The following is from: http://www.rainkc.com/resources/BLOG.ASP
See next to last paragraph in particular.
Monarchs & Rain Gardens
by Dr. Chip Taylor, University of Kansas
In addition to serving as catchments for runoff, rain gardens can be designed as habitats for wildlife. I work with monarch butterflies and direct a program known as Monarch Watch (http://www.MonarchWatch.org
). We have recently developed the Monarch Waystation Program, a new conservation initiative aimed at creating habitats for monarch butterflies. The justification for this program, largely the loss of habitats in the U.S., and additional information can be found at:
Rain gardens can be ideal habitats for monarchs if they incorporate the host plants needed for monarch larvae and the nectar bearing flowers for the adults. In this short entry, I will describe 6 milkweed species that serve as hosts for monarch caterpillars. All are native to this portion of the Midwest and are suitable for rain gardens. In addition, each is known to have colorful flowers that attract numerous butterflies and other pollinators.
Milkweeds can be started from seeds or can be planted as plugs. The latter is preferable as the plants will get a better start but starting plants from seeds or growing young plants indoors and planting the seedlings can be quite rewarding. General texts describing milkweeds and their propagation can be found at:
[The above referenced propagation text describes stratification as a necessary condition for germination of seeds of most milkweed species; however, germination can also be induced by soaking seeds in hot water. To do this, place seeds in a jar, add hot water, let them sit for a day and then drain off the water and add more hot water each day for the next two days. On the fourth day place the seeds in flats with potting soil - making sure to cover the seeds with 1/4 to 1/2 inch of the soil.]
The milkweed species are arranged below on the basis for their tolerance of wet conditions or need for well-drained soils. The first species, Swamp Milkweed, is tolerant of wet conditions while at the bottom of the list, Butterfly Weed, requires well-drained soils. If I were to plant these species in the typical rain garden, I’d plant the Swamp Milkweeds in the center followed by each of the other species as I moved upslope with the Butterfly Weeds planted around the outer edge of the rain garden.