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Yellowing leaves: Root rot can lead to yellowing leaves. Try to pull the plant up. If it come s out of the ground easily and the roots are mushy, root rot is the culprit and the plant should be discarded. Iron chlorosis (iron deficiency) causes yellow leaves; adjust the soil pH and, possibly, apply a foliar feed of iron.

You need to be sure your plant isn't receiving direct sunlight through the glass. Gardenias would be happier outside in semi shade with you bringing them into the house in the winter if you have issues with frost. Fertilizing with an acidifying fertilizer like Miracle Gro Miracid is a good option. This fertilizer will also work for azaleas, camellias, ferns and other acid lovers. To encourage blooms once the plant is healthier use something like water soluble Super Bloom alternating with the Miracid. Be careful not to over feed.