The "rosy wreath" is a bouquet of roses that the speaker has sent to the lady, Celia. The eNotes study guide has a very good interpretation of these flowers:
Jonson uses the rosy wreath, however, in an unconventional way. The speaker admits that his primary motive for sending it was not to honor her beauty, as any lover would with red roses, but for another purpose, which reflects her more intense charms. He does not discount her beauty, noting that he is sending the wreath "not so much" for her honor, but insists that he has a greater purpose. When he claims that the wreath would not wither in his lady's presence, he suggests her power over it.
Briefly, he sends her the roses not to make her happy or to flatter her or even to compare their beauty to hers. Rather, he sends her these beautiful flowers in the hope that in her presence they will not wilt or fade.
But might I of Jove's nectar sup, I would not change for thine.
Jove's nectar makes one immortal. But Jonson's speaker would rather have Celia's nectar to ensure immortality.
I sent thee, late, a rosy wreath, Not so much honouring thee, As giving it a hope, that there It could not withered be.
Celia's beauty is so amazing, Jonson's speaker argues, that it can give immortality. So he sends her a wreath of roses, not so much to honour her, but because it (and he) hoped that, by being near Celia it wouldn't die - it wouldn't wither. Because she has this power of immortality, hopefully the roses would live.
But thou thereon did'st only breath, And sent'st it back to me: Since when it grows, and smells, I swear, Not of itself, but thee.
Celia did simply breath on it, and sent it back to the speaker. Since she did that, the wreath is now growing, and smelling - not of itself (i.e. of roses) but of Celia.
Hope it helps!