Yes, poplar, from the underside, wind blew them up.
Thank you for the translation. A riddle obviously.
Well, a few remarks. As you can see from this angle the poem is still visible, it was designed to be visible from different corners. Secondly there are some letters missing, the i in 'Blind' and the 'e' in 'Zelfs',
On many occasions someone wrote 'e' in between 'zlfs'. It has to be cleaned many times in a year.
Great sky of Bonn, Frank.