The poem uses a metaphor to compare anger to a tree.

A figure of speech is the use of language that is not literal.  In the case of this poem, the entire poem is figurative.  The poem compares a tree to a person’s feelings.  The poem uses metaphor, which is a comparison between two things. The tree becomes a metaphorical representation of the anger.

The poem describes the speaker’s wrath.  When he does not tell anyone that he is angry, the anger just festers like a growing tree. 

And I waterd it in fears,

Night & morning with my tears: 

And I sunned it with smiles,

And with soft deceitful wiles. (Stanza 2)

The metaphor the speaker uses is to say that he watered his anger and his anger grew.  He watered it with his tears, meaning that as he cried and fretted about whatever it was that made him angry, the anger grew and grew. The speaker did not literally plant anger, and it is not literally growing. That is why we call it a figure of speech.