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 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, it is not that simple. For one thing, the official temperature is recorded at one specific location and is normally taken from a thermometer several feet off the ground. If your landscape is in a "low" area at the bottom of a hill, the temperature may have been several degrees lower. If you are on the top of the hill, odds are better that the temperature did not go below -20.

Plants in an open, windswept site may suffer more damage. 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. If the plant was under-watered or undernourished or suffered disease, insect or site stresses last 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 Zone 6 which does not get as cold. These plants may work out fine for many years until a deep freeze hits as it did this winter.

Different tissues on a plant respond differently to extreme cold. Generally, flower buds are least hardy. Flowering that set their buds during the previous fall may not be very showy this spring. 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.

Marginally hardy species such as peach trees located on low sites may experience loss of major branches or the entire plant. Hardy native trees or shrubs may only suffer loss of a few flowers.

What can you do? For existing plants, the answer is, "Nothing." Plants that were not hardy enough for the temperatures have experienced the damage and there is no going back. Inspect the new growth in the spring and remove the dead tissue.

For the future, use plants known to be hardy for Zone 5 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. Plant marginal species on high sites and/or protect them from raw winter winds.


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