Here’s something from the book Innocence and Decadence: Flowers in Northern European Art 1880-1914, (which is a very good book by the way, and can be bought here!) from the chapter “Flowering Symbols”:
In the nineteenth century, flowers became used as a highly sophisticated “code” for expressing emotions and ideas. As a consequence, a close relationship between flowers and language originated, and flower poetry in particular took flight, though in novels, too, flowers were presented as the “messenger” of emotions and central thruths, which could range from everyday feelings, love, faith, and morality to decidedly mystical ideas. Charts were even drawn up in which flowers and scents were assicoated with a particular spiritual condition of the soul.
However, the symbolism of a given flower was not unequivocal. Depending on the situation, flowers could have various symbolic meanings and the same bloom in a different coulour could even mean something entirely different. When giving a lady a bouquet, the young nineteenth-century swain had to be sure that she knew and used the same “code” to avoid any misunderstanding. There would have been few problems with forget-me-nots […] yet the poppy could mean both “why are you so tired?” and “your sleepy, phlegmatic temperament prevents any significant emotions from welling up from your heart.