Flower imagery is commonly associated with motifs of growth and death, as a flower is destined to both bloom and then to wither. In a story, flowers can symbolize the growth/death of a person, a relationship, or some other larger idea.
In this book, the best example of this type of imagery is in the name of Gatsby's love interest, Daisy. As a flower herself, she blossoms under the gaze of Gatsby. The power of the love he shines upon her is parallel to the power of a sun upon a flower. She literally "blossomed for him like a flower" when he kisses her. However, the bloom will fade, and it does so under the dark gaze of Tom. It is then that Gatsby's "Daisy" withers, and he loses her.
This theme of blossoming and withering does not just apply to Daisy, however. Gatsby blossoms in the hope of achieving his flower, and withers when that hope is removed. Nick blossoms in his new life, then withers when he realizes the insincerity of that life. Gatsby describes a woman at his party as a "human orchid", and later must eventually face the truth that a rose (perhaps Daisy, perhaps life) is a "grotesque thing." These repeated images underscore the motif of blossoming or coming of age that drives both Gatsby's narrative and Nick's narrative. The two narratives overlap, and are held together by a golden center (Daisy), but they are separate stories - or, in the flower language, separate petals.