Heat Stress in Cannabis Plants: Symptoms and How to Protect Your Grow

Heat stress hits cannabis plants when temperatures stay above their comfort zone, roughly 20 to 30 °C (68 to 86 °F), long enough to slow growth or cause lasting damage: think curling, taco-shaped leaves, wilting, crispy edges and stalled buds. You prevent it by controlling the environment: ventilation, AC and cooler lights indoors; shade, smart watering, light-colored containers and mulch outdoors. Cold stress is the flip side, and it's covered at the end.

What is heat stress in plants?

Heat stress happens when plants are exposed to long stretches of intense sunlight and heat. The term describes a period when plants deal with temperatures high enough, for long enough, to permanently affect their ability to function or grow normally.

The ideal temperature range for most crops is between 20 and 30 °C (68 and 86 °F). Temperatures outside that range, whether in the air or the soil, day or night, are bad news for plants.

Heat stress is the cumulative result of how intense the heat is, how long it lasts and how fast the temperature climbs.

Signs of heat stress in cannabis and other plants

Heat stress shows up in a bunch of ways, but it can be tricky to spot because the symptoms look a lot like those caused by certain pests or crop diseases.

On top of that, heat and drought stress can leave plants more vulnerable to other diseases. Looking at the following symptoms together makes for a more accurate diagnosis:

1. Leaf curling and cupping: a mechanism plants like corn and tomatoes use to control water loss. The leaf surface shrinks and the stomata (the tiny pores on the leaves) close up. On cannabis, you'll often see the edges curl up into a "taco."
2. Wilting: happens when there's less moisture available than the plant can take up, creating a water deficit. The longer you wait to deal with heat stress, the higher the risk of permanent damage.
3. Dry leaf edges: some crops dry out the outer edges of their leaves to survive. It's especially common in squash and pumpkins.
4. Ozone damage: high temperatures combined with poor air quality can cause ozone damage, which shows up as dry brown spots between the leaf veins.
5. Flower and/or fruit drop: in ornamental plants, heat stress can cause buds and flowers to drop or keep the plant from blooming at all. In veggies like cucumbers, squash and peppers, heat can make flowers and fruit fall off.
6. Bolting: in response to heat stress, some crops flower early, before their usual harvest season. This process, known as "bolting," can be a real problem for cool-weather crops.
7. Sunscald: fruits like apples, tomatoes and melons can burn in extreme heat, especially on the side facing the sun directly. It shows up as discoloration, water-soaked spots, blisters or firm, sunken patches on the fruit's surface.
8. Blossom-end rot: heat and intense sunlight speed up photosynthesis and transpiration, pulling water and calcium away from the fruit and into the leaves. The result is blossom-end rot, a dark, water-soaked spot on the fruit, especially in tomatoes.

How heat stress affects plants

Heat stress dehydrates plants, which slows their development. The drop in photosynthesis is a direct result of how heat stress hits the leaves' relative water content and the plant's water potential.

In other words, heat stress makes plants lose water and wilt. That said, when the heat stress is temporary or moderate, plants can adjust their respiration and transpiration rates to reach a thermal balance.

In response to heat stress, plants adjust their concentration of soluble proteins and sugars to control the osmotic pressure inside their cells. And yield losses from heat stress show up across all kinds of crops, from grains and legumes to root vegetables.

The most critical biochemical and physiological changes heat stress causes in plants include:

1. Stomata closing: When temperatures climb, the stomata close, so the plant can't take in enough carbon dioxide (CO2) for photosynthesis. That can slow growth and cut yields.
2. Protein damage: High temperatures damage proteins, reducing enzyme function and throwing off a lot of metabolic processes.
3. Oxidative stress: Extreme heat can trigger oxidative stress, releasing reactive oxygen species that damage plant cells and hold back growth.
4. Higher transpiration rates: High temperatures ramp up transpiration, cutting the water available to the plant and causing water stress.

How to protect an indoor grow from heat stress

Indoors, you've got a lot more control over the environment, which gives you the chance to take real steps to protect your cannabis plants from overheating.

Below are a few strategies you can use, from picking the right spot for your grow to choosing your lighting setup.

Pick the right spot for your indoor grow

Where you set up your indoor grow goes a long way toward protecting your plants from heat. The more insulated the room is from the outside, the better. If you live somewhere hot and pick a room with sun-baked, south-facing walls (north-facing if you're in the Southern Hemisphere), it'll be a lot harder to control the temperature inside in summer.

Ventilation and exhaust systems

A good ventilation system is essential for keeping air moving through your indoor grow and preventing heat buildup and pockets of humidity.

