While cannabis usually loves warm climates, heat stress damages cannabis plants and can ruin a harvest when not tackled quickly and correctly. Knowing how to prevent, spot, and treat heat stress is crucial for growing top-shelf weed both indoors and outside.
It’s true – cannabis is a hardy plant that grows vigorously like a weed. But that doesn’t mean that it’s immune to environmental stress. Unfavourable variations in temperature, humidity, rainfall (or watering), and ventilation can all have a drastic impact on the growth and health of cannabis plants.
Heat stress, in particular, is a common environmental stressor to cannabis that can affect both indoor and outdoor growers. Left untreated, heat stress can cause significant damage to cannabis plants, stunting their growth, and attracting pests into the grow space.
In flowering plants, heat stress may trigger hermaphroditism, and extremely high temperatures can even cause the death of plants altogether. Keep reading to learn everything you need to know to prevent heat stress from ruining your future harvests.
What is the ideal temperature for cannabis?

Cannabis can be cultivated all around the world, thanks to its ability to adapt to a variety of conditions. However, there are certain temperature ranges within which cannabis plants thrive throughout their different stages of life.
While different strains of cannabis may like slightly different conditions, most growers agree that the ideal temperatures for cannabis are:
- Seedling stage: 20-25°C at all times. Seedlings also like high relative humidity of 65-70 %.
- Vegetative phase: 20-28°C during the day (or “lights on”) and 18-23°C during the night (or “lights off”). Vegetative plants like relative humidity of 40-70% depending on the strain. Indicas typically prefer drier conditions while sativas thrive in more humid environments.
- Flowering phase: 20-24°C during the day (or “lights on”) and 18-21°C during the night (or “lights off”). During flowering, it is best to keep relative humidity at around 40% to prevent the growth of mould or mildew in dense buds.
What causes heat stress?
Heat stress occurs when the temperatures in your cannabis garden rise above the ranges outlined above. When this happens, your cannabis plants will struggle to perform fundamental cell functions.
At first, this may impact their ability to grow new foliage before eventually triggering certain defence mechanisms (such as foxtailing). Eventually, if left untreated for extended periods of time, heat stress may cause cannabis plants to wither and die.
In outdoor gardens, the most common cause of heat stress is high temperatures. This is common all over the world, and we’ve seen outdoor growers in both the northern and southern hemispheres struggle against heat waves and rising temperatures.

Indoors, heat stress is often caused by:
1. High outdoor temperatures
Unless you have air conditioning in your tent or room, you’ll struggle to mitigate the effects of high outdoor temperatures in your indoor cannabis garden.
2. Improper lighting
Grow lights produce heat, and running your lamps too high or too close to your canopy can cause heat stress.
3. Inadequate ventilation
Fans help to keep fresh air moving around your plants as well as dissipate old, hot air, creating a cooler, more stable temperature in your room or tent.
4. Low humidity
Dry, arid air can produce symptoms of heat stress in cannabis. To avoid this, keep the humidity in your grow space between 40-70% during veg and around 40% during bloom.
5. Failed equipment
Failed temperature sensors or automated fans are a very common cause of heat stress in indoor gardens. We’ve seen even veteran growers fall victim to heat stress due to failed equipment.
What are the symptoms of cannabis heat stress?
The symptoms of heat stress will differ depending on the stage of life that a plant is in. In seedlings, for example, heat stress usually causes stunted growth, pale new foliage, and withering/wilting. Cannabis seedlings are super fragile and have very little defences against heat stress.

In vegetative plants, on the other hand, common signs/symptoms of heat stress include:
- Upwards curling leaves.
- Dry, brown spotting.
- Brown and burnt-looking leaf edges and tips.
- Bleached leaves, particularly at the top of the canopy.
In flowering cannabis plants, heat stress may cause all of the same symptoms as in vegetative plants as well as:
- Foxtailing (more info below).
- White pistils, even close to or past your harvest date.
- Bleached or burnt flowers.
- Reduced trichome and terpene production, resulting in flowers that lack potency, aroma, and flavour.
What is cannabis foxtailing?
Foxtailing is a sign of heat stress seen in flowering cannabis plants (particularly plants in their final stages of bloom). It’s characterised by the misshapen growth of flowers and particularly the growth of new flowers on top of already developed ones.
Foxtailed buds may grow upwards in thin spires or form big, bulky new buds atop already mature flowers. This is believed to be a defence mechanism of the female cannabis plant and a final attempt to grow larger bud structures capable of developing more seeds and secure the survival of the species.
How to distinguish heat stress from nutrient burn or deficiencies?

Unfortunately, the symptoms of heat stress sometimes overlap with those of nutrient burn or specific nutrient deficiencies. Some of the nutrient issues that cause symptoms similar to those of heat stress include:
1. Nutrient burn
This causes the tips of leaves and buds to appear burnt and withered.

