The poem is like football, a poem of two halves – albeit the first half is only the first 2 lines.
In the first half the writer says that when he is angry with his friend he tells him and the anger is forgotten – oh, how the power of communication works so well!!!
The rest of the poem deals with what happens when we are angry with someone we actually don’t like so well. We don’t say anything, we suppress the feeling and what happens? The feeling doesn’t go away, instead it grows and grows inside of us, fed by all that negative energy and hatred until it explodes as a mighty and destructive force.
I think the underlying theme behind the poem is that communication is always a good thing. Always keep the lines open!
A Poison Tree
I was angry with my friend:
I told my wrath, my wrath did end.
I was angry with my foe:
I told it not, my wrath did grow.
And I watered it in fears,
Night and morning with my tears;
And I sunned it with smiles,
And with soft deceitful wiles.
And it grew both day and night,
Till it bore an apple bright.
And my foe beheld it shine.
And he knew that it was mine,
And into my garden stole
When the night had veiled the pole;
In the morning glad I see
My foe outstretched beneath the tree.