Sclepias syriaca, commonly known as common milkweed, is a native perennial plant that serves as an important food source for pollinators like monarch butterflies. Here are detailed planting instructions for sowing Asclepias syriaca seeds both outdoors and indoors:
Planting Outdoors (Fall & Spring)
Site Selection
Choose a full sun location, as milkweed thrives in bright sunlight. The plant requires at least 6-8 hours of direct sunlight each day to grow vigorously and produce flowers.
Well-drained soil is essential. Avoid areas where water tends to pool, as milkweed does not tolerate standing water.
Common milkweed is well-suited for meadows, prairies, and naturalized areas. It can also grow along roadsides or in garden beds with adequate space.
Soil Preparation
Prepare the soil by removing weeds and any debris from the planting area.
Asclepias syriaca is relatively tolerant of poor soils, but it thrives in sandy to loamy, well-drained soil.
Milkweed can tolerate slightly acidic to slightly alkaline soils, with a pH range of about 6.0-7.5.
If the soil is compacted, loosen it with a garden fork or tiller to improve drainage.
Planting Depth
Surface sowing is recommended because milkweed seeds require light to germinate.
Lightly press the seeds into the soil or cover them with a thin layer of fine soil or sand.
The seeds should not be buried too deeply—just barely cover them or leave them on the surface.
Watering
Keep the soil moist during the germination process, but avoid overwatering or creating standing water.
After the seeds have germinated and the plants are established, they become drought-tolerant but will still benefit from occasional watering during dry periods, especially in the first few months of growth.
Milkweed plants generally prefer moderate watering and should not be waterlogged.
Stratification & Scarification
Stratification: Asclepias syriaca seeds require cold stratification to break dormancy. For outdoor sowing in fall, the seeds will naturally undergo cold stratification over the winter.
If sowing in the spring, you will need to cold-stratify the seeds by placing them in a moist paper towel or sand and refrigerating them for 30-60 days before planting. This simulates the cold winter conditions that trigger germination.
Scarification is not necessary for Asclepias syriaca seeds, though some gardeners report slightly better germination rates if the seeds are lightly nicked with sandpaper or filed, especially if the seeds are older or harder.
Timing
Fall Sowing: The best time to sow milkweed seeds outdoors is in late fall (October to November), after the first frost. The seeds will naturally stratify over the winter and germinate in the spring.
Spring Sowing: If sowing in spring, ensure that the seeds have been cold-stratified for at least 30 days in the refrigerator. Sow the seeds after the last expected frost and once the soil temperature reaches about 60°F (15°C).
Maintenance
Once the seedlings have sprouted, thin them out to allow for 12-24 inches of space between plants. Milkweed plants can get quite large and need room to spread.
Weed control is important during the first year of growth. Mulch can help suppress weeds while maintaining soil moisture.
Milkweed plants benefit from occasional deadheading to encourage more blooms and reduce seed production if you don’t want them to spread too much.
Cut back dead stems in early spring before new growth begins.
Starting Seeds Indoors (Spring for Later Transplanting)
Planting Containers
Start seeds indoors 6-8 weeks before the last expected frost date.
Use seed trays or individual pots with good drainage.
Fill the containers with well-draining seed-starting mix.
Soil & Planting
Sprinkle the seeds on the surface of the soil. Press the seeds gently into the soil, or cover them with a thin layer of soil or sand.
As with outdoor sowing, the seeds require light for germination, so don’t bury them too deeply.
Stratification
For indoor sowing, stratify the seeds by placing them in a moist paper towel or sand in the refrigerator for 30-60 days before planting. After stratification, sow the seeds indoors.
Germination
Keep the soil moist but not waterlogged.
Germination typically takes 10–30 days under optimal conditions (65–75°F or 18–24°C).
Provide bright, indirect light to prevent seedlings from becoming leggy. Use a grow light if necessary.
Transplanting Outdoors
After the danger of frost has passed, harden the seedlings off by gradually exposing them to outdoor conditions over the course of 7-10 days.
Once hardened, transplant the seedlings into the prepared outdoor garden, spacing them about 12–24 inches apart.
Invasiveness
Asclepias syriaca is not considered invasive in its native range. It is a native species in the USA and serves as an important food source for pollinators, including monarch butterflies.
However, it can self-seed and spread over time, especially if growing in ideal conditions. While it does not typically become invasive in a garden setting, it may naturalize and spread if left unchecked.
Some gardeners may manage its spread by removing excess seedlings or deadheading flowers to prevent the production of seed.
By following these instructions, you can successfully grow Asclepias syriaca, adding a beautiful and beneficial plant to your landscape while supporting pollinators.
