The Xerces Society just released a new database of monarchs' favorite nectar plants:

http://www.xerces.org/blog/to-save-monarchs-we-need-more-than-just-milkweed/

...While restoring the millions of milkweed plants that have been lost is certainly an important strategy, monarchs need more than milkweed to support them throughout the year. Adult monarchs need nectar to fuel them during spring migration and breeding and to build up stores of fat which sustain them during fall migration and winter. Too few nectar plants in the landscape may reduce the number of monarchs that successfully arrive at overwintering sites in the fall.

There are many sources of information about which species of native milkweeds are best for your region (including from the Xerces Society), but information on which nectar plants are best for monarchs has not been available for large areas of the U.S. To address this need, the Xerces Society has created a series of nectar plant lists based on a database of monarch nectaring observations compiled from a wide variety of sources, including published and technical reports, research datasets, and personal communications with monarch researchers, lepidopterists, botanists, and other experts. This database now houses nearly 24,000 reported monarch nectaring observations on 358 native plant species.

Working with the Monarch Joint Venture and the National Wildlife Federation, the Xerces Society used this database to develop monarch nectar plant guides for all regions of the continental U.S. Each of the 15 guideshighlights species that are commercially available, are native to and widely found in the region, and are known to be hardy or relatively easy to grow in a garden setting—although, as with any plant choice, we encourage you to use additional references when making final species determinations for your location.