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Madison’s warm weather may have made the outdoors more comfortable for people, but does it spell trouble for overwintering plants and bugs?

Research says woody trees and shrubs are at their most cold-tolerant in December, according to Laura Jull, associate professor of horticulture.

That’s a trick that wouldn’t have done them much good in Madison in December of 2011, when the mean temperature was 30.8 degrees, about 7.8 degrees above average. January was another warm month, with the mean temp at 25.5, high by 6.7 degrees.

Even those temperatures aren’t high enough to lull trees out of their winter slumber.

“If we were getting up into the high 60s or 70s and then down to the 20s, that’s when we’d get problems,” Jull said. “Trees are tough, especially our native trees. It would have to get really cold — like 20 below — to see some injury on our trees and shrubs.”

Photo: Winter tree buds

Small buds form on the branches of a red-berried elder shrub in Muir Woods.
Photo: Jeff Miller

Some trees and shrubs may look like they’re developing buds, but Jull says bud swelling — preparation for spring growth — isn’t abnormal. Only woody plants fooled into leafing out this early would be in danger of damage.

Jull has seen daffodils and tulips trying to sneak out of the ground in her own yard. There’s more danger in warm spells for plants that grow from bulbs.

“If the flowers are coming out, that’s not good,” she said. “As long as the flower stalk stays below ground, it won’t be damaged when the temperature drops.”

That’s why Jull is hoping for snow.

“If we get a little snow, I make sure to shovel some on the ground where the tulips are,” she said. “It reminds them it’s winter, and keeps them dormant.”

There’s not much near-term danger for insects, either.

“I can’t think of a whole lot of critters that will be harmed by the mild winter,” says Phil Pellitteri, a distinguished faculty associate in the department of entomology at the University of Wisconsin-Madison.

But he adds that the absence of extreme cold is already changing our insect life.

During a warm winter, insects like Asian lady beetles and box elder bugs may arouse briefly and harmlessly from diapause — the insect version of hibernation. On Dec. 29, Pellitteri started hearing about winter cut worms crawling on the snow up north.

“When it starts reaches 40 degrees, they start waking up, but they seem to be adapted to waking up and falling asleep again,” he says.

The weather feature that is changing Wisconsin insect populations is the absence of 20-below temperatures, which can kill overwintering insects, Pellitteri says. The absence of such cold weather is a factor enabling the recent gypsy moth invasion.

A cold snap in the spring can also be harmful to insects that have fully woken from their winter slumber, Pellitteri says.

So far this year, he says, “the lack of significant cold is not likely to do anything crazy. I’m not too worried, and I’m enjoying the warm weather. And if the real cold stays away, we’ll be seeing more preying mantids.”

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