“Nature’s first green is gold….”
In other words, the trees have finally leafed out.
When I was a cheerful young undergrad going to school in Arkansas, I thought that mud-time was something that Frost made up for poetic purposes; likewise, the birches bending “to left and right/Across the lines of straighter darker trees.” Then I moved up here, and realized that he’d been making his poetry out of sober observation all along.
As do we all, even if we’re writing stories set in worlds completely of our own imagining.