Sunday, 18 February 2007

The Big Freeze

February 18, 2007

As you look out your window longing for spring take comfort in knowing that your trees are not really frozen. “That’s impossible with the temperature so far below freezing!” you may think. If your trees were frozen in the traditional sense, like your garden tomatoes, they would be dead.

Hardy perennials and trees have three strategies to prevent them from joining their green cousins every fall. An understanding of why your tomatoes are dead will help you realize why your trees are simply dormant.

In the fall when temperatures drop to the freezing point, ice crystals begin to form in the plants cells. While they are pretty to look at in a snow flake, forming inside a living plant cell

the results are devastating. The sharp water crystals puncture the plant cell causing its contents to leak out. The cell dies as a result. What you see is a wet limp leaf or stem that usually turns black. Commercial food production uses a fast or flash freezing to prevent ice crystal formation and keep your vegetables from getting mushy. Trees are by in large composed of water so how can they survive freezing weather?

Trees have several strategies to prevent ice crystal damage. Even the smallest amounts of

dissolved solids in the water in a tree will lower the point at which ice crystals form. This increases the plant’s ability to survive temperatures slightly below freezing. This is the same reason that salty ocean water freezes at a lower temperature than fresh water. In the case of plants, it is the dissolved sugar that prevents the plants from freezing at just below the freezing point of water.

As the temperature decreases the tree must be prepared by moving water out of the exposed plant cells and into areas that don’t freeze, like the roots. This causes the drying of the exposed upper stems and branches as well as concentrating the remaining sugars in the cells to prevent freezing. Much of the moisture remaining in the cells can be shifted to locations just outside the living cell in the intercellular spaces. Once outside the cell there is no danger that water crystallization will damage the cell.

The entire process reverses itself as the weather heats up in spring. Many plants are injured by rapid warming and freezing that can occur in areas that experience extreme changes in temperature. The ability to survive longer unusually warm spells, while not breaking dormancy, is an issue that affects winter hardiness. Plants that bloom or leaf out early in the spring, are more likely to be damaged by spells of unusually warm winter weather. For more information on winter hardiness contact an ISA Certified Arborist.