The extended metaphor is comparing anger to a plant (the poison tree of the title). The process of cultivating one's emotions (as seen in the line " And I watered it in fears") is compared to cultivating a plant. The emotion is followed through an entire growth cycle, until it blossoms into death.  There is a secondary metaphor implied by the apple; Blake is tied the anger of the poem with the pride of the biblical story of Adam and Eve. This becomes a kind of original sin.