Flying Dragon (Poncirus Trifoliata)has been used for ages by the Chinese and Japanese as a root stock for citrus trees. Of all the stocks for citrus trees, trifoliate is the HARDIEST and it imparts to the variety that is worked upon it *SOME* of it hardiness and its resistance to BOTH HEAT & COLD. Its resistance to heat is quite as important as its resistance to cold, for if the tree responds to heat by starting to grow and flower, the cion becomes very tender. The measure of a variety's hardiness is frequently its resistance to high temperatures by remaining dormant. Besides imparting to the cion *some* of its ability to withstand cold, it has also a tendency to retard growth in the spring. As a general rule, the resistance to cold CANNOT be stated in so many degrees of temperature, because it is variable, and in no two cold periods or in no two locations would it be the same. Much depends on the year and how "dormant" the tree has become before a cold spell arrives. As you are asking about a key lime tree worked onto a FD rootstock, my recomendation is that you should treat it exactly as any other key lime. The FD rootsctock will impart *SOME* hardiness but no where near enough to leave it in the cold. An exact temperature of hardiness cannot be given. As I said ,it all depends on the year, the location, and the thoroughness of dormancy. - Millet

May I add on some, Millet? The big problem with lemons and limes, as far as cold-hardiness goes, is that they lack the ability to go into the complete dormancy that other citrus go into. They may stop growing for a bit, and yes, FD may assist in that, but it is their nature to grow repeatedly and frequently. Once a growth flush starts, all cold-hardiness is lost. And a happy Key Lime tree will almost always attempt a growth flush in mid-winter. It just can't wait.

One very important thing that was not mentioned is the pattern of cooling towards the winter. If the cooling is very gradual, it will enable even the cold sensitive citruses to tolerate much lower temperature, much lower than their published guidelines for cold tolerances. A cold tolerant variety could also become susceptible to a very sudden cold draft from the arctic, but of course, the cold sensitive one would be totally wiped out.

Thank you guys for some VERY informative posts! That is exactly why I asked.