It sifts from Leaden Sieves -
It powders all the Wood
It fills with Alabaster Wool
The Wrinkles of the Road -
It makes an Even Face
Of Mountain, and of Plain -
Unbroken Forehead from the East
Unto the East again -
It reaches to the Fence -
It wraps it Rail by Rail
Till it is lost in Fleeces -
It deals Celestial Veil
To Stump, and Stack - and Stem -
A Summer's empty Room -
Acres of Joints, where Harvests were
Recordless, but for them -
The snow veils trees and stumps (“Stump”), haystacks (“Stack”), and grass and plants (“Stem”) alike. Stump and stack and stem are what is left of the harvest from “summer’s empty room,” which are the farms and fields after harvest season. 
 Note the heavy alliteration.
It sifts from Leaden Sieves - 311