What is ‘a hemlock tree’? Why doesn’t the poet write about a more ‘beautiful’ tree such as maple, an oak, or a pine?

‘A hemlock tree’ is considered to be associated with inauspicious/negative things and is a poisonous tree.
The poet didn’t write about a more ‘beautiful’ tree such as an oak or a pine because he didn’t want to shoe beauty.
He wanted to convey that something as negative as a hemlock tree brought a positive change in him and it isn’t necessarily bad.

Final Answer:
‘A hemlock tree’ is considered an inauspicious tree and is poisonous.
He doesn’t write about any other beautiful tree because he wanted to symbolize regret and sadness feeling.