A purple patch is a passage of writing that is overly ornate, showy, or richly decorated, especially when it feels out of place beside the rest of the poem. It usually calls attention to itself with grand language, elaborate images, or heavy embellishment instead of serving the poem's larger movement.
Writers and critics often use the term negatively, though a vivid purple patch can also be memorable when used with control.
Example
The road was muddy, the gate was broken, the barn leaned left in the wind-
then suddenly the moon spilled silver across the valley,
and every blade of grass became a jeweled spear in the queenly dark.