Milkweeds are the required host plants for monarch butterflies. Milkweeds provide food for the caterpillar, nectar for the butterfly, and chemical compounds that make the monarchs distasteful to predators. Make sure to choose a variety native to California. The non-native Tropical Milkweed may disrupt monarch migration patterns, because of it's longer growing season, so don't plant it unless you're willing to cut it back to 6" or less from Oct-Feb. Learn more at the UC ANR blog posting on Overwintering Monarchs.