White Frost Saturday and Sunday Morning

Saturday morning the lowest temperature I saw on the National Weather Service for Wells, Minnesota, was 34 degrees. Temperatures in Faribault were similar but yet there was a lot of white frost on roofs and lawns. Many call this a white frost but not a killing frost. White frost forms when solid surfaces are colder than the air temperature that is close to but still above 32 degrees. Solid surfaces can be colder than air temperatures because of evaporation. When these conditions occur, water vapor is deposited on the solid surfaces.

A killing frost occurs when temperatures are below freezing or 32 degrees for a long enough period of time to freeze the water in the plant cells. When the water freezes it expands and ruptures the cell walls and the plants die. Some plants are very sensitive to low temperatures while others are more tolerant. Soybeans, for example, have leaves with a waxy covering. Beans can survive temperatures down to 28 degrees for a short time and not be killed.

It does not really make any difference if we had a killing frost at this point in the growing season. Corn and soybeans were mature long ago. Even my beans after peas planted July 15 were almost mature. However, I am happy that we did not get a killing frost yet. The leaves will stay on many trees and flowers and shrubs will long nice a while longer.