Baby, it's cold outside! While we huddle up to the fireplace or pay the oil delivery man for our comfort, trees and shrubs just stand out there and grin. How do water-filled woody plants survive freezing, especially when temperatures fall well south of zero? Let's de-mystify this rather complex process.
Cold hardiness depends upon not just how cold the air turns, but also on rainfall, light intensity, day length, soil fertility, previous high temperatures, and the consistency of temperatures. The shrinking light and dropping temperatures which accompany autumn are the most crucial of these influences which naturally trigger plants to develop cold tolerance. The longer a plant is exposed to these changes, the hardier it gets. Scientists call this "acclimation," while gardeners know it as "hardening off." If frigid temperatures occur before proper acclimation, plant damage may result. For example, August's English ivy plant may die if exposed to 25 degrees F, while it may withstand minus 30 degrees F in January after being properly hardened off. Similarly, a warm spell during a winter might cause a plant to de-acclimate and later suffer if there is later cold snap - just another factor to keep us horticulturists awake at night.
Genetics also play a part. A native red maple (Acer rubrum) from Georgia will be less cold tolerant than the same species from New England, even if hardened off in the same way. That is why we often proclaim it is better to buy young woody plants from northern nurseries rather than from warmer climes. But of course every species, even when properly acclimated and of the hardiest known stock, has its rock bottom temperature it can tolerate. Witness the crepe myrtles in Charleston, SC but not Castleton-on-Hudson, NY.
Forgive me for wandering: My prose has evaded the original question better than a politician before Election Day. How exactly do woody plants survive winter? Down to the microscopic level! Plants are composed of cells and the spaces in-between cells, known as intercellular spaces. If an un-acclimated plant is exposed to freezing temperatures, it will be injured because the water inside each cell freezes. The freezing water expands, the cell walls burst, and the cells die. Not good. The hardening off process stimulates the solutes (the stuff dissolved in the water inside the cell) to become more concentrated, or in other words, it encourages the cells to become less watery inside. These less watery/more concentrated cells don't freeze as easily. It's just like putting antifreeze in your car's radiator instead of water.
An interesting aside is that hardening off also makes the solutes in the in-between spaces less concentrated, or more watery. This might seem dangerous, but freezing water in these intercellular spaces does not normally cause plant damage. Yet even tough plants have their limits, and at some very cold temperature, the differences of more-solutes-in-the-cell/less-solutes- outside-the-cell cannot be maintained. The cells then may indeed freeze and die. Let's just hope it doesn't get that cold this winter.