In the lonely, dark landscape grows one single tree. Still, it's doing more than just growing.
The poplar tree of line 41 works as a symbol for the woman's loneliness and desire for the man to return. But how do we know that this tree is more than just a tree?
For starters, it stands alone: "For leagues no other tree did mark/ The level waste" (43-44). This causes it to become Mariana's focus when she looks out the window, and thus she begins to transfer her loneliness onto it.
Oh…and trees can also be a phallic symbol, and a symbol of fertility and growth. Turns out, that totally applies here. Mariana, who has been waiting for a man we can assume was her lover, is unable to consummate her love. And when, in the fifth stanza, the shadow of the poplar falls on her bed (a place where romance is usually known to uh, bloom), she becomes even unhappier. The tree's shadow serves as a reminder of her lover's absence.
More evidence that the poplar is a symbol for romantic loneliness comes in stanza seven, where the poplar is "wooing" Mariana. That, too, annoys her. The poplar is a symbol of her loneliness and lack of love, and she doesn't like to be reminded of either—sniff.