Lines 1-4 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Nature's first green is gold,

This line gives us the setting of the poem—we're in nature, in case you were snoozing. And we're talking about nature's first green, which makes us think about spring.
The speaker, though, says this first green is actually gold. This makes us think of trees like the willow, which are golden early in spring, before they mature to green.
But it also makes us think of the morning, when the sun rises and makes everything a bit golder than normal, all bathed in the dawn light.

Her hardest hue to hold.

Now that our speaker has told us that nature is gold before it's green, he goes on to say that gold is the hardest hue, or color, for nature to hold, or keep. So the first color we see in spring doesn't stick around very long.
The idea of nature having an easy or hard time holding onto something is an example of personification. And not only is nature personified here, it's actually made into a female figure.
You know what else we notice about this line? Check out all those H sounds. That's some major alliteration right there. And it this short line, all those rapid-fire H sounds right in a row force us to slow down and really ponder over the meaning.
Then there's the rhyme. Hold rhymes with gold, which means we've got a rhymed couplet here.
And since these lines have the same number of syllables, we're gonna go ahead and assume there's a meter in play, too. In fact, there is—iambic trimeter, to be precise. Be sure to click on over to our "Form and Meter" section for more.

Her early leaf's a flower;

The speaker wants to be clear here, so he's going to elaborate on what he was talking about in Line 1. Just like nature's first green is gold, her first leaf is a flower. In spring, trees and bushes bloom with gorgeous flowers, which are replaced by green leaves in the summer.
Frost is really getting into his poetic groove here, when he pops a metaphor into this line. The first flowers of spring aren't actually leaves in disguise; the speaker is using figurative language to intentionally blur the line between flowers and leaves. Eventually, in real life, the blooms die and drop off the trees, making room for the leaves, which come to soak in nourishment from the sun.

But only so an hour.

This line completes the alternating structure of the first four lines. If nature's first green, gold, doesn't stick around long, then it only makes sense that the first version of the leaf, which is the flower, doesn't stick around long either.
As the speaker says in this handy rhyme, the first leaf is a flower for only an hour. This doesn't literally mean that the trees or plants the speaker is referring to bloom for exactly an hour. But blooms, as you'll know if you've ever gardened, only last a few days, or weeks, depending on the plant.
Or, Frost could be talking about how, when the sun comes up, everything is golden and flower-like. But then, when the sun gets high in the sky and everything becomes its normal color, what once looked like golden flowers now look like what they truly are—green leaves.