Emily Hoover, professor and department head, UMN Department of Horticultural Science
Question: What is the implication of the freeze warning on the apple crop?
Answer: It depends on how cold it gets. The temperature within an orchard is not consistent. The "rule of thumb" is about 10% of the fruit on the tree will freeze if the temperature drops to 28 degrees Fahrenheit and remains so a few hours. Ninety percent of the apples will freeze if the temperature drops to 25 degrees Fahrenheit and remains so for a few hours.
However, the level of sugar in an apple also changes the severity of the event. The higher the amount of sugar, the lower the temperature has to be before freezing will occur because sugar lowers the freezing point of a solution. (Think Chemistry 101). Note that if the fruit freezes on the tree, but is not touched until it thaws, the fruit is fine to harvest.
Emily Hoover is a professor and department head in the UMN Department of Horticultural Science. Her research focuses on apple rootstocks.
Original source: "Over the Backyard Fence," blog of Master Gardener state program director, Julie Weisenhorn.