What is the metaphor in "A Poison Tree" describing the speaker's wrath?
This poem is an extended metaphor, meaning that the metaphor is maintained at length, in this case, through the entire poem. The metaphor is that the narrator's wrath is a seed that grows into a tree. The narrator is saying that he has chosen to keep his wrath towards a foe to himself, and in doing so, nurtures it until it grows into a tree that bears a poisonous apple, which kills the friend he is angry with.
In the first verse, he hints at the metaphor, saying "My wrath did grow" (line 4). In the second verse, the metaphor is extended further. The narrator "waterd it in fears" (line 5) and with his tears. He provided it with sunshine in his false smiles, and finally the tree bears fruit in the third verse, "an apple bright" (line 10). His foe sees the shining apple, consumes it, and dies. It is the poison of the narrator's wrath that has caused the foe to die.
This metaphor is a particularly good one, since our anger, held inside, does bear poisonous fruit, although I find that the poisonous fruit often tends to harm the one bearing the anger, rather than the one against whom the anger is directed. Be that as it may, wrath held and nurtured is toxic, and the poem is telling us to beware of this in the form of a powerful metaphor.