“Boutonnieres” are small arrangements of flowers or foliage pinned on a groom’s or groomsmens’ upper left suit lapel. Usually, there will be a button hole on the left lapel yet the purpose of this is to hold a single stem of a flower not a grouping arrangement. This originated from Savile Row as custom suit wearers wore a single flower in their lapel regardless of the occasion.
This has nothing to do with formal wedding attire. The act of wearing a boutonniere is completely separate from wearing a single flower through the buttonhole. In this sense, the boutonniere is NOT pinned through the buttonhole, it is pinned on the outside of it, so that the buttonhole is still visible.
There also has been some debate whether or not a pocket square should be worn with a bouttonniere. I must admit that it will look “busy” and that an obvious effort is being made to accessorize. It’s like when a woman wears earrings, bracelets, necklace – you should only wear 2/3. However, I feel as though if done right through – wearing a plain pocket square then it could work fairly well a la Fred Astaire.