Answer: You didn't say if your gardenia is indoors in a pot or outside in the landscape, but considering your location in Lompoc with all the nurseries/growers I'm going to assume it's outdoors! There are many things that can cause leaf drop. One cause is a common plague to gardenia - they are very susceptible to spider mites, which cause tiny webs on the undersides of leaves and where the leaves join the stems. Another way to detect them is to take a sheet of white paper out to the bush, tap a few branches on to the paper, fold the paper in half and press/rub together. If you see little red, blue, or brown smears, those are spider mites. You can spray them off with a strong blast from the hose (do this daily until they're under control). A product called Neem can also be used to treat for spider mites. Follow product instructions for application.
Gardenias need at least a half day of full sun to bloom, but at the same time, the hot afternoon sun can be tough on them. Morning sun is best, so an eastern location would be ideal. But since you contend with the morning marine layer in Lompoc, you might need to try a southern or western exposure with some filtered shade. Buds that turn black or drop and bottom leaves that are yellowed are sure signs that gardenias aren't getting enough light.
Leaf drop can also be caused by improper soil pH (gardenia require 5-5.5pH - on the acid side.) Yellowing and leaf drop are also signs of various soil nutrient deficiencies such as nitrogen, zinc, and iron. Are you having problems with any other plants? You might want to get a soil test. Speak to your local County Extension Agent for details. 624 W. Foster, Santa Barbara, 93455, 805-934-6240.
If these possibilities don't help, please send another question to the Q&A database and describe the plant's entire condition (e.g., do leaves turn yellow before they drop, how are you caring for it, etc.) and let us know whether it's indoors or outside in the landscape. Thanks for writing!
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