If you want to help monarch butterflies in the Golden State, knowing when to plant milkweed seeds in California is your first step. California’s unique climate, with its coastal fog belts, hot valleys, and mountain chill, means timing is everything for success.
Planting at the right moment gives your seeds the best chance to grow into healthy plants. This guide will walk you through the ideal windows for different regions, from San Diego to Siskiyou County. We’ll cover how to work with your local weather, not against it.
When To Plant Milkweed Seeds In California
There isn’t a single calendar date for the whole state. Instead, the best time to plant milkweed seeds in California depends on two key factors: your specific region and whether you’re starting seeds indoors or sowing them directly outside. The general rule is to mimic nature’s schedule.
In nature, milkweed seeds drop in late fall. They then experience a cold, damp winter (a process called cold stratification) before sprouting in spring. We can replicate this to get better germination.
Understanding California’s Climate Zones
California is divided into several major climate zones. Each one has a slightly different ideal planting window. Here’s a quick breakdown:
- Coastal and Southern California (Zones 9-11): This includes areas like Los Angeles, San Diego, and the Bay Area. Winters are mild, and frost is rare. Here, you can plant seeds outdoors in late fall, after the first rains, or in early spring (February-March).
- Central Valley and Inland Areas (Zones 8-9): Think Sacramento, Fresno, and Bakersfield. These areas have hotter summers and colder winters with fog. The best time is late fall or very early spring, as soon as the soil can be worked. Avoid the peak summer heat.
- Northern California and Mountain Regions (Zones 7-8): This covers places like Redding, Chico, and the Sierra foothills. Winters are colder with regular frost. Planting is best in early to mid-spring, after the last frost date has safely passed. Fall planting is risky if seeds freeze too deeply.
- High Elevation and Desert Areas (Zones 6-7): These zones have the shortest growing seasons. Wait until late spring, when soils have warmed consistently. Starting seeds indoors is highly recommended here.
The Two Best Planting Windows
For most Californians, you have two primary options for timing your planting.
1. Fall Planting (The Natural Method)
Planting seeds in late autumn, from October to December, is often the easiest and most successful method for native milkweed species. The seeds get their needed cold stratification naturally from the winter weather. They’ll be ready to sprout exactly when conditions are right in spring.
Pros: Less work for you, higher germination rates for many native species, and strong root development early in the season.
Cons: Seeds may be eaten by birds or rodents, and you have to wait months to see results.
2. Spring Planting (The Controlled Method)
If you miss the fall window or prefer more control, spring planting is your go-to. This involves either sowing seeds directly outdoors after the last frost or starting them indoors first.
- Outdoor Sowing: Aim for March through early May, depending on your zone. Soil should be workable and no longer soggy.
- Indoor Starting: Begin seeds indoors 6-8 weeks before your last expected frost date. This gives seedlings a head start, which is crucial in shorter-season areas.
Step-by-Step: How to Plant Your Milkweed Seeds
Follow these steps for the best chance of success, whether you’re planting in fall or spring.
Step 1: Choose Your Milkweed Species
Always choose milkweed species native to your region. This is vital for monarch health and your gardening success. Common California natives include:
- Narrowleaf Milkweed (Asclepias fascicularis): Widespread, drought-tolerant, great for most of the state.
- Showy Milkweed (Asclepias speciosa): Prefers some summer water, good in inland and northern areas.
- California Milkweed (Asclepias californica): A fuzzy, native species for dry, hot slopes.
- Heartleaf Milkweed (Asclepias cordifolia): Found in foothill and mountain areas.
Step 2: Prepare or Stratify Your Seeds
For fall planting: No preparation needed! Just plant. The winter cold and moisture will stratify them.
For spring planting: You must stratify seeds yourself. Place them in a damp paper towel inside a sealed plastic bag and refrigerate for 3-6 weeks before planting.
Step 3: Select and Prepare the Planting Site
Milkweed needs full sun—at least 6 hours of direct sunlight per day. Most species prefer well-draining soil. Clear the area of weeds and grasses. Loosen the top inch or two of soil. Don’t amend it too much; native milkweeds are adapted to local soil conditions.
Step 4: Sow the Seeds
Scatter seeds lightly on the soil surface. Press them gently into the soil, but do not bury them deeply. A light dusting of soil or sand over them is plenty. Milkweed seeds need light to germinate. Water the area gently but thoroughly after sowing.
Step 5: Ongoing Care for Germination
Keep the soil consistently moist (not soggy) until seedlings are established. This can take 1-3 weeks. Once seedlings have a few sets of true leaves, you can let the soil dry out more between waterings. They are quite drought-tolerant once established.
Common Mistakes to Avoid
- Planting too deep: This is the number one reason for failure. Seeds need light.
- Overwatering mature plants: This can cause root rot, especially in summer.
- Using non-native tropical milkweed (Asclepias curassavica): It can disrupt monarch migration and harbor disease. Stick to natives.
- Giving up too soon: Some seeds may take a full year to germinate. Be patient.
Regional Calendar Quick Reference
- San Diego / Los Angeles / Coastal: Oct-Dec (Fall) or Feb-Mar (Spring).
- San Francisco Bay Area: Nov-Jan (Fall) or Mar-Apr (Spring).
- Sacramento / Central Valley: Oct-Nov (Fall) or Mar-Apr (Spring). Watch for late frosts.
- Sierra Foothills / Northern CA: Spring only, after last frost (Apr-May). Fall is possible but riskier.
- High Elevation / Desert: Start indoors in Apr for transplant after last frost (May-Jun).
FAQ: Your California Milkweed Questions
Can I just scatter milkweed seeds in California?
Yes, in fall or early spring, scattering (called “broadcasting”) can work well. The key is to ensure good seed-to-soil contact by pressing them in lightly. It’s less precise but mimics nature’s way.
What is the best month to plant milkweed seeds in Northern California?
For most of Northern California, aim for April. This ensures the soil is warming and the threat of a hard frost has passed. In warmer microclimates, late March might be suitable.
Should I soak milkweed seeds before planting in California?
Soaking overnight can help speed up germination for spring-planted seeds that have been refrigerated. For fall planting, it’s not necessary as winter rains provide ample moisture.
Can I plant milkweed seeds in summer in California?
It’s not recommended. The intense heat and dry conditions make it very hard for seedlings to establish. They will require constant watering and often fail to thrive. It’s better to wait for the cooler fall or spring seasons.
How long do milkweed seeds take to grow in California?
After sowing, germination typically takes 7-21 days with proper moisture and temperature. The plants themselfs may not flower in their first year, focusing instead on building a strong root system. You can expect blooms in the second growing season.
Final Tips for Success
Mark your planting area so you don’t forget where you sowed the seeds. Consider protecting the area with a light layer of straw or a piece of burlap to retain moisture and deter birds. Most importantly, plant more than you think you need. Not every seed will germinate, and having a good cluster of milkweed plants is more attractive to monarchs.
By aligning your gardening with California’s natural rhythms, you give these essential plants—and the butterflies that depend on them—the best possible start. Your efforts will contribute to a healthier ecosystem right in your own backyard.