2. Nitrogen deficiency
This causes yellowing fan leaves.

3. Potassium and calcium deficiency
Both cause the tips and edges of leaves to become brown and mottled.

4. Sulphur and boron deficiency
This causes yellow, burnt-looking new growth.
To avoid mistaking heat stress for one of the above nutrient issues, we recommend closely monitoring the temperature and humidity in your garden over at least 1-2 days. Ideally, take note of your temperatures and humidity levels multiple times during the day:
- Once at night or when your grow lights are off.
- Once in the morning or when your grow lights first turn on.
- Once during midday or just before your grow lights turn off again.
If your temperatures clearly fall above where they should be, there’s a good chance that the symptoms that your plants are displaying are signs of heat stress. If your temperatures are fine but your humidity levels are too low, it may be that the arid air is the primary cause of your plants’ symptoms.
Key differences between heat stress and nutrient issues:

- Heat stress will cause the edges of a plant’s leaves to curl upwards – a unique trait that you won’t see in plants with nutrient deficiencies or nutrient burn.
- Burning or drying out from heat stress can happen very quickly – even overnight. The browning/yellowing of foliage caused by nutrient issues, on the other hand, is more gradual.
- Heat stress will toast foliage in the areas of your garden that are hottest – often high up, close to your grow lights. Nutrient burn, on the other hand, affects foliage all over a plant, while nutrient deficiencies will cause yellowing or browning in particular parts of the plant. Nitrogen deficiency, for example, affects old foliage near the bottom of a plant first, while boron and sulphur deficiencies affect young growth tips and shoots.
How to treat cannabis heat stress?
Dealing with heat stress is a lot easier in indoor gardens than outdoors. If you’re growing cannabis in a room or tent, address heat stress by:
- Making sure you have a thermometer-hygrometer set up and regularly tracking the temperature/relative humidity in your space.
- Checking your grow lights. In indoor gardens, heat stress often strikes as a side effect of high light stress. To ensure you’re running your grow lamps properly, check out our article on light stress here.
- Making sure your garden is well ventilated. For the best results when growing cannabis indoors, we recommend investing in intake, outtake, and oscillating fans. Intake fans draw fresh, cool air into your grow space, while oscillating fans will keep that air circulating around your plants. Outtake or extractor fans, on the other hand, draw old, hot air out of your tent/room.
- Keeping humidity levels optimal. The best way to increase humidity indoors is using a humidifier. While some growers keep buckets of water in their grow space to increase humidity levels, we do not recommend this. Spilling water in a grow space riddled with electronic grow equipment can be extremely dangerous.
- Keeping your plants’ root zone cool. The best way to do this indoors is by using mulch or rocks to create a layer of insulation above your plants’ topsoil. This will protect your plants’ roots from high temperatures.
- Watering your plants before your lights turn on. This will keep the water in their soil from evaporating and simultaneously keeps your plants’ roots cool for longer.
- Using air conditioning. If all else fails, use an air conditioner to keep the temperature in your grow space optimal. Keep in mind that this will significantly increase the energy consumption and running costs of your garden.

Unfortunately, dealing with heat stress outdoors is much more difficult, especially during prolonged heatwaves. Here are some tips to protect your plants from rising temperatures:
- Time your grow properly. If you live in a region with extremely hot temperatures, consider starting your grow a little later or earlier to avoid the temperature peaks of mid-summer. Auto seeds come in handy here, allowing you to get in a quick harvest at the beginning and end of the season.
- Use cooler pots. While we generally recommend growing cannabis in smart pots, if you’re growing cannabis outdoors in hot regions, you may want to consider using ceramic or cement pots instead as they provide more protection from hot temperatures.
- Raise your pots. If you’re growing on a terrace or balcony, consider raising your pots to keep them from touching the floor, which can become extremely hot.
- Use mulch and rocks to protect your plants topsoil and insulate their roots.
- Water your plants early in the morning to help keep their roots cool during the day and prevent their water from evaporating under the heat of the sun.
- Create a temporary shelter for your plants using shade cloth. Move your plants into the shade at midday when the sun is at its strongest and temperatures soar.
- If necessary, bring your plants inside and finish them in a tent or room.
Can cannabis plants recover from heat stress?

Vegetative cannabis plants should recover from the damage of heat stress. However, ideally you’ll want to extend their veg time by a few weeks to make up for any lost growth. If your plants suffered from heat stress during bloom, however, you won’t have this option and the quality/size of your harvest may suffer in consequence.
Picking the right strain for growing in hot regions
If you live in a hot region and plan on growing cannabis outdoors, one of the best ways to ensure a successful harvest is by growing a heat-resistant strain. To ensure that the plants you’ll be growing will hold up to the heat in your local area, look for seeds that:
- Are apt for growing in mediterranean or sunny climates.
- Are sativa-dominant. Cannabis sativa originates from warmer, tropical climates and sativa dominant plants typically perform better in hot areas than indicas or indica-dominant hybrids.
- Are fast or autoflowering. Auto or fast-flowering strains allow you to grow and harvest your seeds before and after mid-summer when temperatures are more amicable. While most growers in warm climates choose to grow photoperiod strains, you could opt for an autoflowering strain if you who live in an area where the middle of the summer is too hot for cannabis. Automatics can offer a good solution as they allow you to sneak in harvests at the beginning and end of the season, avoiding the hottest time of year.

Some heat-loving strains from Sensi Seeds include:
Do you grow cannabis in hot conditions? Do you have any of your own tips for dealing with heat stress that you think we’ve missed? Share them with us and our readers in the comments below!