Hey I know a lot of outdoor growers are dealing with high temperatures right now, and I wanted to share some tips for keeping plants healthy even when it gets hot!
Please also share your tips!
1.) Don't Let Plants Dry Out
Make sure plants are well-watered! Plants drink a lot in the heat and if they dry out they can die almost overnight. Having a good amount of water will help them be as healthy as possible.
2.) Don't Let Plants Get Over Watered
It is easy for plants to get over-watered in the heat, because hot water contains less oxygen. "Over-watering" and droopiness are actually the symptoms of the roots getting enough water but not enough oxygen. Therefore, even though you don't want the plant to dry out, you also don't want to soak the soil every day. Making sure that your soil has plenty of drainage (for example incorporating significant amounts of perlite) will help prevent overwatering.
Only water when the top soil is dry up to your first knuckle. Also, when it comes to watering, try to avoid getting water on the leaves as this can cause leaf burns when the sun starts beaming down.
3.) Take Advantage of Evaporative Cooling
Water plants in the early morning if possible, because the evaporating water can help the roots and the area around the plant stay slightly cooler during the day. Some growers will take a shallow but wide tray, fill it with gravel, then fill it with water up until the top of the gravel. Then place the plant container on top. The gravel will hold the container up and prevent the water from being sucked up by the roots through the bottom of the pot (which would cause overwatering), but as the sun evaporates the water puddle it causes the air directly around the plants to be a few degrees cooler. Every degree helps when the temperature gets really high!
4.) Keep Roots Cool
Plants with cool roots are more resistant to heat. If growing in a container, keep roots cooler by putting some sort of barrier between the sun and the outside of your pot. For example, place the main container inside another, larger container. Another idea is to dig a hole in the ground and place the whole plant container inside. These strategies protect the sides of the pot from being baked in direct sunlight. Don't let plants in containers sit on ceramic tiles, or other materials that tend to heat up a lot under the sun. It can also help to have plants in a bigger container altogether, because the extra soil also acts as a buffer for the roots.
Some growers also recommend adding a layer of light-colored mulch (like straw or dried grass clippings) on top of the soil. The light color helps reflect heat back. I personally caution against dark or bark-based mulch for first-time growers, especially those using liquid nutrients (which are very sensitive to pH), because bark can affect pH as it decomposes.
5.) Offer Shade
When the plant is under direct sunlight and the heat gets out of control, try to move plant indoors or offer shade if possible. Some growers will erect a mesh shade net over the top of their plants during the hottest days of summer, if a shadier spot is unavailable. Keeping several plants relatively close together can also help them give each other some shade.
Whatever you do, make sure plant is not put in the dark during their day period! It should still be getting some amount of light, even if it's just a sunny window or a single light bulb. If you mess up the plant's circadian rhythms by keeping them in the dark during their normal day period, it can stress them out further.
6.) Breezy Spot If Possible
If the plant can be moved somewhere slightly breezy (the leaves or stems shouldn't be waving around, but a gentle leaf rustling is good), that is better than sitting in stagnant air. Sometimes different parts of your yard or grow area are more breezy than others, so it can help to pay attention if a slightly different placement may be cooler for your plant. That being said, don't place plants on top of a hill where they're getting beat up by the wind
7.) Seaweed Kelp Extract
Some growers recommend using seaweed kelp extract (available as a liquid or powder) to help plants recover from the stress as well as help protect plants from heat stress in the future. Kelp naturally contains lots of trace elements and minerals that have protective properties.
8.) Silica Supplements
Silica is not a "required" nutrient and your plant won't suffer from Silica deficiencies. However, supplementing with extra silica offers additional support to plant cell walls. This can help the plant be more resistant to heat and other types of stress. Silica supplements made for plants like Botanicare Silica Blast or General Hydroponics ArmorSi. Whenever possible, try to get a silica supplement from the same manufacturer that makes your base nutrients, just to help make sure everything works well together.
9.) Heat-Resistant Strain
Unfortunately this doesn't help growers who already have plants growing outside, but it's just something to keep in mind for future grows, or if you plan to do a second summer grow. 
Please also share your tips!
1.) Don't Let Plants Dry Out
Make sure plants are well-watered! Plants drink a lot in the heat and if they dry out they can die almost overnight. Having a good amount of water will help them be as healthy as possible.
