This is what Brite Leaf Citrus Nursery says:
"If you live in USDA Hardiness Zone 8, you may be able to, depending on your location and the variety you choose, plant your citrus tree outside. However, you will need to protect your tree from frost and freezing during extreme cold periods.
If you live in USDA Zone 7 or North, for survival of your tree, we strongly recommend you plant in a movable container which will allow you to move your citrus tree indoors during the winter."