We only have a couple of nights when it frosts. In winter, I'd say night temps are around 36-40. Even on nights when I know it will frost, I don't bother covering it. Our winters are pretty mild. My lime tree is in a hugh pot and is about 7 yrs. old and still going strong.
NYboy ... There is a commercial 20 acre Meyer's Lemon orchard and several large acrage ( each from 20 to 100 + acres ) commercial Navel Orange orchards about a mile from our ranch in Cal. . The winter tempertures are normally down to 27* every year and sometimes as low as 17*. They have wind turbines and sprinkler systems that come on when the night temperatures are expected to be at the freezing point. Ice and air movement protect the trees. Sometimes they might turn on smudge pots as well as the wind turbines but with the no burn laws this practice is mostly out.
