For plants, heats isn’t the only problem

by Kathleen W. Ward

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,” said Jason Griffin, director of K-State Research and Extension’s John C. Pair Horticulture Center.

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 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.

Temperature does affect almost every physiological and biochemical process, he said. Each plant grows best in a particular temperature range. It also has a high and a low threshold for survival.

But, heat nearly always has partners in crime, Griffin explained. Moisture, wind, cloud cover and plant species can be just as important. Like heat, 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 added. “They’re still trying to recover from 2011’s heat and drought.”

Still, plant owners can take countermeasures to help ornamentals through summer’s searing days.

“You can directly affect the temperature of a plant by shading it. 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,” he said.

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,” Griffin said.

Monitoring soil moisture is always a critical step.

“A plant can’t cool itself without adequate soil moisture,” he said. “At the same time, overwatering can shut down plants’ cooling system as fast as drought can. Not enough and too much are both bad.”

Griffin is in a position to know a lot about how plants respond to weather extremes. At the research center he directs, K-State tests everything from shade and ornamental trees to bedding and medicinal plants.

Plus, the John C. Pair Horticulture Center is located just outside Wichita. Last year that city broke a 75-year record for the most days in a single summer with temperatures above 100 degrees (a total of 53, reached Sept. 1). And, although 2012 isn’t shaping up as a repeat of 2011, plant owners can “rest assured that it will be hot,” he said.

As often is the case, however, last year’s heat wasn’t an isolated event.

“When seasonal temperatures rise in Kansas, rain totals tend to drop,” Griffin said. “Last year, though, they’d almost disappeared by August. Plus, the Kansas wind machine got fired up, drying things out and reducing our cloud cover.

“That four-part whammy greatly reduced plants’ ability to tolerate the high temperatures. Wichita residents who actually lost a weeping willow or quaking aspen are probably giving some thought this year to replacing it with something like a Shumard oak.”

The main way plants cope with heat is a process called transpiration, he said. 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.

Griffin said, 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,” Griffin said.

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,” Griffin said. “That only happens around here, though, if nighttime temperatures cool off. If they don’t, plants have cumulative effects – 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 recommended 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.

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