How Plants Cope with Heat: Lessons for Our Gardens

By Laura Huenneke

Silver-gray foliage of plants adapted to hot environments, like this native Sand Sage or Artemisia filifolia, make them good choices for a heat-tolerant landscape. Photo by Laura Huenneke

On the hottest summer days, we may seek out a shady patio or head up into the mountains for some cooler refuge. But plants are rooted to one spot, and not able to move out of the hot sun. How do they deal with the stresses of heat? Plants do have a variety of coping mechanisms, from short-term responses to evolutionary adaptations. We can help our gardens and landscapes thrive by understanding the plant’s perspective.

Most familiar plants cool themselves in hot sun by using evaporation, the way we use perspiration or sweating. Wet cells on the inside of leaves lose water from their surfaces, and the water exits the leaf through surface pores or stomates; this evaporation cools the air inside and near the leaf. This works, of course, only as long as the interior is wet. Thus most plants require consistent watering to provide moisture in the soil that can be taken up to replace that lost by evapotranspiration (movement of water from soil through plant tissues to the air).

Real-world environments don’t typically have unlimited water, however. When plants start to become dehydrated, they usually close their stomates to avoid losing water. Some plants fold or tilt their leaves, reducing the surface area hit directly by the sun. We can assist by providing afternoon or filtered shade, or even growing some crops under shade cloth.

Plants that evolved in hot regions of the world often have relatively small leaves; this helps them cool by reducing the area exposed to direct sun and by shedding more heat to passing breezes. Another adaptation to hot sunny environments is a leaf covered in silvery hairs or glands to increase reflection of sunlight, reducing heat load. Species with light-colored leaves may thrive in a hot landscape or ornamental bed.

Plant species differ in the sensitivity of their enzymes or biochemical machinery. That is, their important enzymes differ in the temperature at which they work best. These varying sensitivities can be found within our favorite garden plants – we look for cold-tolerant or heat-tolerant varieties depending on local climate.

We might expect that plants in warm humid environments are well adapted to high temperatures, or at least are able to move enough water for their leaves to stay functional. NAU researchers, however, have been working for several years in the top canopy of tropical rainforests, observing that leaf temperatures there can sometimes reach lethal levels. As drought and heat waves become more common, even these famously stable ecosystems may begin to suffer.

Leaves are not the only parts of a plant affected by heat. Roots in very warm soil also heat up, and they have few options for cooling off. We can assist plants by shading or protecting the soil, for example by using deep mulch to protect the soil surface. I use straw mulch in my vegetable beds; fresh straw is very light in color and reflects sun well, keeping the soil cool. Growing plants near each other can also help shade soil and roots.

Sometimes heat is not so much a stress as a signal to the plant. Many cool-season crops bolt (start to flower) when temperatures get hot: cilantro, spinach, and lettuce are prone to this. In my hot Doney Park garden, even supposedly long-lasting varieties of spinach and cilantro can start to flower very quickly after I set transplants out, squeezed between hard freezes and very warm afternoons in the sunny garden. I have some luck growing them under shade cloth or in a shady corner of garden, and harvesting them early.

A recent report from the United Nations’ Food and Agriculture Organization (FAO) and the World Meteorological Organization warned that most of the globe’s major crops are sensitive to warming temperatures, and society should expect sharply reduced food production at or above temperatures of about 85 degrees F. It may seem ironic to worry about heat for our plants when Flagstaff is conventionally thought to have a cool mountain climate. But the specific environment in your garden is what really counts. Be prepared to give your plants some assistance on the hottest days.

Laura Huenneke, coordinator for the weekly Gardening Etcetera column, has been gardening east of Flagstaff for more than 20 years. Gardening Etcetera is written for the community by certified Master Gardeners of the University of Arizona's Coconino County Cooperative Extension Master Gardener Program. To learn more visit https://extension.arizona.edu/programs/coconino-county-master-gardener.