One of the worst things that can happen to a fruit-bearing tree is to be burned by the cold during certain times of year. Once the buds form on these types of trees and the flowers are ready to open, the frost can affect the outcome of the growing season. Blooms that get bit by frost will have to start all over, which can delay fruit production and result in small harvests. Fruit that is not fully formed can be killed by the cold frost and prevent the harvest from happening at all.
Cover little fruit trees with sacks or cloth to keep plants warm and frost from settling on the plants.
Set up lights in small or medium trees. The lights emit heat that will stave off some of the cold. You will need to run extension cords out to the trees from a building or have a portable generator from which to draw power. Hang the lights in the trees. You can also set up heaters on the ground.
Coat the trees with water. This will coat the leaves and freeze at cold temperatures. The ice around the tree will actually insulate the foliage and keep it safe.
Set up fans to create constant air movement between the trees. This keeps frost from being able to settle on the foliage.