Potential freeze damage

Published 8:38 pm Friday, January 15, 2010

The good news is the weather was relatively cool this fall and plants did start to acclimate to the cold weather.

The bad news is it is really cold and for marginally cold tolerant plants acclimating to extreme cold is not possible. This is especially true of most sub-tropical plants and half hardy perennials.

If you are a gardener who likes to push the hardiness zone to extremes, you will be saddened this spring when those marginal plants disappear. Follow this link for the USDA hardiness zone map (http://www.usna.usda.gov/Hardzone/index.html).

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The news is better for more cold hardy species of woody plants. In these plants cool temperature initiates the accumulation of sugars, modification of proteins and changes in cell membrane permeability – all of which increase the plant’s cold hardiness. While most plants require both short days and lower temperatures to develop full cold hardiness, other plants harden only in response to low temperature regardless of the day length.

Once leaves and stems of evergreens harden enough to withstand freezing, becoming frozen makes them even hardier.

Note: the freezing response is strictly localized and is not general to the whole plant. In other words, if lower leaves are acclimated to freezing that does not necessarily mean the upper leaves are also hardened. That is why plants will sometimes burn back from the youngest leaves while the interior has little or no damage.

For woody landscape plants, low temperature injury, often called freeze damage, can be caused by intra- or extra-cellular ice formations within the plant.

When intra-cellular ice is formed, crystals originate inside plant cells. This type of ice formation would be extremely rare in Alabama’s hardy plants and it is unlikely to occur during this cold spell. The other more likely type of freeze damage occurs when extra-cellular ice forms during normal cold winter conditions.

This means that water moves out of plant cells as temperatures approach 32 degrees F to prevent freeze damage and then back into cells for hydration when the temperature rises above freezing.

This type of freeze damage is not lethal to most woody plant species that have been properly acclimated and are cold hardy to at least Zone 7a in North Alabama and Zone 7b in Birmingham and Central Alabama.

Injury can occur, however, if the cells are dehydrated for relatively long periods of time, or subjected to very low temperatures that they cannot tolerate.