People always like to mention flowers when they talk about love ("my love is like a red, red rose," anyone?), and this poem is no exception. Here, the speaker says he doesn’t love his paramour the way he loves beautiful flowers. Instead, he actually compares his lover to a plant that doesn’t flower (in Spanish, florecer), but keeps its beauty "hidden" away. It seems like he's saying that he loves her for her inner beauty.
Lines 1-2: The speaker says he doesn’t love this woman as if she were the salt-rose (a particularly resilient species of rose) or a carnation, two beautiful flowers. "Arrow" is a metaphor used to describe the way in which the beauty of the flower shoots out from within.
Lines 5-6: Instead, the speaker loves his paramour as if she were a plant that doesn't bloom, but rather keeps its beauty hidden away. The word "as" in line 3 tells us that this is a simile, a comparison that uses the word "like" or "as."
Lines 7-8: Something strange is growing inside the speaker. It is an "aroma" that originally came from the soil (from a non-blooming plant, perhaps?). This aroma is a metaphor for the powerful emotion the speaker feels as a result of his lover, that strengthens quickly as it grows.