Heat isn’t the only problem for plants
The best way a plant owner can protect trees and shrubs against heat stress and related injury is to help the plants do a better job of cooling themselves.
“The only problem with that is: Heat isn’t the only problem,” says Jason Griffin, director of Kansas State University Research and Extension’s John C. Pair Horticulture Center. "Temperature affects almost every physiological and biochemical process. Each plant grows best in a particular temperature range. It also has a high and a low threshold for survival."
Heat nearly always has partners in crime. Moisture, wind, cloud cover and plant species can be just as important. All four can affect how well plants’ cooling system works.
“Another factor this year is that many central U.S. landscape plants are unusually vulnerable to any kind of damage,” Griffin adds. “They’re still trying to recover from 2011’s heat and drought.”
Plant owners can take counter measures to help ornamentals through summer’s searing days.
“You can directly affect the temperature of a plant by shading it," says Griffin. "Obviously, this is a labor-intensive step. But, I’ve seen people use everything from bed sheets to screening to umbrellas, in order to reduce the temperature of leaves. And, as ridiculous as this sounds … it’s worked.”
Occasionally misting foliage during the mid-afternoon can also help. Nurseries with overhead irrigation sometimes turn on the water for 5 minutes per hour through the day’s peak heat. This lowers the air temperature surrounding the plants without overwatering.
Monitoring soil moisture is always a critical step.
“A plant can’t cool itself without adequate soil moisture,” says Griffin. “At the same time, overwatering can shut down plants’ cooling system as fast as drought can. Not enough and too much are both bad.”
The main way plants cope with heat is a process called transpiration. Roots absorb water from the soil and send it up through their plant. Some of that water then evaporates from the leaves through tiny pores, called stomata. The evaporating water cools each leaf much like evaporating sweat cools skin. However, a variety of factors can limit or disrupt the transpiration process:
* Dry soil – reduces water availability. It also signals the leaves’ stomata to close.
* Wind – shakes branches and leaves, causing stomata to close. Plus, wind blows away the thin layer of cool air around each leaf and often scatters potential cloud cover.
* Intense sunlight – plays a role in stomata closure and sends leaves’ internal temperature above the surrounding air temperature.
“Each of those factors can take part in a complete breakdown of leaf cells. We may call the result ‘heat scorch,’ but it’s a lot more than that,” says Griffin.
Excessive heat makes things worse by upsetting the normal functions of internal plant cells. One of the first processes affected is photosynthesis – the way the plants make food.
“Many plants can recover from this upset overnight,” says Griffin, “but that only happens if nighttime temperatures cool off. If they don't, plants have cumulative effects and carryover stress from one day to the next. I’ve seen well-established, well-acclimated plants make some amazing recoveries from extreme weather events. Hot weather that hangs on, however, can be sort of insidious. Its impacts can build and then linger. So, you can’t afford to let things slide when it comes to helping your most valuable trees and shrubs -- even for a day or two.”
For the longer term, he recommends that homeowners shop for plants with good heat-and drought-resistance, plus take advantage of any microclimates in their yard that could provide wind and sun protection for more sensitive plants.
Click here for more tips to help your plants through heat and drought
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