A Late Walk

by Robert Frost

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

Up from the tangle of withered weeds

Is sadder than any words.

A tree beside the wall stands bare,

But a leave that lingered brown,

Disturbed, I doubt not, by my thought

Comes softly rattling down.

I end not far from my going forth

By picking the faded blue

Of the last remaining aster flower

To carry again to you.

What is the theme of this poem?