alodal,
Gardenias like morning sun, afternoon shade.
THey like composted cow manure in the soil, and depending on what your soil is like, add alot of it, and mix it up well with your exsisting soil.
They need mulch, VERY important, if you don't use it, don't plant them, that is how important mulch is.
At least 2-3 inches of mulch, not to the stems of the bush, underneath of them.
When a gardenia needs water, it will ask. Use your fingers to check for moisture, and also look at the leaves. Are the leaves all folded, like its saying "I need water".
Never fertilzie a plant right after you plant it, or when it is blooming. It doesn't like to eat when it is stressed out.
Gardenias develope next years flower buds in this summer, so always trim or prune a gardenia right after it blooms, that way you won't miss next years flowers.
That's it.
And enjoy, yellow leaves are fine. DOn't stress over them.
THey really are more hardy than you think.