Many citrus lovers prize navel oranges for their firm texture and sweet flavor, but the fruit trees that produce the orange are susceptible to cold weather. Of the many things that can kill navel oranges, frost damage is something that can do it very quickly. Even if the rootstock survives, it may never produce the quality of fruit it did before the frost.
Superficial frost damage is not much of a problem, aside from the aesthetics of dead leaves. In some cases the frost damage may have the potential to affect all the limbs and even the main trunk of the tree, but this damage will not appear right away. The first signs will be a wilting and dying of the leaves. The leaves may turn brown, or even a dark black. If the leaves do not return, this could indicate that the branch is dead.
A typical frost will not usually damage a navel orange tree, as most varieties of navel oranges are cold tolerant down to 24 degrees. However, if the cold snap happens suddenly, the tree is in danger of succumbing to cold temperatures, possibly before reaching the critical 24-degree point. In such cases, always monitor for frost damage and take preventive action beforehand.
If you have determined that frost damage has killed an entire branch, either by the fact that no leaves have grown back or by the fact that the wood has started to decay, it is best to remove the branch. If you do not, disease could invade the rest of the tree through the damaged wood. Though most pruning is left to the cooler times of the year, pruning frost damage is often done in the summer, once the grower can be sure the tree branch is not alive. Prune, and seal the wound with white water-based latex paint or whitewash, which will prevent sunburn and discourage the entry of disease organisms.
One of the best ways to keep navel orange trees from getting frost damage is to plant them on a south-facing slope where they can get plenty of sun. Another technique is to plant them close to a building, such as a home. Make sure you leave enough room for the plant to comfortably grow, but keeping it close to the building will allow for some radiant heat to reach the tree and protect its cold-susceptible tissues.
Preventing frost damage begins with keeping a close watch on cold snaps. Younger navel orange trees are more susceptible to cold than more established trees. You may try building a mount of dirt around the trunk as insulation on younger trees. If the tree is older, saturating the ground ahead of a cold snap with water, or using large containers of water around the tree, should provide some insulation and radiant heat, as the water will cool more slowly than the air. Using large industrial fans can also help ward off frost.