Make sure you're planting a native species for your area first! Milkweeds have a very wide range, and there's been issues with tropical milkweeds being planted here in California, since they don't go dormant during the fall and winter. That means that:
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Along the coast, they're more likely to be exposed to high levels of moisture that will promote disease growth that can be spread to Monarchs
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Monarchs might lay their eggs on the non-dormant plants instead of migrating, which will likely cause the caterpillars to die, being out of season.
California has some 15 species of milkweed that are native, with one of the more common being the narrowleaf milkweed. You can buy seeds for narrowleaf milkweed online, and now's the time to do it if you plan on directly sowing the seeds, since they need to get cold in order to germinate reliably.
Some fast facts about milkweed:
-It's a perennial, and once established will continue to spread through tubers.
-It's very drought tolerant once established
-The flowers are said to be highly fragrant and are known to draw in a lot of pollinator species besides monarchs.