It was big in the Victoria era, but is much older than that. It gets a mention in Shakespeare's Hamlet:
Ophelia: There's rosemary, that's for remembrance. Pray you, love, remember. And there is pansies, that's for thoughts.... There's fennel for you, and columbines. There's rue for you, and here's some for me. We may call it herb of grace o' Sundays. O, you must wear your rue with a difference! There's a daisy. I would give you some violets, but they wither'd all when my father died....
It's essential a means of coding messages. Each flower has its own special meaning, from "unrequited love" to "suspicion", and by combining them in a bouquet quite complicated messages can be spelled out.
Let's have a look at the herbs in poor damp Ophelia's bouquet:
Rosemary's significance is listed as "remembrance" in all the major flower correspondence texts. That's why you might sport some on your lapel come Remembrance Day.
Pansies are thoughts, sometimes specifically loving thoughts.
Fennel is a complicated one. Today's texts would have you believe "praiseworthy" or "strength", but in Shakespeare's day it was much less flattering. Because it wilts quickly when picked, it came to symbolise flattery and philandery, and a male's passing plays for a female's attention.
Columbines are apparently unfashionable these days, because a lot of modern texts don't even mention them. Katarina Eriksson's analysis of Ophelia's flowers assigns to them foolishness and adultery. It's not all sweetness and sunshine, this floriography business.
Rue can mean regret (as in, "you'll rue the day...") It's also very bitter, and carries that bitterness from its taste to its symbolism. Back in the day, it was also used to induce abortion, which in time tied its symbolism to adultery, suffering and fallen women.
By contrast, pretty, simple little daisies are about innocence, and also imply purity and simplicity.
image by Louise Docker, again
Violets signify faithfulness. More specifically the blue violet denotes fidelity, purple daydreaming and white modesty. One text has white meaning "let's take a chance on happiness".