What is the extended metaphor of the poem "A Poison Tree"?

The extended metaphor in the poem is the metaphor of the poison tree being a tree. The poem explores the malignant nature of hate as a living object which can grow and flourish as seen in line nine, “And it grew both day and night.” Blake subverts the natural image of a tree and growth, using this to emphasize how hatred is also a living and growing force. The description of hate as one which “grew both day and night,” and, “bore an apple bright,” illustrates Blake’s idea that like the slow growth of a tree, anger and wrath, when not controlled, can transform into something as mighty and deadly as a poison tree.