What effect will the unusually mild autumn weather have on our outdoor plants? The answer is, "It all depends on what comes next."

Plants in our region of the world have adapted to living through annual cycles of warm and cold weather. An important part of this adaptation is the practice of developing cold hardiness. Ideally, the level of hardiness increases gradually starting in the fall as the temperatures get colder and reaching its peak by the depths of winter. Then, the process reverses and hardiness gradually decreases as the temperatures warm into the spring.

Woody perennials such as trees and live with much of their tissue exposed to the open air. To survive, woody plants have developed the ability to "harden off." Plant tissues vary in the degree of cold hardiness they can attain. Generally speaking, a tree trunk will be hardier than a branch. A branch can tolerate more cold than a twig. Buds are more easily damaged than twigs and flowers are the least hardy plant tissue of all.

A flower bud produced in late summer may be killed if exposed to a 20 degree night in October. This same bud, when properly hardened off, will easily survive minus 20 degrees in February. Then, in the spring as it loses hardiness, a mild frost of 27 degrees in early May will often kill the bud.

Unfortunately, our weather does not always honor the plant's hardiness schedule. Sudden changes from one extreme to another are the most destructive. A mild autumn retards the hardening process so that a sudden drop to zero degrees in late December will cause as much damage as 30 below zero might in February. Likewise a March warm spell where temperatures rise into the 60's for a few days followed by a return to freezing temperatures can cause great damage to buds.

What can you do to prevent injury from these sudden changes? Unfortunately, for existing trees and shrubs, there are few options. The best approach is to consider the hardiness of the plant before it is added to your landscape. USDA climate Zone 5 means plants must be able to routinely withstand a minimum temperature of -10 to -20 degrees Fahrenheit. Areas to our south  are Zone 6 and do not experience such low temperatures. To our north is Zone 4 where plants must be even hardier.

Too often, people purchase plants from other climate zones and expect them to survive our harsher weather. Care must be taken when purchasing woody plants from the Pacific Northwest or from the South to make sure they are rated as hardy for our climate. Flowering dogwood trees raised from seeds grown in Kentucky, for example, may have difficulty surviving Miidewestern winters.

Ornamental plants such as Japanese maples, rhododendrons, azaleas, boxwood and roses also tend to need protected sites. They may be able to withstand cold temperatures but winter winds will dry tender tissues and cause damage. Natural wind screens by other plants, buildings and temporary windscreens of burlap can help them survive.   

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