The yellowing of the leaves could be a sign that your gardenia need more iron, or that the soil is too wet or that the pot sat in drainage water. It would not hurt to add some Osmocote Plus as it will take care of the lack of iron that could be causing the yellowing. For the tiny pot you showed in the picture you sent I would add just 1/8 of a teaspoon of Osmocote and try to work the granules into the top half inch of soil. Then water well. Remember that gardenias love lots of humidity so mist the plants and set the pot on a saucer of pebbles but do not let the bottom of the pot sit in water. I expect some fragrant blooms from your plant soon. Keep growing, Marianne Binetti