I asked the rarefruit yahoo group what control methods they used on their trees to protect against frost and Roger Meyer from California and Erica from Florida said to cover the tree with old sheets right to the ground. You can pin the sheets together with clothes pins. You must use fabric. Shadecloth does not work. Then you need a heat source under the tree and various methods were given by other members eg. a lighted candle, outdoor Christmas lights, or you can take one of those outdoor extension cords, which also has a hanging hook and light socket at the end and hang that under the sheets on a tree branch. This will keep the temperature a few degrees warmer than the outside. Another of our members in Sydney, Jason Rukowski, has also been affected by frost so he has also purchased Marix fabric which will be used next year for cold protection. Several of our members have also used the product Envy without success. I looked up their website and it says: Envy is a frost protectorent and anti transpirant which can substantially reduce moisture losses by up to 50%, eliminate sun & windburn damage as well as increase frost tolerance by an additional 4 Degrees.
From a rarefruiter: “It was incredible to see the results of our frost protection and the damage that an extra degree can do to plants. Our sprinklers come on when the air temperature reaches 0 degrees, this was at 11.30 pm last night so by sunrise there was a thick coating of ice over everything. The ice prevents the cells inside the plant from freezing and therefore prevents damage by the frost. The sprinklers stay on all night so by the morning there is a thick layer of ice covering the plants. As it melts the plants do not suffer from internal damage and as you can see these young seedlings do not look as though they have been through a -4 degrees frost. What happens is when you sprinkle water on the plant ice forms but when new water goes onto the plants the original ice that has formed around the plants releases heat of up to 10 degrees which protects the plant. The application of water has two beneficial effects regarding frost protection. The drier the air becomes then more heat is lost into the atmosphere, so humidifying the air reduces the amount of heat radiated away and the severity of the frost. The constant application of water to plants maintains the surface temperature around zero-ish degrees which is generally more tolerable and less damaging than if it continued to drop to several degrees below zero.”