I am sorry for this delayed reply to your gardening question. The spring rush has brought a deluge of questions and we are working hard to catch up!
Gardenias like moist soil but not soggy soil. They need some sun but full day may be too much. I suspect the problem plant had a soil moisture problem and this may have let a root rot get to it. There is little to do now but keep the soil moist but not soggy. Hopefully the damage will be minimal and it will recover.
The following web site should also be helpful:
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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