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John Fulton

John Fulton
Former County Extension Director

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In The Backyard

Horticulture columns and tips done on a timely basis

Frost and Freeze Damage - from Martha Smith

Posted by John Fulton -

Spring is barely two weeks old, and we are about to experience how fickle Mother Nature can be. If the forecasters are correct, very cold air will move through Illinois.

"Just last weekend I was commenting on how many buds my tree peony had, anticipating the large, beautiful fragrant blooms that will cover the plant," says Martha Smith, University of Illinois Extension horticulture educator. "With temperatures forecasted to dip below 30 degrees, I wonder if they will survive."

We have been spoiled with early warm temperatures. But, patience this time of year is so important. Average dates of last frost range from April 5 in southern Illinois to April 25 in northern Illinois. That means we still have a 50 percent chance of freezing temperatures.

"We usually recommend a 2-week waiting period before it is declared 'safe' to plant," says Smith. "But, I remember May 3, 2005, when night-time temperatures in western Illinois dipped to 28 degrees --- Mother Nature's way of reminding us of who is in charge!"

There are two types of freezes, radiation and advective.

Radiation freezes or frosts occur on calm, clear nights when heat radiates into the environment from surfaces or objects. Soil, buildings, plants and other objects at the earth's surface act as a heat reservoir by absorbing heat during the day. Plants are damaged when enough heat is lost from this reservoir to lower the temperature at the surface to below critical temperatures. These freezes are generally considered light, and they primarily damage outer tips and expanded buds. Plant damage from a radiational freeze can be minimized by reducing radiant heat loss from plant and soil surfaces.

Advective freezes can occur when cold air masses move down from northern regions, causing a drastic drop in temperature. Windy conditions are normal during advective freezes. Although radiant heat loss also occurs during an advective freeze, Smith says the conditions are quite different from a radiation freeze. The temperatures tend to be much lower and are liable to last longer during advective freezes, and protecting plants is more difficult. Expanded buds and new leaves will be damaged. Tip die-back may occur. Herbaceous plants may be killed completely or die back to ground level.

Plant protection during radiation freezes involves covering plants to prevent heat given off by soil and plants from escaping into the atmosphere. Protective coverings are usually porous material such as newspaper, bed sheets or burlap.

"Use stakes to hold the covering up and off the plant material," advises Smith. "Boxes, buckets, styrofoam containers or milk jugs with their bottoms removed can also be placed over plants to help alleviate heat loss."

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