Very nice pictures!
Have you Mainz, Mannheim, Frankfurt, Bonn, Bochum and Cologne (Koeln) already?
Mainz botanical garden has got three big trees, Frankfurt botanical garden one, Bonn botanical garden one (which is somewhat strange in that it begins flowering at the moment). Bochum botanical garden has got one not very tasty one. And Cologne has got three trees in the "Flora" (the botanical garden of the city). Mannheim "Luisenpark" (a mixure of zoo, park, and botanical garden owned by the city) has got one big tree.
It seems that most botanical gardens in Germany have at least one Poncirus tree.
But I unfortunatelly have no fotos. I have only tasted the fruits and have some hundred seeds. So if somebody wants seeds ask me...