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Protecting Cannabis Against Heat

Warmth, sunlight, overexposure and heat stress can cause serious harm to any cannabis grow. Of course, all cannabis plants need (sun) light for the photosynthesis that fuels their growth and flowering. But you can have too much of a good thing. This blog explains why heat can cause problems both indoors and outside, and why protecting cannabis is crucial for good harvests.

Why Protect Cannabis Against Heat?

In nature, all processes run most smoothly within certain parameters, no matter whether it’s plants, animals, or even single-cell organisms. Every plant species has an ideal combination of environmental factors for growth and flowering. No matter whether it’s air humidity, soil moisture conditions, acidity, amount of light, or temperature: if even one of parameters falls below or exceeds its critical limits, the wellbeing and, ultimately, the existence of the organism is threatened.

Sadly, temperature (heat, that is) becomes more of a risk factor year after year. Anyone who has been growing weed for a couple of years will have noticed how our summer weather is changing. Heat waves are getting more common. That’s fine when you’re spending the day on the beach, but al that heat is far from cool if you’re trying to grow weed outdoors. Heat is problematic indoors as well, by the way, since grow lights emit heat that can be harmful for plants. Just like us, plants tend to do better if they have minimal stress in their lives.

weed burn sun heat

Heat Stress Symptoms

During a cannabis grow, heat can be a total disaster long before harvest time arrives. The damage caused by excessive heat is called heat stress. The plant actually react with a stress response. The usual signs of cannabis heat stress are the following:

  • Dried-out, exhausted-looking plants;
  • Curled-up tips of fan leaves;
  • Brown spots along the edges of leaves;
  • Odd cola shapes.

All of these symptoms can manifest while cannabis plants flower, but they will often appear far before that time. That is why the symptoms visible on the leaves are often the main warning signs.

During the vegetative phase, cannabis plants need lots of light. Light is energy: useful if you want to draw grow power from it by photosynthesis, but considerably less awesome if you get al that energy straight in your face.

In the flowering phase, signs of heat stress can show on the leaves too, but the shape of flower buds can also be an indication. If you see a thriving bud with a kind of extra mini bud growing op top of that, beware: these ‘foxtail’ shapes can be a sign of excessive heat.

Protecting A Cannabis Grow Against Heat

When it comes to heat stress, prevention is better than cure. Once heat has damaged plants, they tend to recover poorly, which seriously threatens the eventual harvest yield. That just makes protection and prevention more important. The best way for a grower to protect their cannabis from heat, however, depends entirely on the causes. Logically, these causes will usually differ between indoor grows and garden or balcony projects.

Growing Outdoors: Sunscreen For Your Crops?

Even though any cannabis plant loves a fair bit of sunbathing, they are much like us in terms of the health concerns of overexposure. Another similarity with humans: as the climate is changing and extreme weather events become more common, our cannabis needs protecting against heat (waves) and intense sunlight. Thankfully, there are several ways to do just that, and they’re not hard to pull off if you know what you’re doing.

outdoor weed

Finding The Best Spot

When growing outside, it is crucial to find the best spot for your plants before the grow kicks off. Think carefully about the influence of both rain and sunlight on different parts of the garden (or balcony). Wind is definitely a factor to consider, too: is there enough cover to survive a summer storm? You can read more about picking the perfect outdoor grow spot in this blog.

Mobile Weed    

One clever way of protecting cannabis against heat is keeping it mobile, i.e., growing in pots rather than in open soil. This way, a plant can enjoy all the sunshine, rain, and fresh air it needs whenever the weather is nice. If it’s going to be a hot day, you can simply move them to a shaded spot until the danger subsides.

Be careful with pots, however, as they make the roots of the plant more vulnerable to heat stress. Black planters in particular tend to heat up fast out in direct sunlight. This will accelerate moisture evaporation while making rot more likely. You can tackle this issue by adding a protective buffer around the pot. Placing the pot inside another, larger pot is one solution, but covering the top soil layer with straw is another easy way of protecting potted cannabis from heat hazards.

cannabis greenhouse protecting heat

Shelter And Shade

Even growers with only modest DIY skills will find it fairly easy to build their plants a home-made shelter. A bit of clever carpentry and some hinges will do the trick, as will a simple frame and some tarpaulin sheeting to pull over it. This latter option is also a means of protecting cannabis from heat in open soil.

