Answer: Gardenias need soil that is organic and humusy, acidic, and evenly moist yet well drained, meaning never soggy or saturated. Most often yellowing at the base is an indication of overwatering or planting in a soil that is not suitable.
I might mention that gardenias generally would not be considered winter hardy in your area (according to your zip code you are gardening in zone 6B) so if you have just planted it in the ground you might want to transplant it back into a container so it can be brought into a protected area for the winter.
If this is a new plant, you might also want to check back with your retailer for advice. Good luck with your gardenia!
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