Trees and some other plants survive thanks to a process called acclimation. Trees start preparing for winter in early fall when the the first stages of acclimation take place and we don't even know it. The short days, but non-freezing nights trigger the process in trees. It triggers dormancy as well.
A key ingredient in the acclimation process is water. Water dictates the outcome. Plants are primarily composed of water. Should water freeze inside living plant cells, the plant cells will die and could potentially kill the entire plant. Therefore, living plant tissues survive low temperatures by suppressing ice formation, or by allowing water to freeze, but only in areas of the plant that won’t be injured by ice crystal formation. Trees are smarter than we thought!
Some of the 'smarter' trees and shrubs such as oak, elm, maple, birch, ash, walnut, hickory, apple, pear, peach and plum have the ability to suppress ice crystal formation in their cells, even at temperatures far below the freezing point. However, it is at -40° degrees that cellular water can spontaneously turn to ice, therefore resulting in the death of the cell.
Plants vary in their degree of acclimation, depending on their locality. Plants that must survive even temperatures lower than -40 degrees, like those native to the subarctic tundra growing in USDA Zone 1, do so by using a dehydration mechanism. Under these extreme freezing conditions, water undergoes a dehydration process where it moves out of the plant cells. Having left the plant cells, the water then freezes in the areas between the cell walls where ice formation is not destructive. This slow loss of water from the cells concentrates the sugars and other compounds. This remaining cell sap results in a lower freezing point. You know how antifreeze works in a radiator? It's the same idea.
The hardier the plant, the greater the capacity for the plant cells to undergo this dehydration process.
As winter progresses, there is a silver lining. If your trees and shrubs became fully acclimated this past fall, they will likely survive even the coldest temperatures, all winter long, 24/7.
- Temperatures fall below a plant’s maximum low temperature limit even after normal acclimation has occurred.
- Late freezes occur in the spring after the plant has de-acclimated.
- Dramatic swings in temperature during the winter cause a plant to de-acclimate before the threat of severe freezing is over.
So, don't fret too much about how your landscape trees will look this spring. It is likely that the curb appeal all your woody plants gave to your home in the fall will still be there in spring. What you should do is start planning now what trees you'll add to your beautiful landscape this spring. Trees give us back way more than we put in to them. Stay warm Chicago!