It’s cold this week: Cold enough to do all the cold weather tricks, like throwing hot water into the air and watching it vaporize, or freezing wet jeans upright; cold enough that you can see your breath and cold enough to get frostbite if you don’t hurry the dog along to do its business so you can both return to the warmth of home.
In a relatively warm winter like this one, Minnesotans groan at the prospect of a week and a half of what’s actually pretty normal winter weather for this part of the country: single-digit temperatures, nights below zero and stinging wind chills.
But the cold isn’t bad for every living thing. Our state’s native trees are pretty well adapted to it, and in fact, cold spells like this — even longer ones with lower temperatures — have benefits for them.
How trees survive winter
Trees that thrive in environments like Minnesota’s, where temperatures can drop below freezing for an extended period of time, have adaptations that allow them to survive the winter, said Matt Russell, an associate professor and extension specialist at the University of Minnesota’s Department of Forest Resources.
Among those adaptations is the way they keep their leaves from freezing. Deciduous trees, like many of Minnesota’s maples, oaks and elms, grow leaves every spring that help them fill up on light from the sun, turn it into energy through photosynthesis. They drop those leaves in the fall, which stops the water in those leaves from freezing and shriveling them up, and go into a dormant state.
Evergreens, like pines and spruce, keep their leaves, or needles, rather, year-round, allowing them to convert sunlight into energy during the winter. Their needles are coated in a substance called cutin that helps lock in moisture, and filled with a substance that acts as antifreeze.
Some conifers — like firs and spruce, your typical Christmas trees — are even shaped the way they are to better survive in places where the sunlight is limited and comes at extreme angles for parts of the year.
“The trees are shaped with a pyramid-like cone, designed to better capture the sun’s rays when the sun angle is really limited,” Russell said.
A natural pesticide
Not such good news for Minnesota’s trees? Pests. One big concern right now in Minnesota is emerald ash borer, a bright-green invasive beetle native to Asia whose larvae winter under the bark of a tree, often doing enough damage in the process to kill them.
The beetle is widespread across the eastern U.S. and the Midwest, but it hasn’t taken hold in Minnesota to the same extent as it has elsewhere, in part because of the cold.
At -20 degrees Fahrenheit, 50 percent of emerald ash borer larva die. At -30 degrees, that number goes up to nearly 95 percent, Russell said. He checked the weather in Grand Rapids this week and saw a low of -26 degrees Fahrenheit.
“You’ve got to think that’s pretty close to being able to kill those invasive insects,” he said.
Emerald ash borer isn’t the only invasive species that doesn’t like the cold.
Japanese beetles, a metallic-looking insect an enemy to gardeners because of its tendency to feed on crops, fruit trees, birch and elm in addition to other plants, die if the soil temperature drops to 32 degrees for two months, even less time when the temperature is lower, said Peter Boulay, assistant state climatologist at the Minnesota State Climate Office.
“If you get cold you can kind of knock them back a bit,” he said.