Shown above with a visiting monarch, swamp milkweed (Asclepias incarnata) grows about four feet tall and bears flat heads of pink flowers in summer or even into early fall. A cultivar called "Ice Ballet" has white flowers, while the cultivars "Cinderella" and "Soulmate" are varying shades of pink. The flowers are followed by attractive seed heads in the fall.
As the name implies, this milkweed is a lover of wet places, making it a perfect addition for any wetland (or just plain wet) garden. Mine is growing in a drainage area that also serves to draw water away from my house. The photo below shows the rock-filled drainage ditch leading away from this mini-wetland, which was just scooped out of our clay-filled soil. As you can see from the photo, the flowers attract hordes of butterflies, and not just monarchs. (These are fritillaries.) Swamp milkweed also attracts hummingbirds, hummingbird moths, and certain beetles.
Milkweeds are essential for monarch butterflies because plants in this family are the only ones monarch caterpillars will eat. The milkweeds contain toxic chemicals (warning: toxic to humans though apparently only in large quantities) that cause the caterpillars to taste bad. This protects them from predators.
In Canada, swamp milkweed is native in Manitoba, Quebec, and Nova Scotia; in the United States, it is native to the Northeast; as far south as Georgia, Texas, and Louisiana; and as far west as North Dakota.
Plant swamp milkweed in a moist spot; heavy soil and/or occasional standing water are not a problem. The site should probably receive full sun, though some sources say that this plant can tolerate part shade. Because it has a long taproot, don't plan to move it after you've planted it. Swamp milkweed will spread slowly by rhizomes, and you can naturalize it by scattering the seeds.
The plant is said to attract aphids, but it's highly unlikely that these insects will kill the plant. If aphids gross you out, try to remember that birds take a different view--"bird ice cream," I call them--and will most likely control any aphid outbreak for you. If deer are a problem in your neighborhood, you'll be happy to hear that they don't like the taste of milkweeds.
For more information on milkweeds, I highly recommend the 10,000 Birds blog entry for July 20, 2007. Ten Thousand Birds is a great blog to follow if you're interested in birds, especially if you're a bird watcher.
For additional information on swamp milkweed specifically, check out "This Week at Hilton Pond" for July 15-21, 2001. While you're there, I hope you'll look around this website. The Hilton Pond Center for Piedmont Natural History sure sounds like a place I'd like to visit. The Lady Bird Johnson Wildflower Center also has good information on swamp milkweed.