Think about every place a flower might show up. Go on, do it.
Done? Good. All of those things you just listed make their mark in Sentimental Education. There are flower markets, flower baskets, fake flowers, real flowers, and bountiful bouquets, all of which seem to point to the novel's grand ideas of romance and love, wooing and seducing.
Marie in particular is a flower-associated woman. Some examples?
Frederick imagines Marie as a sleeping flower.
He watches her throw flowers out a carriage window (a turning point in Frederick's opinion of her, actually).
He visits her in her garden in the countryside.
Roses in particular have a place in the novel. We even have Rosanette as one of our main ladies. Ever heard of the phrase rose-colored lenses? It means that you see life in a kind of romantic and idealized way, which—you guessed it—describes our dear Frederick.