Symbolism, Imagery, Allegory
The first mention of beach roses in We Were Liars comes during summer fourteen. Cadence and Gat start taking walks together, and on one of those walks she notices that "Beach roses lined the path, deep pink. Their smell was faint and sweet" (5.37). It's one of those sensory elements of falling in love that you remember forever—we have no doubt that the scent will take Cadence right back to these evenings with Gat for the rest of her life.
So when she sees him putting a dried beach rose into an envelope in the Clairmont kitchen at the beginning of summer fifteen, she's sure it's for her. "Gat, my Gat," she says. "He had picked me a rose from our favorite walking place. He had hung it to dry and waited for me to arrive on the island so he could give it to me" (6.7). It has to be a symbol of his love for her, right? Right?
Wrong. It's for someone else. A girl named Raquel, to be precise.
Cadence gets one of those dents in her heart that isn't quite a break, but definitely feels like having been kicked. So when, during summer seventeen, she sees a new tire swing outside Clairmont, she's amazed to find that "Inside the tire is an envelope. Gat's handwriting: For Cady. I open the envelope. More than a dozen dried beach roses spill out" (56.15-18). It's what she's dreamed of… but is it just a dream? After all, Gat's dead. So Cadence is either seeing ghosts or experiencing wishful hallucinations.
Ghostly and/or hallucinatory Gat, when pressed for information about the events of summer fifteen, tells Cadence, "I messed everything up […] I shouldn't have kissed you, or made you a tire swing, or given you roses. I shouldn't have told you how beautiful you are" (77.36). He knows he's communicated with her using the symbol that would most tug at her heart, but he'll never be able to be with her, what with his deadness and all. And for wooing her while knowing this impossibility, he's sorry.
Beach roses die at the end of the summer, just as Cadence and Gat's relationship does (though it always resumes the following year). As Cadence's time on Beechwood during summer seventeen draws to a close, so does her life with the Liars—they walk into the ocean and go off to be dead people forever, because that's how death works. The next year there will be new roses, but the dried ones will always represent Gat.