When can you trim up a gardenia bush?
When Can You Trim a Gardenia Bush to Shape It?
Certified GKH Gardening Expert
In order to prune without hurting blooming, you should wait until after all the plant's blooms have faded, which should be in the summer. Here is more information: https://www.gardeningknowhow.com/ornamental/flowers/gardenia/when-to-prune-a-gardenia.htm