I think the stems that bloom will die off gradually by fall, Donald. You should cut them off when they get scruffy looking.
It does take a few years to get mature stems on the plant so it will bloom. My big ones in the front yard shady beds bloom every year now. But if you have a really cold winter that kills back all the top growth, the plant won't die, but it also won't bloom for a couple more years.
So if you're growing them where there is winter chill, it pays to throw a blankie on them when it goes below 40 or so.