How acidic is your soil? Gardenias like a very rich and quite acidic soil. The discoloration of the foliage looks like iron chlorosis, which is a common indicator of too high a soil pH.

I agree, high pH inhibits the uptake of iron. You can use chelated iron but better to lower the pH.

Thanks! What's the best way to lower the pH? I have a feeling my local garden center will be clueless....

You could use chelated iron to help the plant while working on lowering pH. Things like leaf mulch and other organic matter are acidic and help. Sulphur (as an element) is also good but is slow working. Faster acting are things like iron or aluminium sulphate. All these are fairly readily available Make sure you read the instructions.

Does it have a grafted rootstock? Gardenias in Florida are often killed by nematodes in sandy soil. If it isn't grafted and it is planted in normal, sandy soil it will probably be killed off by nematodes eventually. It may well be what was stated above--a problem with the ph level. On the other hand, if you have been fertilizing somewhat with gardenia-specific fertilizer it may be something else, like root knot nematodes.

Thanks, Tom. It is grafted, I made sure of that when I bought it. I rooted some cuttings from a favorite Gardenia before I moved here and they were all done in by nematodes.