And that was my long scythe whispering to the ground.
What was it it whispered? I knew not well myself;
Perhaps it was something about the heat of the sun,
Something, perhaps, about the lack of sound—
And that was why it whispered and did not speak.
It was no dream of the gift of idle hours,
Or easy gold at the hand of fay or elf:
Anything more than the truth would have seemed too weak
To the earnest love that laid the swale in rows,
Not without feeble-pointed spikes of flowers
(Pale orchises), and scared a bright green snake.
The fact is the sweetest dream that labor knows.
My long scythe whispered and left the hay to make.
The idea of leaving the hay to ‘make’ suggests that, after labor, the making of hay — or poetry — is out of the laborer’s hands. It must ‘make itself’.  So, the mowing and scything and the resulting hay connotes passing of time.  And, of course, death. 
 So time itself is  anthropomorphized .  The final line evokes Shakespeare’s  Sonnet 116 : 
 Love’s not Time’s fool, though rosy lips and cheeks Within his bending sickle’s compass come. 
 The speaker in Frost’s sonnet could be mowing through life and, realizing its transitory nature, questions what is most important. 
 Finally, the act of mowing is a known  euphemism  for sex.  Frost was a classicist and would have known that ‘orchis’ is taken directly from the Greek word for testicles.
Mowing