Freezes can be characterized as radiational or advective. Radiational freezes or frosts occur on calm, clear nights when heat radiates from the surfaces of objects into the environment. These surfaces can become colder than the air above them due to this rapid loss of heat or long wave radiation. When the air is moist, a radiant freeze results in deposits of ice or frost on surfaces. Dry radiational freezes leave no ice deposits but cause freeze damage. Reducing radiant heat loss from plant and soil surfaces can minimize plant damage from radiational freezes.

Advective freezes occur when cold air masses move from northern regions causing a sudden drop in temperature. Windy conditions are normal during advective freezes. Although radiant heat loss occurs during an advective freeze, and the conditions are quite different from a radiational freeze. Plant protection during advective freezes is more difficult.

The ability of plants to withstand freezing temperatures is affected by temperature fluctuations and day lengths prior to a freeze. A gradual decrease in temperature over a period of time increases the ability of plants or plant parts to withstand cold temperatures. A sudden decrease in temperature in late fall or early winter usually results in more damage than the same low temperature in January or February. Short durations of warmer temperatures in midwinter can deacclimate some plants resulting in buds breaking or flowering. Deacclimated plants are less prone to withstand chilling temperatures. Preconditioning of plants to withstand chilling temperatures has not been well documented.

Cold injury can occur to the entire plant or to plant parts such as flowers, fruit, buds, leaves, trunks, stems or roots. Many plant parts can adapt to tolerate cold, but fruits and roots have little ability to acclimate or develop cold tolerance. Cold injury to roots of plants in exposed sites or containers is a common occurrence and usually is not evident until higher temperatures stress the plant.

Leaf and stem tissue will not survive ice formation inside the cells (result of rapid freeze), but many plants can adapt to tolerate ice formation between cells (result of slow freeze).

One type of winter injury is plant desiccation or drying out. This is characterized by marginal or leaf-tip burn in mild cases and totally brown leaves in severe cases. Desiccation occurs when dry winds and solar radiation result in the loss of more water from the leaves than can be absorbed and/or transported by a cold or frozen root system. Evergreens on exposed sites are more prone to desiccation injury.

Curt Peterson, Ph.D. landscape

specialist, is from Union Springs.

He can be reached at curt.peters@hotmail.com