Well, it hasn’t happened yet but it’s coming. The annual battle with a frost or a freeze has been waged by gardeners for generations. There are good ways and bad ways, popular methods and snake-oil type tricks to protect plants from frost or freeze damage.
The term frost refers to below-freezing temperatures that are harmful to plants. At such temperatures, the fluids in plant cells freeze and expand, causing the cell walls to rupture.
Frost is a pattern of ice crystals formed from water vapor on grass, windowpanes, and other exposed surfaces near the ground. Frost forms mainly on cold, cloudless nights when the air temperature drops below 32 degrees, the freezing point of water.
Frost and dew forms in much the same way. During the day, the earth absorbs heat from the sun. When the sun sets, the earth begins to cool. The drop in temperature is greater on clear nights than on cloudy nights because there are no clouds to reflect the heat given off from the earth’s surfaces. As cooling continues, the vapor in the air condenses to form dewdrops on objects. Some of the drew drops freeze when the temperature falls below 32 degrees.
Frost injury is caused by an unseasonable cold snap either in the fall or spring. Freeze injury occurs during the winter when temperatures drop below the lowest point that can be tolerated by buds of that particular plant species. The amount of injury from freezing in dormant plants is influenced by the rate at which the temperatures fall, the duration of the low temperature and the rate of thawing.
Whenever frost damage occurs it destroys plant cells. Symptoms of frost damage include blackened leaf parts, split bark and failure for the plant to thrive. Flowers or flower buds when hit with a frost or freeze will either fail to open or drop off.
Some frost protection can be obtained by increasing the amount of heat loss from the soil due to infrared radiation on nights when frost is likely. This may be accomplished by watering the day before a predicted frost.
The additional water in the soil increases the amount of heat in a given volume of soil that can be radiated as infrared rays. This radiation can result in a few degrees protection to plants from frost injury. Frost sensitive plants growing on dry soil, especially freshly cultivated soil, are much more likely to be injured or killed by frost.
Another means of frost protection is by selecting sloping land to plant. Air drainage on slopes helps to prevent cold air pockets from developing due to the movement of the air.
When sprinkler irrigation is available, applying water at 50 pounds pressure through sprinklers having 2 to 6 gallons per minute capacity may prevent plants from being killed by frost. The sprinkler should be turn on when the air temperature drops to 34 degrees.
Maintaining plants at 32 degrees will not kill them. As water freezes, it loses heat. Enough heat is provided by water freezing on plant leaves to keep the leaves at 32 degrees. Because of sugars and other soluble materials in plant cells, the freezing point of cell sap is somewhat lower in temperature than 32 degrees.
It is very important to start irrigating as soon as air temperatures at plant level gets to be 34 degree and continue application until all ice has melted from the plants. If sprinklers are stopped before all ice has melted, the melting of the remaining ice can remove sufficient heat from plants to result in their freezing.
A major problem with this last method of frost protection is the amount of water needed. The gardener has to determine if the plants are worth the water and expense to keep them from freezing.
—Jim Coe lives in Lawton.