Flower Crowns are headbands or other hair accessories created by stringing a number of flowers together. On Tumblr, these floral accessories have become an exploitable photoshop meme by adding them to photos of males who would not normally wear them.
Flower crowns have always been around, but in 2013, they became a widespread meme on Tumblr. According to Know Your Meme, this meme started in One Direction fandom in March 2013, was inspired by a 2011 tweet by Harry Styles, "I wish I was a punk rocker with flowers in my hair.", and spread to other fandoms by May.
Flower crowns became especially popular in the nascent Hannibal fandom, so much so that the showrunners noticed. In July 2013 Hugh Dancy wore a flower crown during a San Diego Comic Con panel. In September, NBC ran a contest for fans to submit photos of themselves wearing (real or photoshopped) flower crowns. The first Hannibal flower crown manip to be posted is thought to be hate everything (except for puppies, I love puppies) by hettiennes posted on April 28, 2013. Although the start of the show coinciding with the meme may be the proximate cause of flower crowns taking root in Hannibal, cleolinda speculated that the meme "caught on as the fandom's way of balancing the darkness and tragedy and gore by being silly and light-hearted and imagining Will Graham Having Nice Days".