A Poison Tree

Poem lyrics of A Poison Tree by William Blake.

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 water'd it in fears,
Night & morning with my tears;
And I sunned it with my 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 veil'd the pole:
In the morning glad I see
My foe outstretch'd beneath the tree

But I thought the wrath a person grows is supposed to poison himself? In any case that'd be a more powerful imagery I think.