When weeds, in wheels, shoot long and lovely and lush; (line 2)
Bringing natural ("weeds") and man-made ("wheels") imagery together sets up a subtle sense of harmony between man and nature.
thrush Through the echoing timber does so rinse and wring (lines 3-4)
The connection between the sound that a thrush makes and this feeling of spiritual cleansing says a lot about how the relationship between man and nature can bring man closer to a heavenly ideal. (If you want to hear the thrush's song, click here.)
it strikes like lightnings to hear him sing; (line 5)
Hearing a thrush sing is like being struck from above. A connection between heaven and earth is made here, with the power of the heavens being felt and expressed through nature.