Asclepias incarnata
Asclepias syriaca
Asclepias physocarpa -- These are large plants; note that the flowers are inverted, and about 1/2 inch across. Plants must be kept free of gold aphids. Best grown in zones 7 to 10 if you want seeds. Season is a bit short for seeds in zone 5. I saw monarch butterflies laying eggs on the leaves, but found no evidence of caterpillars.