WINTER ACCLIMATION OF WOODY PLANTS (or how plants prepare for winter)

Curtis E. Swift, Ph.D., Colorado State University Extension

Cardinal Temperatures

Temperatures at which metabolic activity of the plant is affected.

The High Temperate Growing Point

This is the temperature at which the membrane `melts'.
In this case the membranes go from a liquid-solid phase to a liquid phase and enzymes start denaturing the tissue. This occurs at +50C (122 F). The damage may take several days to cause problems.

Chilling Injury.

This occurs on frost susceptible plants; i.e. African violet and other tropicals between +4 to +17 C (39.2 to 62.6 Fahrenheit, respectively).
We typically talk about 10C (50F) as the temperature at which chilling injury occurs with these chilling sensitive plants. The low temperature growing point is that temperature at which enzymes in the membranes are `frozen' and can't function properly. The damage that results is metabolic in nature. This is called phase separation in which the membranes go from a liquid-solid to solid state.

Super-Cooling

This occurs in hardy woody stems occurs at ~-40 C ~-40 Fahrenheit)

Deep Super-Cooling

The temperature of liquid nitrogen is -196C ( -320.8F).
Woody plants found in boreal regions can tolerate temperatures this low if they properly acclimate.

Acclimating for Winter

Woody plants undergo several stages as they acclimate for winter.

Stage 1

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. (-16.6 F. ) Note: To be correct we should say the plant responds when the length of night is greater than some critical length.

Notes:

Plants native to the area or plants originating from the same latitude respond to this chemical trigger at the proper time. When this trigger occurs, plant growth does not slow down. The following metabolic changes take place: respiration increases, protein levels increase, enzyme activity increases, water level drops, starch levels doubles, level of ATP (the plants energy source) increases, ethylene levels increase and leaves drop.

Plants which originated at a different latitude do not respond to the same photoperiodism (length of night) as plants from our latitude, as a result, they may not acclimate properly. Plants brought into Colorado from different latitudes respond differently. Those from high latitudes (i.e. Alaska) may acclimate sooner than they should, while plants from a more southern latitude may not start to acclimate until after frost.

In Colorado acclimation for plants from southern latitudes is a slow process and must be induced by stress. Withholding water and nitrogen fertilizer helps encourage winter acclimation in these plants. Applying nitrogen fertilizer (or a heavy application of water) in the fall (prior to frost) can stimulate growth and restrict winter acclimation. This succulent tissue can be damaged by frost or the plant killed. Trees that have lost their tops due to winter injury (e.g. Austrian pine) often do so because they did not acclimate properly for winter properly. The resulting damaged tissue also is an ideal spot for disease pathogens and insects the following year.

Even when the same species of tree or shrub are planted next to each other they may respond differently to winter acclimation cues. This difference can be explained by the age of the trees as well as the location of their roots. The more established a tree is, the less vigorous it typically is and better it responds to environmental cues.

Roots extend well beyond the drip line of the tree and the roots of a neighboring tree may be in a soil withs more available nitrogen and/or water. This tree would be more vigorous and not as likely to respond to environmental cues to acclimate for winter.

A tree that does not acclimate properly may have its leaves 'freeze-dried' in place. When winter acclimation proceeds properly the production of ethylene gas in leaf tissue begins the process of leaf abscission. When cold temperatures kill the leaf tissue before abscission can occur the leaves that have not yet dropped will hang on. Sometimes remain all winter until they are finally pushed off the tree as bud growth begins in the spring.

Stage 2

This occurs at first frost.

Plants which responded properly to photoperiodism as described in Stage 1 have already acclimated down to ~-27C. Plants which did not respond properly may be damaged or even killed by the first frost.

In all cases the first frost triggers extracellular freezing. Temperate zone plants 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 with thick cell walls can not collapse and are consequently damaged by the formation of ice. These plants tolerate no lower 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 only 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 are plants of the boreal forest.

Summation

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.

Placed on the Internet September 29, 2000; Updated on December 30, 2009