William Blake's poem "A Poison Tree" is a figurative expression of the relation of the spiritual world with the natural world.
When the speaker of this poem becomes angry with his friend, he communicates his feelings to this friend. As a consequence, his anger is released and the friendship between the two men continues.
I was angry with my friend:
I told my wrath, my wrath did end.
In contrast to this amelioration between the friends, the speaker's failure to release his feelings with his enemy causes his "wrath" to increase and intensify, so much so that the poison tree grows as he waters it with his fears and he suns it with "deceitful wiles."
William Blake expresses in "A Poison Tree" the power of the imagination and the spirit. To Blake, the imagination possesses the capability of perceiving the realities of the spiritual world in its expression. On the other hand, the tree becomes symbolic of the corruption that occurs in the soul when a person suppresses feelings. For when the speaker does not release his emotions toward his foe as he does with his friend, and, instead, nourishes his antipathy, the tree grows and eventually produces an apple. This is the apple of cunning and guile--not unlike the apple in the Garden of Evil-- that has sprung from the energy of hate which feeds the tree.