One of my special winter delights is sipping morning coffee as I bask in the warmth of our wood stove and watch the busy chickadees flocking to the sunflower feeders outside the window.
I never cease to marvel at these little birds. They remain up here in the Green Mountain National Forest throughout the entire winter. And, with only their downy feathers for insulation, every night they endure fifteen long hours in the wind and snow without food. Meanwhile I am indoors, warm, cozy and well fed, and I get to spend each night under a large fluffy comforter.
Chickadees and a few other small birds have developed a unique acclimatization strategy to survive the long cold nights of a northern winter. It is a physiological response called ‘nocturnal hypothermia’ which means that each evening the birds’ body temperatures drop dramatically to conserve precious energy. The next morning, before heading off to their preferred food source, the birds literally shiver themselves awake to raise their internal temperatures back up to the level needed for daytime foraging.
Winter acclimation strategies
Unlike chickadees, most birds rely on behavior rather than physiology to survive winter, many by removing themselves from the worst weather. Some, like robins and bluebirds, just move to a lower elevation such as the Champlain Valley for the coldest months, returning to the mountains in April in time for nesting. Other birds, like swallows and hummingbirds, migrate to warmer climates entirely, which is in itself an amazing feat.
A few birds, notably turkeys, owls, grouse, woodpeckers and bluejays as well as, some years, goldfinch and pine grosbeaks, remain up in the mountains throughout the winter and manage to find sufficient food, whether it be berries, seeds or the occasional rodent.The choice of plants in our gardens can help certinaly provide food for the winter birds as you see from this visitation by a gang of turkeys that enjoyed the persistent crab apple fruit on my Malus ‘Snowdrift’ and M. ‘Selkirk’
I still remember, a few years back, as Dick and I were snowshoeing in the forest with our dog, how we encountered a large grouse that had completely buried itself down into the deep snow; presumably the fluffy snow was an insulation against the biting wind. As the surprised bird flew off with strong wingbeats I am not sure who was more startled——the grouse, the dog or the people!
Sometimes we even have winter visitors from Canada at our feeders. Some years a swarm of feisty redpolls will take up residence for a month or so. And one year we were delighted to have a group of crossbills. Since crossbills primarily eat the seeds of conifers—their beaks are a special adaptation to facilitate this— they were probably attracted to our particular location by the stand of Norway spruce growing up the hill behind our house.
Of course land-based animals, both vertebrate and invertebrate, cannot escape the brunt of winter by just relocating. So the various species have developed their own distinctive techniques to survive winter while staying in place.
Large herbivores like deer and moose just tough it out, browsing on anything growing above the snow-line. Other creatures, such as bears and chipmunks, escape the cold by hibernating in a den or nest.
With cold-blooded amphibians and reptiles, as the external temperature falls, their body temperature drops commensurately. And, as their body temperature begins to fall, they instinctively burrow deep into the mud and become dormant until spring.
And then there are the sub-niveans, little animals like the mice and voles, which create their own living spaces beneath the snow. The snow acts as an insulator against the even colder air above. And as the snow melts in springtime, the observant human can see the remains of their tunnels going hither and thither across the ground, revealing their private winter lives.
Plants acclimatize for winter too
And of course all plants that live in these parts, whether in our gardens or in the wider landscape, have their own physiological techniques of acclimatization which, in my view, are equally interesting. Like the chickadee, the crab apple you see in this picture can survive bone-chilling nighttime temperatures of minus 25℉.
Each fall, triggered primarily by the increasing hours of darkness and, to a lesser extent, by falling temperatures, most plants enter a state of dormancy that allows them to withstand the rigors of our New England winters.
We all know that each year in October all our deciduous trees and shrubs drop their leaves, while the entire above-ground portions of perennials die off completely.
At the same time, although less visible to people, the plants store any excess sugars in their roots and their metabolic processes (photosynthesis, respiration and transpiration) either cease or slow dramatically. Finally the chemical composition of the individual cells changes, reducing the likelihood the cells will freeze (like antifreeze on a microscopic level).
In the spring everything reverses. In the woody plants it starts when the sap returns to the branches and the cells revert to their non-dormant structure. This causes buds to grow into leaves and the plant’s metabolic processes to resume. Meanwhile perennial plants respond to the warmer temperatures by pushing out new growth directly from the roots.
Evergreen trees and shrubs are actually the exception that proves the rule. Since their leaves remain, their winter dormancy is only partial, and they continue to lose some water through their leaves throughout the winter months. This is why it helps if your evergreens start winter well watered and why you should plant them where they will be less exposed to the drying effects of afternoon sun and wind.
Choosing plants for a cold climate
The ability of different plants to withstand cold winters varies hugely, and is heavily influenced on where they naturally grow. Some plants—think about the tropical shrub Bougainvillea— will die if the temperature drops even a bit below freezing (to 30℉), whereas others—such as our Vermont Sugar Maple— can survive bitterly cold winter weather (below minus 30℉). However for most plants the lowest temperature they can survive is somewhere in-between.
So naturally great effort has been expended to determine just how much winter cold a particular plant can take, and a numeric scale—the Plant Hardiness Rating— has been developed to specify this.
In reference books, websites and at the nursery you will see a zone designation associated all long-lived plants—shrubs, trees and perennials— which tells you the lowest winter temperature they can take. The Sugar Maple, hardy to below minus 30℉, is designated Zone 3b. But another New England native, the Eastern Redbud, only hardy to minus 15℉, is designated as Zone 5b.
So you need to choose plants for your garden based on the coldest temperature you anticipate. Obviously in a state like Vermont this varies from place to place. If you live down south or near Lake Champlain, it may be minus 15℉ for a night or two during an especially cold winter. But if you live in the mountains or further north you can anticipate it will may be 15℉ colder. All this has been quantified in the on-line interactive USDA Plant Hardiness Zone Map, based on 30 years of historical data. I talk about this more in Hardiness Revisited
So what about climate change, you may ask? Well, based on my own observations, it seems that while our AVERAGE temperatures are surely rising, we sometimes experience the OCCASIONAL bitterly cold winter extreme— and that is all it takes to decimate a tender plant. The recent ‘polar vortex’ of 2014 is still fresh in our minds, and back in January 2011 there was a similar ‘weather event’ , when temperatures across Vermont temperatures also dropped well below minus 20℉.
And if a cold spell comes behind on the heels of a spell of unseasonably WARM weather, causing plants to start losing some of their winter acclimatization, the sudden extreme cold will be all the more damaging.
So, while I may take a gamble with the hardiness of an inexpensive plant, before I invest in a costly plant, like the dwarf pine above, I like to research whether it will tolerate coldest temperatures I can expect where I live.