There are special liquid Iron fertilizers for roses. They also acidify the soil.

Your yellow leaves are probably from not enough light, moisture or a fungus. I get them too - on the Knockouts that don't have good air circulation all around the plant. I think it's a light problem. Iron is not the answer.

Landdawg,

Watering problems can cause yellowing on older leaves. Do you have a nice mulch on your plants? High pH in the soil can have low iron associated with it, but that is usually not a problem in the South, where pH is very low.I agree with bumblebeez that air circulation is really important for Gardenias.