Be mindful of grow spots on balconies or near walls, as these tend to create heat zones by reflecting sunlight. Try to avoid these hotspots, but if you can’t, at least you’ll know you will need some sort of cover before you start. It will also help protect your grow from the wind and rain, which comes in extra handy during the final vulnerable phase right before the harvest.

Having a greenhouse in the garden makes everything easier, as you’ll already have a frame you’re your tarp cover if necessary. But growing weed in a greenhouse has other benefits too, as this blog explains.

Water And Nutrients

Please avoid the mistake of protecting cannabis from the heat of the sun by using water. Watering plants out in full sunlight turns droplets into prisms that only intensify the rays, which increases burn and heat stress risks rather than reducing them. Of course, plants out in the hot sun will lose more moisture through evaporation, so be sure to give enough water to prevent dehydration. On hot days, the best time to water your plants is either early in the morning or after sunset. This blog explains all about smart watering.

water cannabis burn

Water and cannabis make a beautiful couple.

Adding certain kinds of nutrition can also help make cannabis plants more resilient to heat. Seaweed such as kelp is an example, as are silicon-based supplements and special-purpose root boosters. However, here too, adding too much of a good thing can be harmful. You can find out more about nutrition in this blog.

Indoor Growing: Lighting Up

Let’s start by stating the obvious: when growing cannabis indoors, protecting your pants against  the heat of the sun should not be necessary. Indoors, lighting is the main risk factor, being the sole light source on which any grow depends. High temperatures due to poor ventilation may cause heat stress too.

Adjusting Lights

Make no mistake: grow lamps can emit serious amounts of heat as a by-product of the light they emit. In fact, a lamp’s heat can be just as harmful as the sun at noon on a sweltering day. On top of that, growers are often more aware of the weather outside than of the conditions in their grow room or tent. Be sure to keep track of temperatures and try to keep them between 20 and 25°C. In the flowering phase, you’ll want to try and keep temperatures below 28°C. Remember, though, that these are average temperatures across the entire grow space. You can monitor heat using a regular thermometer, but remember that it gets hotter the closer you get to the light source. And it just so happens that cannabis plants tend to grow towards the light…

grow lamps heat stress cannabis heat

If you notice the symptoms of heat stress manifest mainly around the top leaves of the plant, lighting is your prime suspect. If you see the signs all over your plants, it’s probably the general temperature in the room.

If you have enough room, hoist up your lighting goes a long way towards protecting your indoor cannabis grow from heat. You could also try grow techniques that aim to keep plants short and compact, such as Screen Of Green or Mainlining, for example.

Also keep in mind that led lights emit less heat than most other systems. In addition, they’re usually the cheapest option as well as being more eco-friendly in terms of energy use.


Grow rooms and tents cool down by keeping warm air circulating, and by making sure that warmed-up air is refreshed with cool air from the outside. That means protecting indoor cannabis from heat can be a matter of putting up enough fans for ventilation plus adequate exhaust capacity. Do make sure any exhaust fans have filters installed, though, to prevent odour problems and related issues.

cannabis protecting against sun and heat

Cannabis Strains Capable Of Protecting Themselves Against Heat

At the end of the day – hot days included – cannabis is a tough and resilient plant species, well-equipped to cope with warm weather or hot grow rooms. Still, some strains can handle heat better than others.

Even though most modern strains are hybrid crossing between indica and sativa genetics, sativa-dominated strains tend to handle heat slightly better. This is largely due to their tropical origins, since they evolved to thrive in warm areas near the equator.

Check Out Grapefruit Superstar With 90% Sativa Power

Still, a cannabis seed’s genetic background is never a guarantee for its heat protecting capacity. Of course, it always helps to order your seeds from a high-quality supplier. You’ll find our best sativa-dominant strains in our online catalogue, but don’t let anyone discourage you from growing indica instead. Or perhaps a robust sativa autoflower like Dynamighty is a practical alternative for you? No matter what genetics you pick, at least now you know that protecting cannabis against heat ain’t no sweat if you have the right knowledge handy!

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