Fans inside the grow tent or room keep the air circulating evenly and help prevent pathogens from taking hold. They push fresh air into every corner of the grow, which is key to healthy plant development.

Choosing a good exhaust system so the air in your grow room or tent gets properly replaced is a must. That means measuring your grow space carefully so you know how much exhaust capacity you need.

Cooling systems

If you're growing indoors in summer in a cool region with short summers that never get very hot, you might be able to keep your grow in range with your exhaust system alone. It'll need to be powerful, though, and ideally you should be able to adjust it with a controller so it ramps up as the temperature climbs.

In most cases, though, that won't be enough to keep your grow's temperature under control, because if it's 30 °C (86 °F) outside, there's no way your grow room will be any cooler on exhaust alone.

That's why, most of the time, growing indoors in summer calls for air conditioning.

Managing lights and the heat they put out in an indoor grow

Grow lights give off heat, which pushes up the room temperature. Choosing the right lighting is key to minimizing their impact on the temperature in your grow room or tent.

Keep in mind that the distance between the light and your plants makes a big difference in how much heat they get. If your plants are too close to the lamps, the tops may start to burn; if that happens, raise the light to give them more space.

HPS (high-pressure sodium) lights ruled the market for years, but they also run the hottest. So if keeping your grow cool is the goal, they may not be your best bet.

LED or LEC (light-emitting ceramic) lighting, on the other hand, runs cooler. They cost a bit more upfront, but that pays for itself over time with a lower electric bill.

How to protect an outdoor grow from heat stress

Controlling the environment outdoors is tougher, but there are still some steps you can take to protect your cannabis plants from the heat.

Shade your plants during the hottest hours

Too much sun is something you want to avoid. That's why shading your cannabis plants when the sun is at its strongest is key if you live somewhere where summer temps top 35 °C (95 °F). A simple shade cloth or awning is enough to give your cannabis grow the cover it needs during the hottest part of the day.

Watering and humidity

Proper watering and the right humidity are essential for keeping your plants hydrated and heading off heat stress. Timing plays a big role in how well your plants absorb water, so it's best to water first thing in the morning or after the sun goes down in the evening. During the cooler hours, plants take up water better and watering is more effective.

Use the right containers

Keeping the root zone from overheating is also important for preventing heat stress in cannabis. That's why it's best not to use black pots, since they soak up more heat, or, if you do, to wrap them in something like white plastic.

Another option is fabric pots, which also give the roots more airflow. And if you already know you'll need to move your plants out of the midday heat, pick a container size and type you can actually haul around.

Use mulch, green manure or other material to cover the soil
Mulch is a layer of loose organic matter, like straw, spread over the soil around your plants. It protects the soil and helps keep conditions where they should be.

You can do the same with "green manure," a cover crop that serves the same purpose. With these kinds of coverings, you'll cut down on evaporation and keep moisture in the soil of your outdoor cannabis grow.

Choose heat-tolerant strains

Not all cannabis genetics handle high temperatures the same way. Cannabis is a pretty tough plant that's adapted to all kinds of climates and conditions, but picking the right strain here will make a real difference.

Some genetics handle heat better than others, so here are a few ideas if you're growing cannabis outdoors somewhere really hot:

• Moroccan Beldia Kif
• Sweet Afghani Delicious Auto
• Amnesia
• High Level
• Jamaican Dream
• Acapulco Gold
• LSD-25 Auto
• Lemon King

Cold stress in cannabis plants

Cold stress, the opposite of heat stress, kicks in when air and soil temperatures drop below the ideal range for plant growth, which can seriously hurt your crop.

Under cold stress, plants go through a range of biochemical and physiological changes that can show up as stunted growth, lower yields and, in extreme cases, even death.

Cold-regulated genes (COR genes) and the buildup of cryoprotectants, like sugars, proteins and amino acids, have evolved as ways for plants to tolerate cold, minimizing dehydration and protecting cell membranes.

How well a stressed plant can handle the cold depends on the species and on how fast and how far temperatures drop. A gradual drop lets the plant acclimate more efficiently, while sudden swings cause more stress and damage, especially in fall and early winter.

Growers can protect their crops from low temperatures with careful strain selection and good upkeep. Watering properly before a frost can help protect plants during a cold snap.

Well-watered soil stores more heat and releases it later, making plants more resistant to cold stress. Mulch also helps hold moisture in the soil through the cold season.

To manage abiotic stress (stress from the environment rather than pests or disease) effectively, growers need accurate, up-to-date weather data, a way to assess frost and heat damage, and a solid understanding of how their crops grow and how the microclimate shifts in their garden. That lets them act fast to protect their plants from whatever's stressing them out.