Milkweed (Asclepias species) is the sole host plant for the monarch butterfly caterpillar, which cannot complete its life cycle without it. The Western monarch population, which migrates to the California coast for the winter, has faced extreme declines. Understanding the appropriate species and timing for planting in California is necessary to support the recovery of this insect. Success depends on aligning the plant’s biological needs with the state’s varied climate conditions.
Selecting the Right Species for California
The choice of milkweed species is important in California, where native varieties should be favored over non-native options. Native milkweeds naturally go dormant in late fall or early winter, providing a necessary cue for monarchs to begin their southward migration and reproductive diapause. This natural die-back helps break the cycle of the protozoan parasite, Ophryocystis elektroscirrha (OE).
Tropical milkweed (Asclepias curassavica) is a non-native species that does not die back in the mild California winter, allowing the OE parasite to persist and accumulate on the leaves year-round. Caterpillars that feed on this parasite-laden foliage can develop infections, often resulting in deformed wings and reduced lifespan, which interferes with the monarch’s ability to migrate effectively. To avoid these negative impacts, gardeners should choose regionally adapted native species, such as narrow-leaf milkweed (Asclepias fascicularis) or showy milkweed (Asclepias speciosa).
Optimal Timing Based on Climate Zones
The best time to plant milkweed in California varies depending on whether you are using seeds or starter plants, and the specific climate zone. Native milkweed seeds require cold, moist conditions—a process called cold stratification—to break dormancy and germinate successfully. For seeds, the most effective planting window across most of the state is late fall or early winter, allowing for natural stratification outdoors with winter rains and cold temperatures.
In the Inland and Central Valley zones, planting seeds in late October through December is ideal for natural stratification, which leads to springtime germination. If starting seeds indoors, they should be artificially stratified in a refrigerator for 30 days to three months and then planted inside in late winter for transplanting outside in early spring. For those using starter plants, or plugs, the safest time to plant is in late winter or early spring (February to April), after the danger of a hard frost has passed.
Coastal and Mild Southern California
In the Coastal and Mild Southern California regions, the threat of frost is minimal, allowing for a longer transplant window that extends into late spring (April to June). Fall planting of transplants is also a good option, especially around October, as the milder climate allows the plant to establish roots before the following summer’s dry season.
Desert and High Elevation Zones
For the Desert and High Elevation zones, planting should be timed to avoid intense heat and severe cold. Transplants are best set out in early spring or late fall, ensuring consistent moisture during the initial establishment period.
Planting and Establishing Milkweed
Once the correct species and timing are determined, successful establishment requires attention to site conditions. Most milkweed varieties thrive in full sunlight, requiring a minimum of six to eight hours of direct sun exposure each day. The planting site must have well-draining soil, as milkweed is tolerant of rocky or infertile ground but susceptible to root rot in waterlogged conditions.
When planting seeds, they should be scattered on the soil surface and covered lightly with no more than a quarter-inch of soil. For starter plants, dig a hole just large enough for the root ball, ensuring the top of the plug is level with the surrounding soil line. Milkweed needs consistent moisture immediately after planting until the plant is established, which may take up to three months. After this initial period, native milkweed varieties become drought-tolerant, requiring minimal supplemental water, even during the dry summer months.