Arboretum

I spotted this flower on a tree while walking in the arboretum. It reminded me of a rhodedendrum bush but it was quite a tall tree.  I have no idea what it is and there was no label against this specimen.  There flower was about a foot across so quite impressive. It has been another hot day so after yoga in the morning I spent the afternoon down in the cool of the basement and surfaced periodically to keep an eye on the Tour de France.