How area trees survive the winter
The make up and structure of trees which grow in our zone 3 environment are different than trees which need a zone 4 or a above to survive. Jeff Gillman, associate professor of horticulture at the University of Minnesota, writes about this in his book, "How Trees Die." The minimum temperature which trees can survive is only one factor. The wind also plays a major factor in a tree's ability to endure this climate. During the winter, the tops of trees become dormant to avoid potential damage. Plant roots, in contrast, are not truly dormant and can take up water any time there is water present. During the winter, however, most water has become ice, so not much water is taken up. Evergreen trees, unlike deciduous trees which have lost their leaves, have lots of exposed leaves in the form of needles during the winter. This makes evergreens look attractive; however, it also makes them more susceptible to drying wind because of the great surface area from which water can evaporate. Throughout the winter, the total amount of water lost by an evergreen can be very significant, but the tree does not show it, remaining green until spring. In spring a host of needles will turn brown and fall from its limbs. Deciduous trees can suffer from the same drying out, but the water loss usually manifests itself in the spring by an absence of leaves on the tips of the branches as opposed to the browning of the windward side of an evergreen. However, a higher zone tree's branches may be completely dried from the wind. This stress may cause the tree to eventually die.
Minimum temperature also plays a factor. When a tree is exposed to temperatures that are cold enough, the water in the tree will actually freeze. The freezing will cause the water to expand and the cells will swell and rupture. When the cells that make up the vascular tissues burst, that part of the tree that was frozen is essentially dead. The tree does not want its cells to freeze and it has a number of ways to stop this from occurring.
One way is to get rid of excess water. Cells within the tree pump water out of the cells and into the spaces between the cells where water is less likely to cause damage.
Some trees employ antifreeze. When sugar or salt is added to water, it gives the water a much lower freezing point. The tree may use these soluble chemicals to lower the temperature at which water within its cells will freeze.
Finally some trees may get rid of "nucleating agents." When ice starts to form it tends to form first near a crack if one is available. Ice crystals also like to form on edges. If a plant removes edges from its cell (via removing particles, also called nucleating agents) the water will freeze at a lower temperature.
These methods are called supercooling. They are strategies which allow trees to survive the low temperatures that winter will bring. Trees that use these methods most effectively are the ones rated for lower temperate zones.
While trees may look dormant in winter, certain important chemical reactions are taking place inside them. When the temperature is between 33 and 45 degrees, certain chemicals are produced in most trees. The longer the tree spends between these temperatures the more of these chemicals are produced. Only when the chemicals reach the right level is the tree ready to respond to the warm air rushing through its branches.
People who grow fruit trees refer to the time a tree spends in this narrow temperature range as "chilling hours," and select trees based on their requirement for chilling hours. Trees in Minnesota might require 1,200 chilling hours, while a tree in Florida might require 150. So if a tree requiring less chilling hours is planted in Minnesota, it will receive all the chilling hours it needs early in the winter. If we have just a few warm days in late winter the tree will break bud and leaves and flowers will begin to emerge. If the temperature drops again, all the young growth on the tree could be killed. If the tree is strong, it may be able to produce new buds and continue for the season. However, if the tree is young or weak or if such an event happens too often, the tree will probably die.
Even after they have met their chilling requirements, trees such as oaks wait until we have had plenty of warm days before opening their buds. This ensures that they will avoid late frosts, because they are not good at producing new buds in the same year. Maples, however, break bud soon after their chilling requirements are satisfied, and if they lose leaves to a late frost will produce more buds the same spring.
Kyle Schulz is a Wadena County Master Gardener from Sebeka, and the regular gardening columnist for the Wadena Pioneer Journal.