It is possible to grow citrus trees outside in many parts of the United States however north of a certain line you need to protect orange, grapefruit and lime trees from freezing. One way to do so is to place a light bulb in an enclosure around the citrus tree. A freeze protection thermostat, like those used to turn heat strips on, cana turn the light bulb on when needed and off when the temperature rises. Here is more on how to protect your citrus tree this winter.