gardenia

Question

A once healthy gardenia is now dropping all its leaves.

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Answer 1

This is most likely a plant nutrient deficiency. The yellowing leaf symptoms look a lot like a magnesium deficiency. These articles will help you:
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/fixing-magnesium-deficiency.htm
https://www.gardeningknowhow.com/garden-how-to/soil-fertilizers/epsom-salt-gardening.htm