Both crow and the hemlock tree represent sorrow. Frost has used both the negative creatures (crow and the hemlock tree)as the carriers of positivism and strength that transformed his day for the better.
Sorrow and happiness
Well… Frost has presented nature in an unconventional manner. He was expressing some sorrow through the bird like a crow and a tree like hemlock..A crow can be associated with something dark, black or foreboding.And a hemlock tree is a poisonous one. These are the two sources through which Frost wanted to symbolise the feelings of sadness and regret. The dust(particles) of snow that is shaken off the hemlock tree by crow stands for joy that Frost experiences. He has used an unconventional tree and bird in order to contrast them with joy in the form of snow.