Written by: John Wayne Farber, Board Certified Master Arborist (BCMA) WI-0877B, Certified Treecare Safety Professional 01462
We weren’t sure winter would ever come, but better late than never. Even though it has been an extremely warm start, once the temperature drops and the snow begins to fall, we are often asked how trees survive winter. The simplest answer is certain trees have adapted to the environment in which they grow. Diving a little deeper into the question, it’s all about water management.
Water is required for all life. We need water and so do trees. Trees use water to move required elements inside of them and it is one of the main ingredients in photosynthesis, which produces sugars to sustain life. We all know in winter, when temperatures drop below 32F, water freezes. So if the water is frozen how does life go on? Trees have come up with some pretty awesome schemes to overcome freezing water.
Water is required in constant supply within trees. If the water column is broken the system stops functioning. In winter, it is very difficult to pull more water from the frozen ground to supply the system. In response, trees tackle the issue from the other side of the equation and try to use (and lose) less water when the ground is frozen. The main way water escapes trees is through their leaves/needles. In deciduous trees, in fall, trees drop their leaves and therefore reduce their water use. In evergreens, they too drop needles annually but also have less surface area then broad-leaf trees, which reduces water loss. Evergreens also narrow the holes in the needles (stoma) to reduce water leaving from them. When water requirements exceed availability, trees (or parts) die, and we recognize this as winter burn or desiccation. This can be caused by tree species not being adapted to the climate where they are planted, excessive salt application, or an early warm-up followed by a cold snap in spring.
Water is a friendly enough molecule in the liquid state, but when it freezes, it expands and crystalizes. Expanded ice crystals are pointy and sharp, capable of tearing through cell walls, which in turn means cell death. Trees again have to be clever to keep the water inside their cell walls from freezing during winter. One intricate way trees decrease the likelihood of cellular freezing is moving the water out of the cells by increasing compounds inside the cellular space. The result is twofold:
1) The compounds inside the cell drive down the temperature at which water freezes, creating a sort of antifreeze. This antifreeze can be tolerant of -40F. The remaining water outside the cell is then allowed to freeze if needed. Freezing water outside the cell is far less detrimental.
2) The water that is moved outside of the cell is very pure, which allows the water to "super cool" (well below 32F) before ice would begin to form. When the water outside the cell changes from liquid to solid, heat is released, further protecting the cell.
Another tree defense mechanism championed by evergreens is to have thicker cell walls. Like a well-insulated house the thicker the wall the more protection from cold and physical damage.
So how do we tell if our tree is evolved to handle our winters? Each species has a designated “hardiness.” Hardiness is the lowest average annual temperature each species can tolerate. It is usually labeled on the plant at the nursery (or easily looked up) and will be referenced by “zone.” Zones are climate maps overlaid on geographic maps, which shows the likely average low temp in a given region. The east side of Wisconsin near Lake Michigan, including Milwaukee County, is in zone 5b, which translates to between -15F to -10F average low temperature. As we travel northwest in the state, the zone number decreases along with the average low temperature.
When you buy a new plant for your landscape, the best defense against winter damage or death is to put the right tree in the right spot, and a primary factor to always consider is plant hardiness. One last note on this, hardiness zones are not static and will move and change as our climate shifts. Someday, we may have Wisconsin streets lined with palm trees.