The speaker describes a love they never chose and cannot control. It arrives against their will, like flowers blooming "out of spite / in all the wrong places," and keeps growing no matter how often it has been "said" and "unsaid." The garden imagery carries most of the feeling: thorns that hurt the person who tended them, weeds pushing through cracked cobblestones, ivy that no garden can hold down. Love here is less a gift than a wild, slightly painful force.
Blue seems to be the beloved's colour, and it spreads like ink through the speaker's nerves and arteries. The willow image suggests that bending toward this person would mean wilting. The speaker expects to be "the first to fall," and calls the feeling both "irresolute" and "irrefutable."
The mood is yearning and resigned, tender but bruised. The speaker seems to accept the hurt as the price of a love that stays undefined.