A Poison Tree is a short and deceptively simple poem about repressing anger and the consequences of doing so. The speaker tells of how they fail to communicate their wrath to their foe and how this continues to grow until it develops into poisonous hatred.
The speaker describes how when they were angry with a friend, they talked to their friend about the issue which helped them to overcome their anger. However, the speaker was unable to do the same with an enemy and this leads to developing resentment and an even stronger degree of hatred. An extended metaphor of a tree growing in the speaker's garden demonstrates how the anger continues to grow. In the lines 'And I water'd it in fears' and 'And I sunned it with smiles' the speaker actively cultivates the tree/anger.
Eventually the anger blossoms into a poisoned fruit, the enemy eats the fruit and dies and the speaker seems to be glad of this. However, there is also a sense that they see the destructiveness of what has occurred. As the first lines acknowledge, we can easily overcome our anger if we communicate it properly.