I am sorry for this delayed reply to your gardening question. We are transitioning to a new Q&A system and your question was unfortunately lost in the transition. I have discovered it now and hope this reply is not too late to be of help.
Yes they can although the trimming may adversely affect blooming by cutting away bloom buds. Try to do any pruning after a bloom cycle and not in late summer or fall. Gardenias may not form a very formal, boxed hedge but they can be kept fairly well in a hedge form.
Thanks for the question. Please stop in again soon!
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