We can have tremendous temperature fluctuations in Minnesota. I remember February days in short sleeves and April days where I froze to death. It doesn’t seem to matter with the teenagers next door, they wear shorts and flip flops regardless of the weather.
The bitter cold, extending into Texas this winter, has me thinking more about climate variability and what effect this will have on the trees. What are trees doing to prepare for winter? What physiological changes do to they need to go through? They can’t put on a coat.
The first thing trees do is get rid of the water in their cells, because it is going to rupture like a frozen water pipe. They have to turn into a Pringles potato chip. Better not have a lot of water in the cell, because if that freezes, then it’s certain death. This is why I stopped a young woman leaving Home Depot with a cart of houseplants on an 11 below zero afternoon and suggested she cover them up.
Trees need to be warned of approaching winter before it arrives. This process of acclimating in autumn starts as soon as the days begin to shorten, somewhere around June 21. Trees are reading daylength but are also sensing temperature and looking for frost exposure. Warm days and cold nights below 28 degrees is what they want. Get enough of those conditions and trees are ready for winter.
How do trees know to come out of winter? They need to be exposed to a certain number of hours between 33- 45 degrees. This is known as the chilling requirement. For example, a tree grown in Minnesota might require 1,200 chilling hours while a tree in Texas might require 50. Once they’ve got enough hours at cold temperatures, they are ready to grow. This dormant period ensures that trees don’t start growing during a warm autumn or a mild mid-winter spell. What are they looking for then? They are looking for warm temperatures. That’s all they need.
Now comes the variable climate, with meandering jet streams and polar vortexes. I’m not worried about the deep cold of January, but the spring and fall that are atypical. Like the aircraft carrier effect; most accidents happen at takeoff and landing.
How often do we get trees’ leafing out in the spring, then we get that frost that knocks them back? Or the warm autumn till mid -November, then a blast of snow while the leaves are still on?
You want a tree that goes to bed early and gets up late, in other words, more influenced by daylength and less influenced by temperature. Tree such as oak, ginkgo and Kentucky coffeetree. Avoid trees such as pagoda dogwood, magnolia and quaking aspen which are experiencing severe declines with climate change. Make sure your trees are healthy going into winter by giving them plenty of water. Dry conditions going into the winter can make plant tissues more susceptible to cold damage. Cover sensitive shrubs with burlap.
The Texas palm trees are probably ruined, but what about the oaks? Are they adaptable to withstand zero degrees? Only time with tell. Climate change is a reality affecting not only people but our plants that live with us too. Let’s hope they can adapt fast enough.