It is difficult to pinpoint a threshold temperature at which it becomes necessary to protect citrus trees from freezing temperatures. There are many variables that can change and alter how trees will respond to these temperatures. Health of tree, temperatures before freezes occur and root stock, can effect how a tree can survive and at what temperatures they can sustain.
It is important that trees are hardened with enough pre-conditioning cold temperatures to halt their growth. Length of time citrus trees are exposed to sub-freezing temperatures is another factor that can make or break its survival status.
With this said, there are generally four factors involved in determining the susceptibility of citrus trees to cold weather.
These ranges in temperature refer to leaf or wood damage. For fruit damage, 30 degrees Fahrenheit or below for 2 or more hours can damage satsuma fruit. On navels and other citrus, 26 degrees Fahrenheit or below for 2 or more hours can cause fruit damage.
Varieties in order of decreasing cold hardiness are - Satsuma, kumquat, sour oranges, tangerines, sweet oranges such as navels and Louisiana sweet, grapefruit, lemons, and limes.
If temperatures are predicted to dip below 26-27 degrees for an extended period of times (more than 2 hours), then you should protect the tree.
Also, remember that citrus trees should have bare ground underneath the canopy. The heat from the ground can radiate up to 3-5 degrees Fahrenheit.
The LSU AgCenter and the LSU College of Agriculture