Here on the 8b / 9a line in Louisiana we had one of the coldest winters in my lifetime, at least when talking about absolute low temperatures, where we hit 14F two night in a row (Our all time record low here is 13F set back in the 1940's). I covered all my 8 citrus trees ranging from second year in ground 4 ft to 15 ft tall x 20 ft wide 25 year old Satsumas, and provided heat with incandescent C7 Christmas lights or heat lamps. Even with this all suffered some damage to limb tips, and one of my smaller Satsumas was killed back to about 6 inches above the graft line, time will tell how well it will recover, but it is growing fast, and now has a new 6-8 inch tall shoot with 30-40 leaves, which is much better than the 3 leaves it was down to in early February.