The use of the rose as a symbol for silence or secrecy goes back to the ancient Egyptians or Greeks, where initiates into the “mysteries,” or pre-Christian religions, wore roses during ceremonies to indicate that they would keep inviolate any secrets that were passed on to them. In many countries it was customary to hang a rose over secret meetings or gatherings—hence the term sub-rosa (literally “under the rose”) to mean confidential.

Source: Harlow Gardens