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Should we sprinkle plants to cool them off?

Editor’s Note: Don Tapio retired in December from the WSU Extension Office. He had written a gardening column for the past 20 years. On a trial basis, he has decided to return. But here’s the catch: He’s retired. That means he’s imparting his knowledge on his own time. And he really would like to enjoy his retirement without having everyone pick his brain at any moment. Let’s enjoy Tapio’s trial return for what it is and learn what we can. Those with gardening questions should contact a WSU Master Gardener volunteer at pnwmg@yahoo.com.

There’s no question with temperatures approaching 90 degrees F. we are in the middle of a “Thermal Blast.” The intense heat is not only uncomfortable for us, but tough on our garden plants as well. Some garden publications suggest sprinkling the foliage on rhododendrons and other plants on hot days to help cool plants and prevent sun scorch damage on the leaves. On the other hand, science tells us that water drops can act like magnifying glasses and by sprinkling you could actually increase the amount of leaf damage on a hot day. Should we or should we not sprinkle plants on hot days?

Research conducted by Professor Harris, a horticulturalist at the University of Arizona, found that it is difficult to establish whether “sun-scald” and other solar-induced leaf injuries are caused by solar radiation alone or by a combination of factors such as extreme temperature fluctuations, excessive temperatures, and adverse water relations.

“Sun scalds,” physiological disorders characterized by red or brown pigmentation of the plant tissue followed by internal breakdown and death of damaged tissue, are prevalent after hot, dry periods followed by sudden applications of a large quantity of water.

According to Professor Harris, landscape plants in dry soils often have leaf temperatures 10 to 25 degrees F. above ambient air temperatures. Leaf temperatures are often as high as 104 degrees F. with little or no permanent leaf injury at these elevated temperatures. Professor David Martsolf, of the University of Florida, hypothesizes that mid-day sprinkling during hot weather may be detrimental due to rapid decrease in plant temperature. If sprinkler irrigation water is applied during a hot, mid-day period when the leaf may be stressed and the stomates closed, the stomata guard cells may regain turgor and the stomates open. The leaf interior might lose so much moisture as to be more severely damaged than if the stomata had remained closed.

Additional research conducted at the University of Arizona however, was not able to document any harmful effects associated with mid-day sprinkling practices. Plants growing in full sun were sprinkled during mid-afternoon for periods of from 1 to 10 minutes. Without exception, applying water to a leaf caused its surface temperature to drop. In general, the higher the initial leaf temperature, the greater the temperature reduction following sprinkling. Sprinkled leaves remained cooler than dry leaves for 15 to 45 minutes. No detectable damage occurred.

Based on all of this research, it would appear that both statements are correct. Sprinkling done when temperatures rise does indeed “cool-off” plant leaves. At the same time, it may well result in some leaf damage. Related research conducted at Oregon State University found that maintaining adequate soil moisture at all times around rhododendron plants will prevent sunburned foliage. Irrigation to cool leaf temperatures was not necessary to prevent sunburn and may cause leaching of fertilizers from the root zone.

Donald D. Tapio is a regional gardening and farming specialist for Southwest Washington, having retired from Washington State University’s Extension Office. This column expresses his personal opinions and tips after decades of research. Contact him with non-gardening questions by emailing the editor at editor@thevidette.com. Those with gardening questions should contact a WSU Master Gardener volunteer at pnwmg@yahoo.com.