Explain why fruit growers spray their trees with water when frost is expected to protect the fruit from freezing.

It's going to be cold enough to freeze water no matter what the fruit growers do, but they can control where the freezing is going to occur. By spraying water on their trees they do two things. 
1. The water they're spraying on the trees is relatively pure. So it's freezing point is close to 0 degrees C. However, the water in the fruit trees isn't pure and has a lot of solutes dissolved in it. So the freezing point of the water in the trees and fruit is a few degrees below 0 C. Because of this, the water sprayed on the trees will freeze FIRST. And while there is some liquid water still on the trees, the temperature of that water will remain constant at 0 C. So the water in the trees won't start to freeze until all of the water sprayed on the trees has already frozen.  
2. The water sprayed on the trees has to lose quite a bit of energy before it's completely frozen. The energy needed is about 333 Joules per gram. And as implied in #1 above, until all that water has frozen, the water in the tree won't start to freeze.

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