Answer: You didn't mention what part of the country you're gardening in so it makes giving advice a little more difficult. It's normal for newly planted trees and shrubs to show some signs of stress which is what's happening with your newly planted gardenia. If you followed the planting instructions and if you're not in the desert southwest, full sun should be just fine for your gardenia. (If you are gardening in the desert, your plant might do better with some protection from hot afternoon sunshine.) Once the roots become established, your gardenia should perk up and replace the lost leaves. In the meantime, you can help it out by watering deeply once or twice a week. Don't feed it for 2-3 months - it should spend its energy building a root system rather than growing new stems and foliage. Feeding will force new growth on top rather than underground.
In a few weeks you can prune your plant back by about one-third. This will encourage new growth, including healthy new leaves and flower buds.
Best wishes with your new gardenia!
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