The best answer is to choose whatever is native to your area. Here in California one of the loveliest natives is showy milkweed (Asclepias speciosa). Scientists recommend choosing native milkweeds over more decorative non-natives because the foreign plants may cause Monarchs to linger in an area longer, leaving them more vulnerable to parasites. This is especially important if you consider the migration area of Monarch Butterflies, which spans from South America to Southern Canada. The showy insects can also be found in Australia, Great Britain and Bermuda. It covers so much territory in the U.S. that it is the state insect of Alabama, Idaho, Illinois, Minnesota and Texas, but also the state butterfly of Vermont and West Virginia.
Wherever you live, start milkweed from seed, if you can, to avoid systemic pesticides that some growers apply. To encourage leafy growth, cut milkweeds back after they bloom. Adult monarchs need nectar, so entice them with plantings of flat, upward-facing flowers.
There's no guarantee you'll be graced with monarchs, but Southern Californians have a good shot. Plus, milkweed flowers feed a variety of butterflies.
(Image: Francine Orr / Los Angeles Times)