As potted plants, Gardenias are very fussy and difficult. Once in bud, cool temps in the 55-65 degree range are very important. Repotting, fertilizing and relocating a Gardenia while it is in bud will cause the buds to blast or fall off prematurely. Of course, so will improper watering and too much or not enough light. The most common reason for bud blast is temps that are too high.
When kept as foliage plants, they are not nearly as fussy. They just need moderately bright indoor light and a thorough watering about once per week. Bonsais do need regular pruning to maintain their carefully cultivated shape.