Heat Stress in Cannabis

Just like every year, we're entering a tricky time for many outdoor growers. Heat stress in cannabis is just around the corner. Remember last year’s heatwave? It was brutal. While marijuana plants are generally tough and can handle high temperatures, they're not immune to heat stress. And this can be fatal.

What is Heat Stress in Plants?

Heat stress refers to the damage plants suffer when exposed to extremely high temperatures for extended periods. Basically, when the thermometer shoots up, plants can't perform basic functions like photosynthesis and nutrient absorption. And that makes survival pretty tough.

When you're under the scorching sun, you can seek shade, take a dip in the pool, or chill indoors with the AC. But plants don’t have that option, and that's the big problem. In these situations, plants are literally fighting for their lives. Trust me, you wouldn’t want to be a plant in that scenario.

Signs of Heat Stress in Cannabis

How do you know if your plants are suffering? Here are the most common symptoms you’ll notice:

Leaf Wilting: When the heat is on, leaves lose water faster than they can absorb it, becoming limp and droopy. It’s a clear sign they desperately need to cool down. Sometimes, these symptoms are mistaken for underwatering.

Leaf Curling and Discoloration: Leaves curl up on themselves to try and reduce water loss. If they also start turning yellow or brown, it’s a sign of entering the severe damage phase.

Leaf Drop and Tip Drying: In an attempt to survive, plants might start shedding leaves to reduce the surface area exposed to the sun. Dry, brown tips mean the problem is getting very, very serious.

Decrease in Bud Production and Quality: The heat doesn’t just affect the leaves but also the buds. Production drops, and quality suffers, with lower resin and terpene output.

Bud Burn and Spots: Buds exposed directly to the sun can show burn marks, appearing as brown or black spots, similar to what happens indoors from light burn.

Effects of Heat Stress on Cannabis Cultivation

Heads up! Those were the symptoms. The effects of heat stress on cannabis are much worse. The most significant are:

Water Stress and Stomata Closure: To conserve water, plants close their stomata, which are the pores that allow gas exchange in the leaves. This reduces photosynthesis and stunts growth.

Photosynthesis Issues and Reduced Growth: Without enough carbon dioxide, photosynthesis gets tricky and plants grow more slowly.

Cellular and Oxidative Damage: High temperatures generate reactive oxygen species that damage plant cells, weakening the plant.

Changes in Resin and Terpene Production: Heat can reduce resin amounts and alter terpene composition as many evaporate, affecting the final product quality.

How to Prevent Heat Stress in Cannabis

To minimize heat stress damage, here are some tips from the pros:

Temporary Shade: Use shade cloths or similar structures during the hottest parts of the day to protect your plants from direct sun, even if just for a bit.

Proper Watering: Water early in the morning or late in the evening to avoid rapid evaporation. Drip irrigation is ideal as it ensures water reaches the roots directly.

Mulching: Mulch helps retain soil moisture and reduces the temperature around the roots. Straw, dried leaves, or grass clippings are good options.

Weed Control: Weeds are not mulch. In tough conditions, weeds will always survive by stealing water and nutrients from the soil. Get rid of them all.

Reduce Fertilizer Use: During heatwaves, avoid fertilizing. Fertilization under these conditions can damage roots and stress the plants even more.

How to Help Cannabis Plants Recover from Heat Stress

If your plants have already taken a good hit from the heat, they’re still alive but not in great shape, you can still try to save them, keeping in mind that some damage might be irreparable:

Install Temporary Sunshades: Use shade cloths or umbrellas to reduce direct sun exposure and lower the temperature. If possible, move the plants to the shade.

Deep Watering: Water deeply in the mornings or evenings to ensure the water reaches the roots. Drip irrigation is the best option.

Apply Biostimulants and Silicates: These products strengthen the plants and help in their recovery, improving their resistance to heat stress.

Avoid Heavy Pruning: Don’t prune during heatwaves. Only remove dead leaves and flowers to avoid further stressing the plant.

Last Tips to Combat Heat Stress in Cannabis

You still have time to take some measures to prevent a potential heatwave. For example, you can still do a final transplant into a fabric pot. These containers improve root aeration and prevent overheating.

Are you using dark-colored pots like black, brown, or green? Well, THAT’S A BIG NO. Fix it as soon as possible.

We talked earlier about silicon and biostimulants. These are cheap products that increase the plants' resistance to heat and even drought. Use them to help your plants withstand high temperatures.

And lastly, if you live in an area where heatwaves are frequent, you might want to go for some heat-tolerant strains. Amnesia Haze, Sour Diesel, or Super Silver Haze are usually safe bets under extreme heat conditions.