Being a fan of winter, this weather has been an absolute blast, but even I must admit- darn it's cold out there. One question I have been hearing a lot is "What about our plants?" Well, if you religiously adhere to the USDA cold hardiness zones then you should have nothing to fear. More than likely your trees, shrubs and perennials will emerge and leaf out to greet the spring. But who are we kidding? Gardeners are notorious for pushing the envelope when it comes to growing plants that are not necessarily suited to our local climate. As I sit here in USDA hardiness zone 5, I wonder, did my zone 6 magnolia and crape myrtles make it? What about the garlic I planted in my containers? Only time will tell.
My oldest son recently asked me, "Why don't the trees die in the winter?" I am likely the best or worst person to answer this question, depending on your perspective.
While this may have been uttered from the mouth of a child, scientists have been thinking about trees and winter for a long time. After all, the Earth used to be much warmer with a predominately tropical climate. So how did plants acquire the ability to survive the big cool down?
One of the biggest stresses a plant faces in a frigid winter is drought. Yes, drought! The majority of tree roots occupy the top four to eighteen inches of soil, and if the soil is frozen to that depth, the water is locked in the ice and not available to plants. Oddly enough, the droughts experienced by plants in our prehistoric world favored the evolution of plants that are better adapted to winter conditions. Over those millions of years, deciduous and evergreen plants have, developed creative survival strategies. For instance, the rugged and evergreen rhododendron leaf has developed a waxy coating that protects from freezing temperatures and reduces water lost to evaporation. Needled evergreens have minimized their leaf surface area curtailing water loss during sunny and windy winter days.
Narrower xylem tissue (the plumbing in the plant that transports water from roots to shoots), developed in response to prehistoric droughts. These narrow xylem tubes are also is very useful for trees to reduce the size of air bubbles formed when water freezes in their vascular tissue.
Moreover, trees learned to deploy one of the most effective tools, sugar. Sugar acts as a form of anti-freeze in plant tissues. As winter approaches evergreens use sugars in their leaves to prevent water in their cells from freezing. Deciduous trees also employ this technique. Remember those red fall colors? That's partly due to the sugars in the plant protecting the leaf long enough for the tree to resorb its energy.
We have been fairly lucky that with each severe cold snap we had an insulating layer of snow. Hooray! Snow is good! For the most part, I am not worried about my zone 6 plants, having sited them in areas with southwest exposure, blocked from desiccating winds and the advantage of radiant heat from adjacent hardscapes and buildings. For my perennials and garlic, I made sure to apply shredded leaf mulch as an insulating barrier to the cold temperatures.