Intellectually, I always realize summer is coming. But, I never actually believe in it until the temperatures soar and then, suddenly, I miss spring. Heat not only affects us, it also affects plants. During excessive heat events, plants struggle to survive, trying to pull enough water out of the ground quickly enough to match what is being lost by evaporation out of the leaves. Many biochemical reactions necessary for growth and reproduction are temperature sensitive. Plants have evolved physiological, biochemical, morphological and molecular responses to heat stress.
When high temperatures recur year after year, plants can develop a heat stress memory (thermo-priming), that, when triggered, sets them up to minimize temperature damage (thermotolerance). They can also adapt their leaves and other structures to better deal with heat. Some heat acclimation mechanisms include a changed leaf orientation, rolled leaves or small hairs that shade the leaf surface; all of which reduce sun exposure and heat absorption. Or, leaves might be covered with a thick waxy cuticle that keeps water in.
Reduced leaf size or change in leaf shape or color are common adaptations. Smaller leaves have less surface area exposed to the sun, lose less water and stay cooler than larger leaves. Gray leaves absorb less sun than dark green ones. Plants often lose leaves when suffering from extreme heat stress. Fewer leaves means less water loss through the pores called stomates, which are found in leaves, stems and other plant organs. Buds and developing fruit may drop. In addition, plants may alter their metabolism in the face of heat stress. This may involve changing cell water content, cell salt content, proteins and phytohormones.
Some species simply avoid growing in the hottest time of year by going dormant. Heat stress responses and adaptations vary with species and also with stage of development. Researchers are breeding plants for heat tolerance in order to maintain worldwide crop production in the face of longer hotter seasons.
As a caretaker of plants in a summer landscape, help your plants survive the heat. Water deeply early in the morning. Deeper roots tolerate the heat better than surface roots. Mulch the soil to reduce soil moisture evaporation and to keep the soil cooler. Avoid fertilizing with anything but slow release fertilizers or compost when it’s hot. Quick release fertilizer causes new growth, which is tender and extremely susceptible to heat damage.
With a little extra care, you can reduce heat stress for plants.
JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at firstname.lastname@example.org.