In Midsommar, flowers represent family, with one clue to this symbolism being the floral wallpaper seen in the bedroom of Dani's parents just before they die. The metaphor can even be extended to plantlife in general — the first time we see Dani after her family's deaths, it's months later, and the houseplants in her apartment are all dead. From a plot perspective, she's been too depressed to water them, but the imagery works as symbolism too.
In the early minutes of the movie, Dani's world is dark and frigid, but things begin turning around for her during the trip, where the sun rarely stops shining and flowers grow in abundance. Before Dani even consciously realizes it, she's being introduced to a new family. The first person to wish Dani a happy birthday — notably, the first birthday she's celebrating without her immediate family in her life — is Pelle, the kind-eyed member of the Hårga.
Dani's suffusion into the cult begins occurring on the movie's final day, when she joins in the drug-fueled dance around the maypole. For the competition, she is gifted a flowered wreath. After she becomes the May Queen, she is literally up to her neck in flowers, covered in them, nearly smothered by the Hårgas' adoration — her brothers and sisters, if she'll have them.
By the end of the movie, it seems like she will. Her old family was violently ended — her new family has a violent start.
