You should also ask on the Fruit and Orchard Forum. Some of them live to grow fruit. They could probably tell you all kinds of stuff about this.

An easy way to prevent the roots from freezing is to water the plant down just before sunset and then wrap it with any kind of plastic or linen you have handy. Because the water out of your tap will be 50+ (F) it will add an additional 20+ degree "buffer" to the temperature of the soil before freezing can begin. Or in other words, before the roots can freeze, all the water you laid down has to freeze first.

I just did this to my front lawn (and i'll do it again around 3AM) because I just renovated it and the poor little germinating sproutlets are only a 1/8th of an inch tall and wouldn't otherwise survive a flash freeze like we've got coming tonight.

Arjo,

There are two issues here. First, it's not anywhere near cold enough for roots to freeze. Ground temperatures aren't the problem with brief freezes like some places are experiencing right now.

The other thing is that watering plants to keep them from freezing is probably not a good idea. In just the right conditions, orchards sometimes spray a mist of water on their trees to form a layer of ice for insulation, but this is not the same as watering the trees. Many plants die when there is too much moisture in the ground when ground temperatures reach freezing or below. The root cells absorb more water, freeze, and are destroyed. The roots turn to mush.