When I go up through the mowing field,
The headless aftermath,
Smooth-laid like thatch with the heavy dew,
Half closes the garden path.
And when I come to the garden ground,
The whir of sober birds
A  tatch  is straw or a similar material used for a roof covering, wich in this sense, can be used as a metaphor for the brain. His understanding. 
 A  dew  can be tiny drops of water that form on cool surfaces at night, when atmospheric vapor condenses. With that being said, the tiny drops here can be used as his Words. The  Heavy  part explains that his Words had huge impact on his surface /his appearance or face .
A Late Walk