In milder regions, gardenias can be grown in containers in a sheltered space out of direct sunlight. This means they can easily be moved somewhere warm, such as a heated conservatory, when temperatures drop. For extra peace of mind, look for hardy varieties such as 'Kleim's Hardy'.
In reliably warmer areas, such as the southern states of the US, they can be planted straight into the ground in soil that has good drainage and a low pH.