I live near the California overwintering grounds. Should I be planting milkweed in my yard?
No! Instead of planting milkweed, plant fall-, winter-, and spring-blooming native plants which provide nectar resources for monarchs and other pollinators.
While monarchs require milkweed for egg laying and larval development, during the overwintering period (~October-February/March), the butterflies are usually in reproductive diapause--meaning they stop mating and laying eggs. During the winter, monarchs don't need milkweed, but they do need plants which provide nectar for energy and water to keep hydrated.
Historic records suggest that milkweed was largely absent from most coastal areas of California. Because of the mild winter temperatures, milkweed planted close to the coast can often escape hard frosts--delaying or preventing these species from going dormant in the fall. This may disrupt the monarch’s natural cycle, encouraging them to continue to mate and lay eggs into the winter. This phenomenon is well-documented in non-native, tropical milkweed (A. curassavica) which stays evergreen and is associated with winter breeding in Florida, the Gulf Coast, and Southern California. And winter breeding has costs-- OE levels of winter breeding monarchs are 9 times the level of non-breeding, overwintering monarchs (Satterfield et al. 2016). In coastal California, even native species may act like tropical milkweed--staying green late into the fall-- and cause similar issues as tropical milkweed. For these reasons, the Xerces Society does not recommend planting milkweed (non-native or native) close to overwintering sites (within 5-10 miles of the coast) in Central and Northern coastal California where it did not occur historically (see State of the Overwintering Sites Report for additional information).
Check out Xerces Monarch Nectar Guide for coastal California for ideas of what to plant.