Growing Up: Last year’s warm fall has caused many area trees to winter-kill
JIM GAINAN Growing Up | Posted: Sunday, August 8, 2010 12:00 am
We received an email question about winter kill and Ash trees. In this case, this spring, the leader or center stem did not foliate, leaving a dead center section. I turned this reader's query over to Arlene at our Garden Center for causes and treatment. The question and her answer are worth sharing, because the problem is widespread in our area this season.
This problem with trees and shrubs in the Billings area was due to the strange weather we had in October of last year. As you may remember, it was very nice, warm weather in September and early October. Then "BAM!", it was cold and snowy and stayed that way. This combination of conditions caused the problems seen this spring and summer. The generic term for this is winter kill.
As the temperature drops in the fall and days become shorter, leaves begin to fall off. Essentially, leaves are plant food factories. Plants and other photosynthesizing organisms have a very special talent. They turn sunlight into food.
In order for plants to make food energy, they need water, carbon dioxide (CO2) and sunlight. From this special combination, a plant is able to make its own food, in the form of glucose, a type of sugar. Plants then use the glucose as food energy to live and grow. In order to harvest sunlight energy, plants have a green pigment called chlorophyll. This pigment is what makes a plant's leaves appear green.
As winter approaches, the days get shorter and cooler. This change in day length and temperature triggers some trees to go dormant, basically hibernating for the winter. A tree's woody roots, branches and twigs can endure freezing temperatures, but most leaves are not so tough.
It is also very energetically expensive for a tree to run its leafy food factories in the winter, when there is often little sunlight and freezing temperatures make water transport (from the ground into the tree's trunk and leaves) a problem. So it's more energy efficient for a leafy tree to close down operations in the winter and go dormant.
A tree is full of vascular cells that transport water and sap throughout, from root to leaf tip. As the amount of sunlight decreases in autumn, the veins that transport sap into and out of a leaf slowly close off. Then a layer of cells, called the separation or abscission layer, develops at the base of the leaf's stem. When this layer is completely formed, the leaf falls off. The food energy and chlorophyll components are removed from the leaf and stored elsewhere in the plant. This is what the plant uses to start its growth the following spring and why leaves turn colors in the fall. The different pigments that we see as brown, red or yellow are always in the leaf, but are masked by the green pigments of the chlorophyll.
This process happens in all deciduous trees (trees that annually shed their foliage), with oak leaves as a notable exception. With last year's warm fall weather, the trees were not triggered to complete this process. Many trees still had most of their leaves attached. The cold weather killed the more fragile leaf tissue; thus, the tree's food energy was lost. You may have noticed this, as leaves remained attached to trees throughout the winter. Maples and Ashes appear to have been the most affected. Now, let's fast forward to this spring.
The tree starts to grow, using its stored energy from the previous year. However some trees did not have sufficient stored energy to fully leaf out and start their growth. Some trees budded, some trees started to leaf and then stopped, some had branches died off, some had the entire upper portion die. Some trees had the upper portion die, with the roots sending out new growth. All this is considered winter kill. There is not much hope for this section of the tree. A certified arborist should be contacted to determine what, if any parts of the tree can be saved and if what pruning techniques could be used to train another 'leader' or top section of the tree.
Jim Gainan is VP/Shareholder of Gainan's Flower and Garden Center in Billings.