Re: Why does washing the frost off plants before sunrise save them?

Good observation!! Let me explain.

The coldest temperatures of the spring occur at night, usually an hour or two after midnight when the air and the ground have had a chance to cool down. Sometimes the weather conditions become right for cold temperatures and frost. The coldest time of the night is the time when growers need to take action to prevent damage to their crops. Spraying water on plants is one way to protect them from damage due to low temperatures.

Plants are injured by the cold when the water inside the cells turns to ice. As the temperature goes down, the cold air draws the heat energy out of a plant. We use water just like you would use a jacket. The water being sprayed is warmer than the plant. The water will do three things. First of all, the water will warm the air around the plant. Second, some of the heat will be transferred to the plant, keeping it from freezing. Third (and most complicated), water will give off a large amount of heat energy as it changes from liquid to ice. This heat energy also serves to keep the plant from freezing. Farmers continue to spray the plants with water until the temperature rises and the crops are out of danger.

This method of protecting the crop is not without its problems. First, a large amount of ice may weigh down the branches of a tree and cause them to break off. Another problem occurs the next day as the ice melts and the water evaporates. Evaporative cooling may chill the plant enough to cause damage even after the coldest temperatures are gone. (Think of when you get out of a pool. Your skin is still wet and the air feels really cold - that is another example of evaporative cooling).

I found a couple of web sights that offer a much more in depth explanation.

"Irrigation as a Tool for Frost/Freeze Protection"

"Frost/Freeze Protection for Horticulture Crops"

I hope this helps. If you have any more questions, send me an e-mail!!

Eric J. Biddinger
Graduate Student - Department of Horticulture
Penn State University
ejb190@psu.edu