Feelin’ Hot, Hot, Hot: How to Help Your Plants Deal With Summer Heat

Summertime can elicit some of the most blissful thoughts of all seasons. Our childhoods were spent swimming, camping, carelessly running through sprinklers and laughing. Sometimes we got too much sun, but that’s a small price paid for the years of responsibility-free hot days and warm nights that seemed to never end.

As adults, summertime doesn’t necessarily conjure the same thoughts and emotions. We have responsibilities—spouses, bills, jobs—and, unlike our former selves, don’t get three months off to bask in the longest days of the year. We also moan about the heat and oftentimes forget it’s these crucial months that make our gardens grow.

Still, summertime gardening bears its own unique set of challenges. While the long, sunny days get the tomato plants popping and peppers rocking, extreme temperatures does indeed affect our precious gardens and crops. Like any living creature, plants have their own set of instincts when faced with environmental changes, and a long period of high temperatures is one thing they don’t handle well.

The Simple Science

The science behind plants in hot summer weather is pretty simple: The rate of photosynthesis (the process plants use to make sugar) starts to rapidly drop after a critical high temperature is reached, according to the Iowa State University Department of Horticulture. The “critical” high temperature varies from species to species, but generally anywhere between 90ºF and 100ºF will slow the food-making process for most plants.

On that note, respiration (the process that releases the energy of stored sugars to fuel growth) will continue day and night, depleting the plant’s food reserves. Severe water loss—known as desiccation—can also occur when transpiration (the process by which leaves release water vapor to the atmosphere) exceeds the moisture absorbed by the roots, according to the university.

An obvious sign of desiccation is wilted leaves. As the leaves’ water content decreases, they wilt to slow the rate of water loss; however, this also causes leaf temperatures to increase due to reduced evaporative cooling.

Along with high temperatures, a large temperature fluctuation (more than 15 to 20 degrees) cause slow growth and stunts the process. A greenhouse can act as a buffer for such major changes, if you’re container gardening.

And just as outside temperatures heat up, so does the soil surrounding your plants’ roots. While not a realistic range for the outdoor grower, the ideal soil temperature for optimal chemical activity is between 65ºF and 70ºF.

Soil temperatures above 75ºF dehydrate roots and can literally cook them as the soil gets hotter. So, when roots get too hot they will send stress signals to shut the leaves down (wilting) before too much damage can occur when high temperatures persist.

To sum it up, plants can die from starvation if extremely high temperatures continue for weeks at a time. However, this is almost completely avoidable if you’ve got some handy tips up your sleeve. Broken down by growing space, below are some useful tips for keeping your plants healthy and happy during the summer’s hottest days.

Climate Control Tips for Traditional Outdoor Gardening:

• Areas of the garden that face west or south will naturally be hotter and get the brunt of direct sunlight. This can be wonderful for plants that require full sun, and detrimental to those needing only partial sunlight. Keep in mind the sun’s habits when planting your crops.
• Water slowly to prevent runoff, and do so deeply and regularly. Some people say it’s best to water in the morning or evening in order to prevent shocking the plant roots. However, if you forget and need to tend to your plants during the heat of the day, it’s better than not watering them at all.
• Adding a thick layer of clean mulch (3 in. or so) on your garden beds will help conserve moisture and protect plants’ roots from extreme temperatures. Mulching can be done at any time of the growing season, but doing so early on—just after preparing the soil and planting—is ideal.

Outdoor Container Gardening:

• Containers can be more susceptible to temperature fluctuations than any other growing medium. Soil over 90ºF will harm roots, and oftentimes soil in containers will heat up to well above 100ºF when the air temperature is lower.
• As stated above, mulch!
• Thanks to their innate ability to be mobile (thanks to the help of their humble humans), container plants can be moved to shaded areas when the heat really kicks in.

Greenhouse Gardening:

• Use proper ventilation. Crack windows and doors and use fans, especially if signs of plant stress pop up (scorched or wilted leaves).
• Apply shade using blinds, netting or paint where necessary. During heat waves, remove panes from the greenhouse to increase air flow.
• Make sure plants aren’t touching any siding or the roof of the greenhouse because they can easily burn during high temperatures.

Grow Room Gardening:

• Use an exhaust fan to remove hot, stale air and to bring in cooler air. This should be placed on a timer and set to turn on once an hour for at least five minutes.
• Another way to combat high temperatures indoors is to hook the fan up to a thermostat. This way you can have the fan kick on when the temperature exceeds a specific degree.
• If using artificial lighting, make sure to not place the light or heat source too close to the plants. This can quickly heat up the outer layer of soil, where most of the feeder roots live. If these roots are destroyed, they take a week or two to return.
• Maintain an optimal nutrient temperature between 60ºF and 75ºF. The ideal is 65ºF, as water retains the most oxygen in this state. Anywhere above 85ºF and below 50ºF is moving toward the danger zone, as root damage can easily occur at these temperatures. High nutrient/water temperatures also lead to evaporation and create a breeding ground for disease.
• Use a reservoir chiller to better control the temperature of the water/nutrient solution.
• Install an electronic ballast. They give off minimal heat compared to that given off by magnetic coil ballasts.
• Consider installing T5 fluorescent lamps. They emit very little heat, have built-in electronic ballasts and produce twice the light of a T12 fluorescent.

Consider treating your plants as you would a small animal or baby—leaving them to fend for themselves during heat spells would be criminal! So, make sure they can breathe, have enough water (but, as always, don’t over do it) and nutrients.

As for yourself, the grower, don’t forget to treat yourself right this summer. Drink plenty of fluids and drape a damp-wet hand towel over your neck to keep cool while gardening.