Heat Wave Gardening: What Your Plants Need to Survive in Extreme Temperatures

Watering deeply encourages the roots to grow deeper into the soil, where they are less likely to dry out. (FamVeld/Shutterstock)

Although summer is prime vegetable gardening season across the country, many gardeners may be surprised to learn that many of the most common plants grown prefer temperatures in the 59- to 86-degree F range, with even “summer plants” such as tomatoes, peppers, and melons thriving best in the 70s and 80s.

Because of that, things get tricky when the temperature goes over 90 degrees for long periods of time. Tomatoes and peppers may not pollinate properly and drop flowers instead of setting fruit, while the sugar content of melons and sweetness of carrots will be reduced. In fact, most plants will start to show some signs of stress.

As the thermometer rises, plants need more care to survive without wilting. Here’s how.

Common Problems

Plants talk to us, particularly when it comes to sending out distress signals. The key is to speak their language. Many scary signs of heat stress are simply the plant trying to adapt and survive, such as rolled or cupped leaves, which is common on tomatoes and corn. In these instances, the plant is trying to minimize leaf surface area and stomata (plant pores) to reduce moisture loss.

Dry leaf edges are a survival mechanism typically found on large-leaf plants such as squash and pumpkins. It may look as if the plant has a disease, as the leaves’ outer margins are brown and their inner sections look healthy, but if there are no other signs of fungus or bugs, then it should be safe to assume it’s heat-related.

Wilting is one of the most obvious signs of heat stress and falls into two sub-categories. Some plants, such as sweet potatoes and cucumbers, may look wilted during the day and perk up at night—that’s heat stress, which goes away as the temperature drops after the sun goes down. Some plants will remain wilted throughout the night. The longer a plant remains wilted, the greater the overall damage. Water them immediately.

Additional Concerns

Other heat- and sun-related conditions include flower and fruit drop; plants such as tomatoes, peppers, squash, and cucumbers pull back to just the greenery in order to conserve resources. In most cases, regular production will resume as the heat wave passes.

Sunscalded fruits and vegetables, although otherwise healthy, might get a sunburned spot or area, dry out, shrivel, or blister.

Blossom end rot, a tomato condition associated with calcium-deficient soil, is exacerbated by sunscald.

Plants that have bolted—the premature flowering of a plant so it can go to seed and the species will survive—may not be able to recover. On the upside, this is most common in “cool weather” crops such as broccoli, cauliflower, lettuce, spinach, and similar plants that should have already provided a­ good harvest. Simply pick their last crop before a heat wave arrives.

Happily Hydrated

The simple act of increasing watering is the most important factor when it comes to heat wave gardening. Watering the soil in the morning does much more than just help plants avoid mid-day wilt, because well-watered soil is cooler than dry soil. Add a generous layer of mulch to cool the soil and the plant’s roots further.

Wilting is one of the most obvious signs of heat stress; these plants should be watered immediately. (LisaCarter/Shutterstock)

Note: Black plastic mulch retains heat and can cook the soil and the roots below. Plants with shallow roots will dry out faster, while those with taproots may be able to find their own water source. Young plants need more water than their established cousins. Container plants may need to be watered twice a day, but they have the advantage that they can be moved to a shadier area of the yard if needed. Wilted plants should be watered immediately, no matter the time of day.

Mulch can cool the soil and plant roots, alleviating heat stress. (Bartek Winnicki/Shuttertock)

Nutrition and Chemicals

Plants that are planted in good soil that offers proper nutrients tolerate hot weather better. As a rule, fertilizer should be applied during initial planting or soon thereafter. If it wasn’t, a liquid fertilizer can be applied to give the plant a boost. However, at all costs, resist the temptation to fertilize during a heatwave, as it will cause new growth, making the plant more vulnerable. Insecticides should be used only when absolutely necessary, as even the hardiest plant can be damaged during extreme weather—even organic options, such as neem oil and insecticidal soap spray, can cause damage.

Let’s Make Shade

Something as simple as putting up some old white sheets will lower the temperature, reduce moisture loss, and reduce the amount of light hitting the plants. An alternative is shade cloth; 30 to 50 percent sheets work for a wide variety of plants. Be sure that any covering does not come in contact with the plant, as this can cause leaf burn.

Small-scale gardeners, particularly those in areas with temperatures over 100 F, may want to consider installing a mister system, too.

Hands On

Drip irrigation systems and soaker hoses on a timer go a long way to keeping plants happy as the mercury rises, but there are some less expensive suggestions as well.

Eliminate Competition

Most weeds are weeds because they thrive in harsh conditions. Get out the sun hat, garden gloves, and a cold bottle of water, and get busy pulling. Every weed is a thief of precious water and soil nutrients from the garden crop.

Love Me Tender

Do not transplant any plants—whether to replace a dead plant or to get a jump start on the fall season—until the extreme heat has moved on. Likewise, don’t prune plants unless there’s significant damage; doing so can cause new growth to be vulnerable.

Look and Take Note

Note which plants are doing well, which need near-constant water, which are flagging from too much rain in a low spot, and which have sunscald. The solution may be as simple as putting plants in better spots next year based on this year’s observations.