A few years ago I planted one swamp milkweed in a garden at the edge of my father's pond. It has since spread like, well - a weed! It loves the wet soil and does very well in the heavy clay. I don't mind that it has spread around the pond, it's a beautiful plant. Much prettier than the regular milkweed. The monarch caterpillars love it just as much, too. I brought one plant home with me and put it in my flower garden, where it is thriving. It has yet to spread and when it does, it may end up in one of my neighbor's yards. Not to worry, they can just mow it down. If you have a butterfly garden, I suggest you plant a swamp milkweed in it! The bees love it too, as you can see, although my picture is a little blurry. This bee was all over this flower like, well - a bee to honey :)
Clicking on the title of this post will take you to a site with more information on the swamp milkweed.