I have had great luck with Butterly Weed (Asclepias tuberosa) and Swamp
Milkweed (Asclepias incarnata) in both attracting nectaring butterflies
and breeding Monarchs. I do not use Common Milkweed as it is not
well-behaved in a formal garden, whereas the previous two species are
(more or less). I have also tried Prairie Milkweed (Asclepias
sullivanti) but never had much luck with it. Prairie Milkweed is the
well-behaved brother of the free running Common Milkweed. It creeps
slowly by rhizomes, making it a superior candidate for the butterfly
garden. Mine, however, never established itself.
In addition to the New England Wildflower Society, Prairie Nursery in
Wisconsin is a good source of butterfly plants.
You can order online at:
http://www.prairienursery.com/store/
Eddie