Retrieve the rhyming words from the end of each line, and group together comma-separated with a semicolon between each pair. 

Nature's first green is gold,
Her hardest hue to hold.
Her early leaf's a flower;
But only so an hour.
Then leaf subsides to leaf.
So Eden sank to grief,
So dawn goes down to day.
Nothing gold can stay.


gold, hold; flower, hour; leaf, grief; day, stay