As winter approaches its Groundhog Day midpoint, this season might have seemed unusually cold to people. But native and invasive plants and insects have hardly shivered.

"It hasn't been cold enough to come anywhere close to killing insect pests," said Lee Frelich, director of the University of Minnesota's Center for Forest Ecology.

The emerald ash borer, for example -- the most high-profile exotic now threatening Minnesota trees -- is believed to start dying of the cold at 20 degrees below zero, with minus 30 killing off entire populations. Recent readings of 30 below and colder across far northern Minnesota will deter the bug's northward advance, but it's not likely to be stymied by cold in the metro area, said Rob Vennette, research biologist at the U.S. Forest Service's Northern Research Station in St. Paul. Going into Thursday night, this season's low in the Twin Cities was minus 12. It's been four years since the Twin Cities experienced 20 below, and 17 since the mercury dropped to 30 below.

Winter is where Minnesota's climate has been warming the most in recent years. Currently, it's normal for the Twin Cities to experience 23 days with temperatures of zero or below each year; the current "normal" is based on readings from the 30-year period of 1981 to 2010. But that's seven fewer than the previous normal, built on readings from 1971 through 2000. Friday is expected to be the eighth subzero day this winter.

This winter's extremes probably won't be enough, Frelich said, to slow down the expansion of red maple trees into the pine forests of the Boundary Waters Canoe Area Wilderness. Similarly, Southern tree species such as sycamore and tulip (the state tree of Indiana), which are becoming popular ornamentals across southern Minnesota, should revive nicely, he added. Likewise for a buckthorn he knows of in International Falls. Buckthorn can withstand temperatures down to minus 60.

Geir Friisoe, plant protection director for the Minnesota Department of Agriculture, added that Grecian foxglove, an attractive but toxic warm-climate ornamental that has been spreading wildly through Washington County for the past 15 years, might also be undeterred.

Indeed, this winter might take out only experimental plants, said Peter Moe, operations director at the Minnesota Landscape Arboretum. Hybrid grape species recently descended from plants grown in Italy or Greece might be among the casualties, he said. But native plants -- and many hybrids -- should do just fine.

"I don't think it will kill off anything we do want," he said. Last week's 12-below reading, he added, was "really what we kind of expect in January."

The U.S. Department of Agriculture plant-hardiness map, updated last year, indicates that this winter's extremes aren't approaching any critical limits. For the Twin Cities and most of the southern one-third of Minnesota, the average coldest temperature -- based on readings from 1976 to 2005 -- falls between 25 below and 20 below. Through Wednesday, St. Cloud had reached minus 18 twice. Rochester, outside the metro area's urban heat island but 80 miles south, had dipped only to minus 10.

Tough on worms

There is one counter-intuitive threat at work, though. The lack of snow across much of the state for much of the winter has allowed frost to penetrate farther into the ground than it would even in a colder but snowier winter. That might kill off insect larvae that live in the soil, not to mention adult earthworms, which Frelich regards as a serious threat to Minnesota's forests.

It also might hamper a crop that's regarded as symbolic of the North: blueberries. The low shrub actually needs deep snow cover to protect its roots, Moe said. So the scant snow cover for the first half of this winter might limit next summer's crop for bears and campers alike.

Meanwhile, Frelich noted that if any species might not have adapted to this winter's cold, it would be humans. "My experience is that warmer winters get people to complain about how cold they are," he said. "In the old days, they dressed for it and got used to it."

Bill McAuliffe • 612-673-7646