I am sorry for this delayed reply to your gardening question. We are transitioning to a new Q&A system and your question was unfortunately lost in the transition. I have discovered it now and hope this reply is not too late to be of help.
Your Gardenia may just be suffering some transplant shock from which it should recover pretty soon with good care.
Nitrogen deficiency can cause yellowing in older leaves. The solution to this is simply to start feeding them with a complete fertilizer product.
I suspect your problem may also be not enough or too much water. Both cause problems for the roots. Temporary droughts, especially in a sandy soil, can cause the symptoms you describe. Over watering creates a waterlogged root system and similar symptoms. The best solution is to feel the soil periodically and water only when it starts to dry out a bit. If the area is low and prone to soggy conditions you may need to build a raised planting bed and reset the plants.
Thanks for the question. Please stop in again soon!
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