Why would a layer of ice prevent frost damage to fruit? I know a family that grows fruit such as blueberries in a climate that frosts on occasion. When this happens, they turn on sprinklers which covers the buds and fruit with a layer of water which turns to ice.
Why would this layer of ice prevent damage to the plants/fruit? Perhaps it insulates them, keeping temperature at 0 degrees but now below it?
 A: Well I don't know anything about blueberry crops, but I'm familiar with what is done (in California) with grape crops, having built a vineyard to do just that.
Since the liquid in the grape is basically water with sugars in it, the freezing point of the liquid in the grape is considerably below zero deg C.
So the vineyards are equipped with sprinklers, and also usually a water storage pond, of say one acre for an 80 acre vineyard.
When the air temperature gets down to near zero, they turn the sprinklers on; and these produce a quite fine water mist, which settles on the crops, and starts to freeze once the Temperature gets to zero.   The sprinklers remain on during the low Temperature period, and pumps also run, to replenish the water in the pond.  Typically the ground water will be in the 20 deg C range, as will the pond water.
So some "heat energy" comes from the water Temperature, but so long as they keep the mist spray going, the crop surface Can't drop below zero deg C, and the grapes are safely unfrozen.
You can't just grow an ice layer, and then stop spraying; it is the continuous freezing that hold the Temperature at zero.   Such systems can easily hold off a freeze for ten hours, in typical California situations.
Biggest problem is keeping the spray level from getting too high because you don't want to weigh down the vines with too much ice weight.
So it is not insulation, but the Temperature regulation at the freezing point of sugar free water, that stops the grapes from freezing.
A: It's the "latent heat of fusion" produced when water freezes that protects the crop, see p. 3 here: http://fruit.wisc.edu/wp-content/uploads/2011/02/Understanding-Frost-in-Fruit-Crops1.pdf
