If I had to pick the great American poet, I'd hate you for asking, but on any given day, Frost would be one of the folks I'd likely name (as would Whitman, Lowell, and Williams). This isn't one of his major poems, but in just eight lines, conveys some of the melancholy of spring that few poets seem to get as well as Frost did (most tend to go with fall/winter for that mood), starting with the beauty of the butterflies and flowers, and ending on the note that they're inherently ephemeral.

(His poem The Hyla Brook is another in this vein.)

Blue-Butterfly Day by Robert Frost

It is blue-butterfly day here in spring, And with these sky-flakes down in flurry on flurry There is more unmixed color on the wing Than flowers will show for days unless they hurry.

But these are flowers that fly and all but sing: And now from having ridden out desire They lie closed over in the wind and cling Where wheels have freshly sliced the April mire.