2.) Don't Let Plants Get Over Watered
It is easy for plants to get over-watered in the heat, because hot water contains less oxygen. "Over-watering" and droopiness are actually the symptoms of the roots getting enough water but not enough oxygen. Therefore, even though you don't want the plant to dry out, you also don't want to soak the soil every day. Making sure that your soil has plenty of drainage (for example incorporating significant amounts of perlite) will help prevent overwatering.
Only water when the top soil is dry up to your first knuckle. Also, when it comes to watering, try to avoid getting water on the leaves as this can cause leaf burns when the sun starts beaming down.
3.) Take Advantage of Evaporative Cooling
Water plants in the early morning if possible, because the evaporating water can help the roots and the area around the plant stay slightly cooler during the day. Some growers will take a shallow but wide tray, fill it with gravel, then fill it with water up until the top of the gravel. Then place the plant container on top. The gravel will hold the container up and prevent the water from being sucked up by the roots through the bottom of the pot (which would cause overwatering), but as the sun evaporates the water puddle it causes the air directly around the plants to be a few degrees cooler. Every degree helps when the temperature gets really high!
4.) Keep Roots Cool
Plants with cool roots are more resistant to heat. If growing in a container, keep roots cooler by putting some sort of barrier between the sun and the outside of your pot. For example, place the main container inside another, larger container. Another idea is to dig a hole in the ground and place the whole plant container inside. These strategies protect the sides of the pot from being baked in direct sunlight. Don't let plants in containers sit on ceramic tiles, or other materials that tend to heat up a lot under the sun. It can also help to have plants in a bigger container altogether, because the extra soil also acts as a buffer for the roots.
Some growers also recommend adding a layer of light-colored mulch (like straw or dried grass clippings) on top of the soil. The light color helps reflect heat back. I personally caution against dark or bark-based mulch for first-time growers, especially those using liquid nutrients (which are very sensitive to pH), because bark can affect pH as it decomposes.
5.) Offer Shade
When the plant is under direct sunlight and the heat gets out of control, try to move plant indoors or offer shade if possible. Some growers will erect a mesh shade net over the top of their plants during the hottest days of summer, if a shadier spot is unavailable. Keeping several plants relatively close together can also help them give each other some shade.
Whatever you do, make sure plant is not put in the dark during their day period! It should still be getting some amount of light, even if it's just a sunny window or a single light bulb. If you mess up the plant's circadian rhythms by keeping them in the dark during their normal day period, it can stress them out further.
6.) Breezy Spot If Possible
If the plant can be moved somewhere slightly breezy (the leaves or stems shouldn't be waving around, but a gentle leaf rustling is good), that is better than sitting in stagnant air. Sometimes different parts of your yard or grow area are more breezy than others, so it can help to pay attention if a slightly different placement may be cooler for your plant. That being said, don't place plants on top of a hill where they're getting beat up by the wind
7.) Seaweed Kelp Extract
Some growers recommend using seaweed kelp extract (available as a liquid or powder) to help plants recover from the stress as well as help protect plants from heat stress in the future. Kelp naturally contains lots of trace elements and minerals that have protective properties.
8.) Silica Supplements
Silica is not a "required" nutrient and your plant won't suffer from Silica deficiencies. However, supplementing with extra silica offers additional support to plant cell walls. This can help the plant be more resistant to heat and other types of stress. Silica supplements made for plants like Botanicare Silica Blast or General Hydroponics ArmorSi. Whenever possible, try to get a silica supplement from the same manufacturer that makes your base nutrients, just to help make sure everything works well together.
9.) Heat-Resistant Strain
Unfortunately this doesn't help growers who already have plants growing outside, but it's just something to keep in mind for future grows, or if you plan to do a second summer grow.
- Each plant is different, and some strains can get really stressed by heat while all the other plants in the same environment are just fine!
- Sativa, Haze, African and Hawaiian strains all tend to be more heat-resistant since they originate from hot climates.
- In general, auto-flowering strains to be relatively sensitive to heat since they originated in Siberia, but some strains have been mixed with heat-resistant strains which makes them more suitable for warm climates
- Many Indica plants, which also come from cool climates, can be surprisingly sensitive to the heat.
What are your heat stress secrets?
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