First step - get your soil tested - both for pH and for nutrients. Your county's Cooperative Extension office can provide you with the forms and a box. Send the sample with $7 to Auburn University, telling them you want to grow gardenias, and they can tell you what, if anything, can be done to improve the soil. Gardenias like an acid soil (5.5 to 6.0 should be OK) and moderate to light shade. I myself just had to move one that was getting to much sun.