Orange trees can tolerate short periods of temperatures that dip into the 20s (Fahrenheit), but when the mercury drops into the teens and stays there for the entire night, they can suffer significant damage from the cold. If you live in an area that gets frequent freezing temperatures, it's wise to grow your orange tree in a large container, which you can move indoors before your first fall frost. But even in Florida and Southern California, where orange trees are grown commercially, late spring frosts can sometimes occur and cause extensive damage, but you can easily help them along.
Protecting Your Orange Tree from Frost
Set up a lawn sprinkler or two next to your orange tree and let it run all night. Moving water cannot freeze---this is a method commercial growers use to save entire orchards.
String Christmas tree lights over and around your orange tree. The small amount of warmth they generate will warm the air around them just enough to keep your tree from sustaining frost damage. Do not use this method if you are using sprinklers on your tree.
Build a frost frame from 2-by-2 boards and clear plastic and place it over your tree if it isn't too large. Be sure the plastic doesn't touch any leaves, branches or fruit. Take it off after your final spring frost date passes.
Spray your tree with an anti-transpirant product such as "Cloud Cover" or "Moisturin." These are chemical formulas that prevent plants from becoming dehydrated, which is what frost causes.
Plant your orange tree in a large pot with a drainage hole. Use a slightly acidic potting soil and keep your tree in an area that receives full sun most of each day. In fall, move your potted tree under cover, indoors, or into a greenhouse.