In the Victorian "language of flowers," sunflowers represented "haughtiness" or "adoration," and these terms certainly fit the unabashedly snobbish Wilde and his besotted sunflower-brandishing followers.
Wilde may have had less conventional reasons for choosing sunflowers, though. He frequented London's Grosvenor Gallery, where paintings by leading Aesthetic artists had found a home after having been rejected by the Royal Academy for being insufficiently moralistic. The Grosvenor's green-and-gold interiors were customized to showcase the paintings' color palette. Gallery regulars followed suit—literally—by dressing in shades of green and yellow, with color-coordinated flowers. Sunflowers were the perfect complement.
The unconventional Aesthetes were lampooned in Punch magazine and satirized in the 1881 Gilbert and Sullivan operetta Patience, which included the following lines:
A pallid and thin young man,
A haggard and lank young man,
A greenery-yallery, Grosvenor Gallery,
Foot-in-the-grave young man!
The success of Patience in England inspired Wilde's overseas tour and launched him as a celebrity, but its stereotypical characters caricatured the sexuality of Aesthetic leaders. So some of the fans who wore sunflowers to Wilde's lectures surely wore them as the 19th-century equivalent of the gay-pride rainbow flag. The modern connotation of the word gay may stem from the fact that it was an acronym for Green And Yellow. Greenery-yallery.
Sometimes a sunflower is more than just a sunflower.
Check out Nancy Rutman's collection of Sunflower Ephemera.