Butterfly weed seeds are dormant when released from the pod and need a chilling period before they will break dormancy. You can mimic the cold period by refrigerating the seeds. Enclose them in a sandwich bag full of moist peat moss, seal the bag and refrigerate the seeds for 12 weeks. This process is known as cold-moist stratification.
Because butterfly weed (Asclepias tuberosa) is a host for the monarch butterfly, it is considered an important native plant. It is also an essential component of a butterfly garden. Easy to grow, butterfly weed is drought-tolerant and grows in any soil as long as it drains freely. Sowing butterfly weed seed directly into the garden in August is the best way to plant it. Butterfly weed grows in U.S. Department of Agriculture plant hardiness zones 3 through 9.
After the flowers fade and fall from the butterfly weed, it produces long, beanlike seed pods. These turn from green to brown as the seeds within mature. When the seeds are ready, the pod bursts open, releasing the seeds. The seeds are covered in feathery white silk to help them take to the wind. For this reason, you will need to keep an eye on the pod and grab it from the plant as it starts to pop open.
Preparing the Seeds
Butterfly weed develops a long tap root and doesn’t tolerate transplanting, so sow the seeds directly into the garden after stratification. Choose a planting spot that receives full sun. Because butterfly weed tolerates even poor soil, there is no need to amend the soil unless it’s heavy. In that case, add a 3-inch layer of compost and dig it into the top 6 inches of soil. Set the seeds on the surface of the soil and cover them with a 1-inch layer of sand.
Water the butterfly weed seed bed carefully after sowing the seed, so the top 3 inches of soil are moist. Although at maturity the plant tolerates extended periods of drought, the seeds require consistently moist soil during germination. Once they emerge, thin the seedlings to 15 to 18 inches apart and allow the soil to gradually dry out until it is dry between waterings.
When sowing seeds outdoors, work up the soil in a protected location in early to mid-November. Scatter the seeds over the prepared seed bed and then cover the seeds with approximately 1/4 inch of soil. The cold, moist conditions over winter improve seed germination. Seedlings should emerge in spring. Carefully transplant the seedlings to their permanent locations when the seedlings are 3 to 4 inches tall.
Harvest the seed pods of butterfly weed when the pods begin to split. Seeds can be sown directly outdoors in late fall or started indoors.
To start seeds indoors, fill a flat with a commercial germination medium (such as Jiffy Mix). Moisten the medium. Scatter the seeds over the surface of the germination medium and lightly press the seeds into the material. Cover the seeds with an additional 1/4 inch of the germination mix. Carefully moisten the additional material. Slide the flat in a plastic bag and place the bagged flat in the refrigerator. Leave the flat in the refrigerator for 4 to 6 weeks. After 4 to 6 weeks, remove the flat from the refrigerator and place it in an area with a temperature of 70 to 75 degrees Fahrenheit. Seeds should begin to germinate in 3 to 4 weeks. (If no seedlings appear after 4 weeks, place the flat back in the refrigerator for another 4 to 6 weeks and repeat the process.) Take the flat out of the plastic bag as soon as seedlings appear and place the flat under fluorescent lights in a 60 to 65 degree Fahrenheit location. Transplant the seedlings into individual pots when the seedlings are 1 to 2 inches tall. Continue to grow the seedlings indoors under fluorescent lights for several more weeks. Prior to planting outdoors, place the seedlings outdoors in a shady, protected location and then gradually expose the seedlings to longer periods of direct sun. Plant the seedlings in their permanent locations after they have hardened outdoors for 10 to 14 days.