Citrus fruit trees provide a great deal of personal enjoyment and are an important commercial crop in locations such as California and Florida. These trees prefer warm weather and full sunshine, but there may be times when the temperatures dip too far below freezing that the tree becomes in peril. While frost is generally not a concern for more mature trees, the stress it puts on younger trees is significant. At the same time, frosty conditions, coupled with extremely cold temperatures, can affect nearly any citrus tree.
Watch the weather forecast with each cold front that comes through the area. In most cases, the weather in zones where citrus normally grows will not reach a critical point but it may happen once or twice per season. Frost may be a concern for the younger trees, but temperatures at 24 degrees or below for an hour or more could be big problems for all trees.
Use large industrial fans to keep air circulating and protect from frost if the event is expected to take place when orange blossoms are blooming. Blossoms are very susceptible to the cold weather and could easily be killed.
Cover young citrus trees with sheets blankets if frost is predicted and if the air temperature is expected to stay at or higher than freezing. The sheets should provide adequate protection against frost and more drastic measures are not needed.
Use a portable incandescent light along with the blankets or sheets if temperatures get below freezing for hours during the night. Make a tent using the blankets or sheets and put a light inside, running extension cords from a power source to the light. The heat from the light will create a micro-climate sufficient for the tree's survival in most cases.
Use a sprinkler system on larger trees if the temperature drops into the mid 20s for any length of time. While this may seem counter intuitive, it may be the only way to save the tree because the ice that forms on the foliage and fruit will actually act as an insulator and protect the tree from getting as cold as the outside air temperature.