JoAnne Skelly: Help plants survive the heat
July 19, 2018
Our poor trees, shrubs and other plants are stuck in place, "standing" in scorching sun and dealing with desiccating winds and high temperatures. Humidity is low and there's little chance of a significant rain event until the fall. It's up to us to provide what our plants need to survive the heat.
Plants contain more water than animals do. They can be up to 95 percent water. Water is essential to move nutrients throughout the plant, to keep cells turgid and to regulate plant temperature. Turgidity allows plant cells to be firm enough to hold up a plant. When cells lose water, they can collapse. This is what you see when a plant wilts.
Plants cool as moisture is drawn up from the roots and carried through the plant out small pores on leaves. This process is called transpiration. When the water then evaporates and cools leaf surfaces, the process is referred to as evapotranspiration.
Transpiration rates fluctuate depending on temperature, humidity, sunlight intensity, precipitation, soil type and soil moisture, and wind. High temperatures cause the pores in leaves to open wide, releasing additional moisture for cooling. Low humidity makes the transpired water evaporate more quickly. Wind also increases evaporation. All these factors demand the roots send up more water. However, there must be sufficient soil moisture available for this to occur. Without enough water, plants start to lose leaves. Fewer leaves means less area for water loss due to evapotranspiration. In extreme water deficiency, plants die.
Plants transpire at different rates. Cacti and succulents lose less water than many broadleaf plants and many even store water in large stems or juicy leaves. "During a growing season, a leaf will transpire many times more water than its own weight. An acre of corn gives off about 3,000-4,000 gallons of water each day, and a large oak tree can transpire 40,000 gallons per year." (US Geologic Survey, 2016).
What this means is we must irrigate more when days are hot for plants to survive. Maintain soil moisture for trees and shrubs to a depth of 15 inches out to the drip line of the tree or shrub. You can measure this with a large screw driver, which will slide easily into moist soil. Lawns also require more water when temperatures are hot. For information, read the "All-Seeing, All-Knowing Lawn Care Manual" https://wrcc.dri.edu/washoeEt/docs/sp9302.pdf.
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Water before the sun comes up or after it goes down, but avoid watering when it's windy. Mulch around plants to conserve soil moisture. Water deeply.
JoAnne Skelly is Associate Professor & Extension Educator, Emerita at University of Nevada Cooperative Extension. She can be reached at firstname.lastname@example.org.