and move them out soon after the spring equinox. Make sure you don't introduce them to extreme heat in this stage.
Vegetative
If your plants are in pots, move them during the hottest part of the day and when heatwaves hit. If you've planted in the ground—this is a good time to make use of that temporary shelter.
Flowering
Watch out for foxtails during this phase. Your plants will not produce any new leaves. Be extra wary of plants with heat stress symptoms. Once again, make use of your temp shelter or move the pots away.
Marijuana Plant Heat Stress
Preventing or at least slowing sudden climate changes is your secret weapon against cannabis heat stress.
Don’t ever take your crops for granted; constantly monitor them to look out for signs of difficulty. A sure-fire tip is to invest in a thermal hygrometer to easily measure humidity and temperature. It's a cheap and effective instrument to keep an eye on your grow room climate. Using the tips mentioned above, you can easily recognize heat stress in weed—and treat it to save your yield.
