killing frost

killing frost A sharp fall in temperature which damages a plant so severely as to cause its death, or prevents the reproduction of an annual, biennial, or ephemeral plant. As the falling temperature approaches freezing point, some water is lost from the vacuole into the intercellular spaces, where a further drop in temperature causes it to form small crystals of ice. If the temperature then rises slowly, the water will be reabsorbed by the cell as the ice melts and the cell will recover, but if the thaw is rapid the water will be lost and the cell will die from dehydration. If the freezing temperature is prolonged, ice may be lost by sublimation (i.e. the direct change from the solid to the gaseous phase). This will also cause dehydration. Reproduction will be prevented if freezing causes such damage to flowers or to developing fruit that has not yet produced viable seed. Seeds themselves contain little water and are seldom damaged by frost. See also growing season.