Apparently with no surprise To any happy Flower (1-2)
Aww, a happy flower. It's just hanging out, being pretty, and playing in the field all day. What could be more innocent? Flowers are often used as a symbol of beauty and innocence, so it's no surprise to see it here. Wait a second though. The first line hints that the flower isn't surprised by the death that's coming for it. If this flower knows what's coming, we guess it's not that innocent after all. It's totally aware that the killer frost is coming, but it chooses to play anyway. Really, what else is there to do?
The Frost beheads it at its play— In accidental power— The blonde Assassin passes on— (3-5)
At first glance, the frost seems like some kind of mass murdering psycho. The verb "beheads" makes us think about an executioner, and it's straight-up called an "Assassin." But what's up with calling the frost's flower-slaying power "accidental"? Seems like this diffuses the evil villain thing a bit. The frost doesn't mean to kill; it just does it because it's frost, and, ya' know, that's what frost does.
The Sun proceeds unmoved To measure off another Day (6-7)
A lot of times the Sun is shown as a jolly, life-giving figure in the sky. In this poem, however, the Sun is kind of cold—even though it's a blazing ball of super hot exploding gas. Flower murder is going on down below, but the uncaring Sun just goes about its job. This depiction of the sun could be meant to show that ultimately nature itself doesn't care about anything. It's all about inevitable cycles that everybody'd better just learn how to accept since there's nothing we can do to change things.