Certain Plants Equipped to Take the Heat

Pity outdoor plants this time of year.

We can jump into some cool water, sit in front of the air conditioner or duck into the shade. Plants aren’t so lucky. They’re tethered in place.

Searing heat can dry out plants and cause them to burn up energy faster than they can replenish it. But if they have water, they cool themselves through transpiration. This evaporation through the leaves can cool a plant by about 5 degrees Fahrenheit.

That’s why more than 90% of the water taken up by a plant returns to the air via small leaf pores. Carbon dioxide and oxygen also pass in and out through these pores. These gases are needed for photosynthesis, which enables a plant to store energy.

This cooling process works only if there’s enough water in the soil. If not, the pores close, transpiration and photosynthesis stop, and the plant warms. Even with moist soil, the pores might close if the leaves are losing water faster than the roots are drinking it in.

Cacti tolerate hot climates by opening their pores at night. It doesn’t do much to keep them cool, but it allows for photosynthesis. Other plants, such as corn and crabgrass, tolerate heat by being so efficient that their pores are open less.

What can we do to help plants weather the heat? Careful siting helps. So does keeping the garden watered and offering a bit of shade from a temporary frame covered with wooden lathe.

Sprinkling or misting plants could keep them cool and keep them from having to pull water up from the soil. But the average 30 gallons of water that runs up through a tomato plant in a season, or the 50 gallons for a corn plant, is more than a cooling agent. It also draws in minerals from the soil. Plants constantly misted might not absorb enough minerals, and the wet environment would predispose them to disease.

A better alternative is to select plants adapted to the weather. Lettuce, spinach, peas and radishes are not plants to harvest in August. Tomatoes, cucumbers, peppers, melons and squashes can take some heat. But vegetables like corn, purslane and amaranth and flowers such as portulaca, sedums and, of course, cacti, thrive on it.