The rose image in this sonnet symbolizes immortal truth and devotion, two virtues that the poet associates with the young man. Likening himself to a distiller, the poet, who argues that his verse distills the youth's beauty, or "truth," sees poetry as a procreative activity: Poetry alone creates an imperishable image of the youth.
Stylistically, the sonnet's form follows the now-familiar model of most of the sonnets, with lines 1 through 8 establishing an argument or situation, and lines 9 through 12, beginning with "But," contrasting that original argument or situation. The first four lines describe how a rose is outwardly beautiful, but its beauty extends to the "sweet odor which doth in it live." Likewise, lines 5 through 8 describe canker blooms as also being externally beautiful. The dissimilarity between these two flowers, however, is evident in lines 9 through 12, in which the poet notes that canker blooms contain no inner beauty. Unlike roses, which "Of their sweet deaths are sweetest odors made," canker blooms leave no such lasting impression when they die: "They live unwooed and unrespected fade, / Die to themselves."
The concluding couplet makes clear the poet's purpose for this extended botany lesson. The young man is like the rose, outwardly beautiful and inwardly sweet-smelling, two qualities that the poet characterizes as the youth's "truth"; the poet's sonnets are similar to the perfume made from dead roses, for after the youth's beauty fades, the poet's verse "distills" — immortalizes — that former beauty for others to enjoy.
canker blooms dog-roses.