Too happy, happy tree
Thy branches ne'er remember
Their green felicity:
The north cannot undo them
With a sleety whistle through them;
Nor frozen thawings glue them
From budding at the prime
In drear-nighted December
Too happy, happy brook
Thy bubblings ne'er remember
Apollo's summer look;
But with a sweet forgetting
They stay their crystal fretting
Never, never petting
About the frozen time
Ah! would 'twere so with many
‘Crystal fretting’ beautifully describes the frozen bubbles, ‘frets’ being the ridges formed when water freezes to ice.  The word ‘stay’ in this context means ‘remain’ or ‘be unchanged’.