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.
Working with the Monarch Joint Venture and the National Wildlife Federation, the Xerces Society has created a series of nectar plant lists for the continental U.S. based on a database of nearly 24,000 monarch nectaring observations. Each of the 15 regional guides highlights 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.
Read more about this project on the Xerces blog, or find a nectar plant guide for your region here.