The connotations of flowers with love and beauty have been established since storytelling began. From poems to plays to literature, by 1931 people could look at a bouquet of flowers and immediately associate them with romance without a single word being said. Many people give flowers when words are not enough, so why should films be any different?
J.K. Rowling imbued her magical world with inspirations from the celestial (the entire Black family) to the religious (St. Hedwig is often associated with orphaned or abandoned children) and, of course, the biological (plants and flowers).
What is a witch or wizard without their wand? Wands are vital to defining witches and wizards so it's no surprise that for the Harry Potter series, J.K. Rowling didn’t just assign wand combinations arbitrarily. Like everything else she did, she planned and imbued each of her choices with symbolism that enriched both the characters and the story.