What is the moral of the poem "A Poison Tree"?

The moral lesson is a lesson on the dangers of holding in angry feelings about a person.  

When the poem begins, the narrator of the poem tells his readers that he was angry with a friend. We do not know the cause of the anger, but we know that the narrator spoke to his friend about his feelings. He got his feelings out in the open, he was no longer angry, and the relationship was kept whole.

Contrast that with the second time that the narrator is angry with someone. This time the narrator does not get his feelings out in the open. Instead he feeds those angry feelings, and his bitterness grows. The narrator cultivates his angry feelings in the same way that a person would care for a growing plant. His anger grows so much that it eventually poisons the entire relationship, and the relationship is forever destroyed.

The moral of the poem is to be open and honest with your feelings because nursing your anger will only bring more harm.