Asclepias speciosa - Milkweed
I'd never noticed this plant before coming to MN and was going to eliminate it from the yard until (whew!) a colleague who's amazing with plants and who has contributed mightly to this garden (yay for mature gardens that need dividing!) told me that a)milkweed is ambrosia for monarch butterflies and b)the flowers are pretty and smell good. She was right on all counts. Imagine a nice lily scent with hints of gardenia. Oh, it started blooming mid-June and plants will still be putting out flowers through the summer.