I think the poet you are talking about is Robert Frost:
A Passing Glimpse
I often see flowers from a passing car
That are gone before I can tell what they are.
I want to get out of the train and go back
To see what they were beside the track.
I name all the flowers I know they weren't:
Not fireweed loving where woods have burnt --
Not bluebells gracing a tunnel mouth --
Not lupine living on sand and drouth.
Was something brushed across my mine
That no one on earth will ever find?
Heaven gives it's glimpses only to those
Not in position to look too close.
He's referring to "fireweed" (Epilobium angustifolium) flowers which sprout up soon after a wooded area is burned (i.e. in a forest fire). They are beautiful pink or purple flowers.
So the flowers he saw he knew were NOT "fireweed".
I hope that helps!