Why do you think Fitzgerald uses flower imagery in the novel?"The Great Gatsby" by F. Scott Fitzgerald

In his novel, "The Great Gatsby," F. Scott Fitzgerald employs colors to convey impressionistic images.  Yellow, traditionally the color of evil, is used strategically by Fitzgerald with the most salient example as the yellow car of Gatsby's, the death car, which contrasts with the white car Daisy has driven when she first meets Jay Gatsby.  Interestingly, her name is a flower, a flower which has both these colors contained in it, but at its center is yellow, the color of corruption and sin.  So, while the reader has the first impression of Daisy through the eyes of the infatuated Gatsby as an innocent debutante, fresh, young, even somewhat naive--the "golden girl" of his dreams--the reader later learns of her self-serving nature as she rejects Gatsby, even betraying him as the driver of his yellow "death-car."  Like flowers that are ephemeral, the love of Daisy for Gatsby withers and dies.

Other flowers are employed in Fitzgerald's novel, as well. For instance, at one of Jay Gatsby's parties attended by dissipated and sordid and dissolute people, one actress stands in sharp contrast to the other guests as she is described in her refinement and rarity, like the flower which symbolizes her:

Gatsby indicated a gorgeous, scarcely human orchid of a woman who sat in state under a white plum tree.

After Gatsby has died, Fitzgerald uses the rose, a symbol of love and passion, when Nick ponders why Gatsby died:

He must have looked up at an unfamiliar sky through frightening leaves and shivered when he found what a grotesque thing a rose is and how the raw sunlight was upon the scarcely created grass.

In this passage, the passion that Gatsby thought was love--earlier when he kissed her "she blossomed for her like a flower"--had withered and he saw Daisy for her cupidity and selfishness, the "raw sunlight" (yellow/corruption,evil ) on "the scarcely created grass" (new money of Gatsby).

Like Gatsby's American Dream, the flowers symbolize a transitory beauty and the mere illusion of what does not really exist.  A tragic figure, Jay Gatsby realizes the illusion before he dies.