I believe that you are referring to ASCLEPIAS TUBEROSA, a member of the milkweed group that is grown for it's attraction for butterflies. In my garden, it is the latest of my various milkweed plants to bloom. I am in MD, zone 7 and in my garden it is just starting to bud out. Others of my milkweeds are either in full bloom or nearly finished blooming. Being in zone 6, I would imagine that yours will bloom a few weeks later than those here.

A photo of my plant now.
{{gwi:486370}}

Bob

Bob,

Are your A. tuberosa plants first year plants?

Mary