Gardenias are like Rhododendrons, they need acidic soil, moist, but well drained. Plant them "high" like Rhodies and Azaleas, with the rootball about 2 inches above soil level and add mulch so that water drains away from the trunk of the shrub. They also need high humidity.
When you fertilize them, use Miracid water soluble type, which contains Sulphur.
Use rainwater for watering them, if possible. If your tap water is alkaline, it can be neutralized by adding a few drops of vinegar to a gallon of water, before applying to the Gardenia.
Gardenias need some direct sunlight or very bright light to bloom well, but protect them from sun during the hottest part of the day. When the Clemsen Botanical Garden were testing the 'Kleim's Hardy' cultivars, they planted them in full sun exposure. They produced many flowers, but the foliage really suffered!
Cool day and night temperatures are required for best flowering. High temperatures will cause "bud drop" and few, if any, blooms. In their sub-tropical habitat in Asia, they are winter blooming plants.
This is the time of the year that they shed some of the old leaves, like the Southern Magnolias, so some yellow foliage is normal.
Keep the soil moist, they aren't drought tolerant, protect from direct sunlight during the hottest part of the day, provide acidic soil or growing medium, and plant high for good drainage. It has worked for me and keeps my old Gardenias(20+ years old) happy. The roots survived during the record low temperatures in the 1980's, of -12°F, during two successive nights. I sheared them, along with every shrub in the garden, to ground level the days afterward and almost everything survived.
Good gardening!
Rb

Most people kill their gardenias with kindness. They do not really need any special watering in my experience. Many people see the yellow leaves that are typical of spring as the old leaves are replaced by new, they then overdue it trying to "fix" the plant when there is really nothing wrong. They will not look perfectly green in the late winter or spring months.