Giving flowers in appreciation or congratulations is one of the greatest and best-known traditions of the theatre. This common place gesture, like much about the theatre, has specific rules so as not to jinx a performance.
Flowers should be presented to an actor only after a performance and, where possible, in front of the audience. Historically, flowers were thrown to the stage when the actors took their final bows. It is considered by theatre folk to be bad luck to give flowers prior to the performance as it was, and still is, thought that the actors would not perform at their best if they receive their reward prior to the performance.
Bouquets of roses are the traditional choice, however irises (admiration), carnations (respect & pride) and lilies (hopes and dreams) are happily received.
On closing night, it is considered good luck for actors to present their director with flowers, after the performance of course. Here is where it gets a little weird - the bouquet must be one stolen from a graveyard, a macabre symbolic gesture to denote the end of a production. Theatre folk love their symbolism. It is said that this tradition likely originated with actors, historically poorly paid, that wished to gain favour with the director for the next audition.