Temperatures at which metabolic activity of the plant is affected.

Acclimating for Winter

There are several stages woody plants undergo when they acclimate for winter.

Stage 1

Acclimation to ~ -27C (-16.6 F.) occurs in response to photoperiodism. When the length of day reaches a certain length, a chemical trigger occurs in the plant causing the plant to acclimate to withstand temperatures down to -27 degrees C. (To be correct we should say the plant responds when the length of night is greater than some critical length).

In native plants or plants originating from the same latitude, the plant responds to this chemical trigger at the proper time. When this trigger occurs, the plant does not slow down. The following metabolic changes take place:

In the case of plants which originated from a different latitude, they do not respond to the same photoperiodism (length of night) as plants from our latitude, as a result, they may not acclimate properly. Plant from high latitudes (i.e. Alaska) may acclimate sooner than they should for our area, while plants from a more southern latitude may not normally acclimate until after the date frost occurs in our area.

Acclimation for the southern latitude plants is a slow process and in our area must be induced by stress. Withholding water and nitrogen fertilizer helps induce stage 1 in these plants. With these plants applying nitrogen fertilizer (or applying a heavy application of water) in the fall (prior to frost) can stimulate growth. The succulent tissue resulting is then damaged by frost or the plant is killed. Trees that have lost their tops due to winter injury have often done so because they did not acclimate for winter properly. The resulting damaged tissue also is an ideal spot for disease pathogens and insects the following year.

Occurs at first frost.

Plants which responded properly to photoperiodism are already acclimated to ~-27C.

Plants which did not respond properly are damaged or killed by the frost as indicated above.

In all cases the first frost triggers extracellular freezing. Temperate zone plants can survive freezing by the formation of ice outside the cells. Water within the cells moves through the cell membrane and cell wall to airspaces between the cells. Plants that can survive the lowest temperatures, typically have the larger airspaces (a place for the ice to form). They may have as much as 50 - 75% air space in their tissue. The cell walls in these plants also can collapse like an accordion and the cell content (protoplasm) is able to dehydrate without damage. As indicated earlier, these plants may be able to tolerate temperatures as low as -320.8 Fahrenheit.

The cells of those plants which have thick cell walls can not collapse, and are consequently damaged by the formation of ice. These plants can tolerate no more than -40C (-40 F).

The freezing point of water = (0c) 32F. Many plants die at 0c (32F) unless properly acclimated.

A second freezing point for water in woody plants occurs at ~ - 40C (- 40 F). This is known as the point of homogeneous ice nucleation. This varies from -41 to -45 C in many trees. This is the lowest temperature limit for most hardwood trees. Not all tissue, however, responds the same to this low temperature. In the case of apple, the living xylem (wood) cells die at ~-40C with very short exposure. This temperature causes the water to freeze intracellularly (not extracellular as discussed previously). The buds and bark of apple, however, can withstand -196C. The American elm can withstand -35C for as long as 6 months, but can withstand -41C for about 1 second.

Other common woody plants which can survive temperatures no lower than ~ -40 are stone fruit, pear, grapes and roses.

Woody plants which are known to survive liquid nitrogen [-196 C (-320.8F)], if properly acclimated, include many conifers to include White spruce (Picea glauca), black spruce (Picea mariana) , Englemann Spruce (Picea engelmannii), Tamarack - Larch (Larix spp.) , white birch (Betula papyrifera), as well as Arctic willow (Salix arctica). These basically are plants of the boreal forest.

As many of our landscape plants originated in different latitudes, they do not acclimate at the proper time for our area based on photoperiodism. Consequently, fertilizing these plants with nitrogen in the fall (often after July 15) could result in stimulation of growth, and plant damage or plant death at the first frost.