My gardenias seem to be happy all year around, but when they bloom, they turn half yellow. Is there a way to keep them green through the bloom phase?
Looks like it is quite a common problem, with nutritional deficiencies being the leading cause. one of the better descriptions can be found here :
Note that there are different kinds of yellow leaf deficiencies. You might be deficient of manganese, magnesium, or iron. An acidic fertilizer is recommended.
Other causes listed include poor drainage and temperature. I don't think drainage would be responsible for the symptoms only appearing when in bloom - that sounds more like borderline nutritional deficiencies (blooming can take quite a few resources in many plants).