As imperceptibly as Grief
The Summer lapsed away —
Too imperceptible, at last
To seem like Perfidy —
A Quietness distilled
As Twilight long begun
Or Nature spending with herself
Sequestered Afternoon —
The Dusk drew earlier in —
The Morning foreign shone —
A courteous, yet harrowing Grace
“Sequestered” means hidden. So nature is hidden away. 
 At this stage it is worth looking at the construction of the stanzas and the lines.  The frequency of dashes may indicate a desire to convey the essence of the speaker’s thoughts; a wish to not even include the ‘scaffolding’ of sentences with full syntax. The frequent pauses,  caesurae , enable the reader to infer what hasn’t been said.
As Imperceptibly as Grief