I sent thee late a rosy wreath Not so much honouring thee As giving it a hope that there It could not withered be" (9-12)
The speaker sends Celia a wreath. The wreath is a symbol of the poem itself so it is almost as if the speaker is sending Celia the poem he has written about her. Since he wants to see if it will live forever in her presence the poem reflects on the importance of an audience to keep art alive.
But thou thereon didst only breathe, And sent'st it back to me" (13-14)
Celia "breathes" on the wreath the speaker sends her; it's almost like she is blessing it, or inspiring it (the word "inspire" originally meant "to breathe or blow upon"). Celia definitely is the inspiration for this poem; she is the speaker's Muse, in a sense. The breathing on the wreath dramatizes this fact, or at least suggests the importance of inspiration for the survival (the continued life or "growth" of) art.
Since when it grows, and smells, I swear, Not of itself but thee! (15-16)
The fact that the wreath continues to "grow" – even though it should be, by definition, dead (you can't make a wreath without killing the leaves and flowers you use) – suggests the importance of audience to the survival of art. The wreath is like a work of art, Celia breathes on it, and then it magically continues to live and grow as result of the life she "breathes" into it.