When temperatures are expected to fall below freezing, you need to protect your orange, lemon and other citrus trees to prevent frost damage. Watch the local weather forecasts for predictions of possible frosts. Most citrus will be damaged if temperatures drop below 28 to 30 degrees for several hours, but hardiness varies by citrus variety. Observe the weather.
Frosts are most likely on still, clear nights.
Coldest temperatures will occur just before dawn. Construct a wooden box-like frame that encloses the tree and cover it with plastic sheets or burlap. Provide additional protection by running sprinklers at the base of the tree or stringing Christmas lights through the tree to generate heat. Don't use both sprinklers and lights, as you could get shocked. Uncover the tree during the day.