Let's face it: this isn't the happiest poem in the world. It opens with sorrow, moves on to grief, and ends with a longing for death. Hooray! Or… not. But hey, what else would you expect from a poem written in the voice of a widow?
Line 1: The poem opens with a metaphor comparing our speaker's sorrow to her yard. While it doesn't technically qualify as an extended metaphor, from here on out you can't really separate descriptions of nature from our speaker's grief. The two go hand in hand.
Lines 2-3: In saying that the "new grass / flames," our speaker creates a metaphor, comparing the grass to fire. This strange and beautiful image is also, frankly, menacing, especially when we arrive at the "cold fire / that closes round me this year" In lines 5-6.
Lines 14-16: Our speaker takes a handful of normally inactive, physical things (flowers) and pits them against an abstract emotion (grief). It's an epic battle between flowers and sorrow; somehow, across the border of emotional and physical, these two sides are able to interact. No matter how happy those flowers appear, though, they will never be able to overpower the speaker's own grief.