A boutonnière is a floral decoration worn by men, typically a single flower or bud. The word comes from the French boutonnière, or buttonhole, which is the British term. The flower itself is often a carnation, which is most formal white, while red remains a classic alternative. Other colours may also be chosen to better coordinate with whatever else is being worn, such as a blue cornflower.
Traditionally, a boutonnière was worn pushed through the lapel buttonhole (on the left, the same side as a pocket handkerchief) and the stem is held in place with a stem loop at the back of the lapel. However, on many recently made coats and jackets, the lapel is made without the stem loop required, which would normally sit on the reverse of the lapel, beneath the buttonhole. Sometimes, the lapel buttonhole is in the 'keyhole' shape, as opposed to the traditional straight cut, or is not even pierced through, in which case the boutonnière may be pinned onto the jacket lapel, although this may be considered unsightly.
While worn frequently in the past, boutonnières are now usually reserved for special occasions for which formal wear is standard, such as at proms, homecomings, funerals, and weddings. (Women who wear jackets on these occasions also often may wear 'buttonholes', but more typically a woman would wear a corsage.)
Source : http://en.wikipedia.org/wiki/Boutonni%C3%A8re
See Also : Flowers Melbourne, Flowers Sydney, Floral