I have found through observation that Tropical Milkweed is the favorite Monarch Butterfly host plant....the plant in which the female lays her eggs.
It is an annual in all but the warm Southern areas of the United States. Ideally, you should start the seeds inside 2 or 3 months ahead of the last frost date in your area. That way the plants will be a decent size for the butterfly garden.
I also have Swamp Milkweed seeds, the other favorite of the Monarchs in my garden. Swamp milkweed is a perennial, so starting them will require some patience and the seeds either need to be refrigerated for 1 month or planted in your garden in the fall.
Tropical Milkweed Seeds
Swamp Milkweed Seeds
If you start tropical or swamp milkweed seeds, and put several of these plants in your garden, you should attract many Monarch Butterflies!!