This winter has started out as being long and cold. With cold weather, we often see what is called frost. Frost is something we see, talk about, and even predict, but do we really understand it.
Since frost can affect plants, I believe it is good for gardeners to have a basic knowledge of how cold temperatures affect plants. Basically, temperatures are hot, cold or somewhere between these two extremes. The temperature of an object indicates the amount of heat energy associated with that object.
Although we can sense if something is hot or cold by touch, thermometers must be used to accurately determine how hot or cold something is.
When water vapor is cooled to a temperature of 32 degrees or less, it freezes and forms frost. Note that water vapor must be exposed to freezing temperature in order for frost to form. The temperature of the surface, on which frost forms, must cool to freezing (32 degrees or below).
When air that contains a lot of moisture (water vapor) physically contacts a cold surface (colder than the air) the air in direct contact with the surface (such as a plant leaf) is cooled. As this humid air cools, water vapor from the air is condensed onto the cool surface as (1) dew — if the temperature of the solid surface is above freezing or (2) frost — if the temperature of the solid surface is below 32 degrees.
Liquid water is not directly involved in frost formation. During frost formation, water vapor changes from a gas directly into ice crystals without going through its liquid phase.
I think that we sometimes get the wrong idea about frost occurrence because we often see frost on plants when the air temperature is above 32 degrees.
However, the air temperature (as indicated by your thermometer or reported by your local weather forecaster) does not have to reach 32 degrees for plant leaves to get cold enough to cause frost. During radiation cooling on clear, cool nights, solid surfaces (including plant leaves) lose heat more rapidly than the surrounding air. As a result, temperatures of solid surfaces, such as leaves, rooftops, etc., may fall below 32 degrees and cause frost to occur even though the air temperature is above freezing.
Plants are injured by freezing of plant liquids (sap) within plant tissues. This injury occurs in susceptible plants when the temperature within the tissue drops low enough to cause freezing. Warm season vegetable plants are injured when this occurs, regardless of the presence or absence of frost.
Frost can be considered a visual confirmation that the temperature of the leaf or other surface on which it formed dropped to 32 degrees or less.
However, keep in mind that when there is relatively little water vapor in the air, plant tissue can freeze without frost occurring.
Clark Beusse is the Dawson County extension agent. For more information, call (706) 265-2442.