The poem is about how anger and revenge can grow over time like a poisonous tree. The speaker describes watering and nurturing his anger toward a foe by keeping it inside instead of expressing it. This causes the anger to grow larger, represented by a bright apple on a tree. When the foe sees the apple, he recognizes it comes from the speaker's anger. The foe then eats the poisonous apple and is found dead under the tree, murdered by the speaker's vengeance that grew unchecked over time.