After enduring three days of intense, record-breaking heat, our trees and shrubs are still feeling a serious burn, according to Seattle Master Arborist Steve Lambert. He pointed out a withered rhododendron which was sun-scalded from extreme heat, and said it could take a year to recover—if it survives.
Trees around our region can be scalded by 100+ degree heat and intense sun because they release as much water vapor through their leaves and needles as they can to survive. Lambert said you can actually feel that cool vapor around a large tree when it’s extremely hot.
“It actually creates a microclimate,” says Lambert. “It loses water vapor through the leaves in a process called ‘transpiration’ that cools the tree, and it cools the surroundings.”
Lambert said transpiration in trees works exactly the same way perspiration does for a person. And in hot weather, trees can release up to 90% of the water they pull up from their roots in vapor though their leaves.
When that water runs out, you’ll see leaves start to curl and dry up—a sign the tree’s in serious distress. “You can run the risk of doing some really long-term damage to the tree,” he said.
And fire experts with the Washington State DNR believe those trees suffering from heat stress could become dry fuel for explosive wildfires, like the unprecedented Western Washington fires around Sumner last September.
Lambert said you must water your trees to keep them healthy.
“For your trees in your landscape, you’re better to prepare them ahead of time and start hydrating them before we get to the heat waves and replacing that water loss as quickly as you can,” Lambert said, adding that the heat wave we just had is part of a long-term Pacific Northwest trend of hotter, drier summers—which he believes could cause some tree species to become extinct.
“I’ve lived in Seattle my entire life and I’m seeing some major tree species really declining and dying off in mass numbers due to these hotter, drier summers,” he said.
Lambert recommended consulting TreesAreGood.org for guidance on hydrating trees and diagnosing trouble in extremely hot, dry weather.
“If your soil is dry near the tree four inches below the surface, you should water the tree lightly for about an hour every week,” he said.
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