"To Celia" is both an exercise in how to borrow from obscure writers (ancient dude named Philostratus, anyone?) to make a good poem, as well as a reflection on the reception and continued transmission of art over time. The flowery wreath that the speaker gives Celia isn't just a symbol of art in general, but of poetry in particular. Both the wreath and poetry require a certain amount of delicacy and pruning to create, and both depend on an audience or viewer to keep them alive.
"To Celia" is as much about love as it is about art and how art is experienced by an audience.
"To Celia" suggests that art can only survive or continue to "grow" if it is inspired by someone; Celia's breathing on the wreath, which causes it to stay alive, is the critical moment for examining this point.