Plants can be injured in many different ways during the winter months. Extreme cold damages plants by freezing the contents of cells and causing them to burst. Once a plant cell bursts, it is dead. If enough cells in a tissue such as a bud or twig are killed, that tissue dies too.

In United States Department of Agriculture (USDA) Hardiness Zone 5, for example, the average lowest winter temperatures are expected to reach between -10 and -20 degrees. Plants rated as hardy for Zone 5 should survive this temperature without damage to their cells.

Note: In January, 2012, the USDA released a web based hardiness zone map that updated some areas based on temperature data from the past 30 years.

Unfortunately, applying the principles of the hardiness zone concept are not always straight forward. If you do live in Zone 5 and they predict -10 degrees one night, are your plants safe?

One factor to consider is that the official weather forecast temperature is usually recorded at one specific location and is normally taken from a thermometer several feet off the ground above where lower growing plants reside.

Also, if your landscape is in a "low lying" area at the bottom of a hill, the temperature may drop several degrees lower than the prediction. On the other hand, if your landscape is located near the top of the hill, odds are better that the temperature did not go below -10 degrees.

Plants in an open, windswept site may suffer more damage than those in a protected area. Cold, dry winds will cause desiccation (drying out) of the plant tissue. This can make buds and twigs more susceptible to low temperature damage.

The overall health of the plant going into the winter will also be a factor in how much damage occurs. If the plant was under-watered or under-nourished or suffered disease, insect or site stresses the previous summer, it is more likely to be damaged by the cold.

Many of us "stretch the zone" and use plants that are really suited to the next higher numbered hardiness zone which does not get as cold. These plants may work out fine for many years until a severely cold winter happens. It only takes one night of temperatures below the minimum for which your plants are rated to cause damage or even death.

Another factor is that various tissues on a plant respond differently to extreme cold. Generally, flower buds formed the previous autumn are least hardy. If these are damaged or killed, flowering shrubs or trees that set their buds during the fall may not be very showy in the spring following a severe winter.

Vegetative buds which will become leaves are hardier than flower buds. Twigs are hardier than buds and major branches and the trunk are the hardiest of the above ground parts of the plant.

What can you do if winter temperatures are predicted to drop below the hardiness level of your plants? For existing plants in your landscape, the answer is, "Nothing."

If you or someone in the past selected and installed plants appropriate for your hardiness zone, they should do just fine. However, if you "stretched the zone" with more tender trees and shrubs, only time will tell if they will survive.

For the future, use only plants known to be hardy your particularly hardiness zone or lower. Keep plants as healthy and stress free as possible during the growing season but do not stimulate lush new growth in the fall by pruning or fertilizing late in the growing season.

If you feel a strong urge to grow marginally hardy plants, located them in the high ground on your site and/or provide some type of protection against the raw winter winds.

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