Way back when courtships were carried out under the watchful eyes of others and lovers were not allowed to be alone, flowers and poems were often the only thing you could give to the object of your affection. It is no wonder then, that flowers became secret messengers of someone's true feelings. Floriography, sending cryptic messages through flowers became very popular in the Victorian era.
Whether someone wished to convey feelings of love or affection, or wanted to let someone down gently or even send them an insult, they could do so with a simple bouquet of flowers.
No matter what you wanted to say, there was a flower (or plant) for everything. Combine flowers together and you could say so much without uttering a word to the object of your affection and this form of communication became particularly useful for conveying forbidden messages of love.
However, this language was not new to the Victorians but had been around for many years, with flowers, herbs and plants being significant for the Egyptians, Ancient Greeks and the Chinese (as well as many others). They became particularly useful in the arts with Shakespeare and Charlotte Bronte using flowers in their work to convey messages to their audiences and communicate much more than mere dialogues could in the stories.