Butterfly Milkweed (Asclepias tuberosa) ready for it's cold stratification period!
Asclepias incarnata stratifying in a cold/moist chamber at the Nebraska Statewide Arboretum. Photo by Rebecca Chandler.
If you are planning to start your Milkweed seeds indoors, now is the time to start cold stratifying them. (Click here to order your Milkweed seeds) Cold stratification is just a fancy way of saying cold exposure and this technique will greatly increase your Milkweed's chance of germination and growth. Most Milkweed varieties need 30-60 days of cold exposure to break their dormancy cycle and this can easily be done in your own home. In nature, this keeps Milkweed plants from germinating at times when conditions are not favorable for growth.
*Note that the warm weather species such as Tropical Milkweed (Asclepias curassavica) do not need cold treatment. There are other pretreatment techniques such as: scarification, direct sowing and hot treatment. However, in this blog we will go over cold stratification since it is the most common way of treating most Milkweed species.
Step 1: Cold stratification Place your Milkweed seeds in a damp paper towel inside a ziplock bag. Don't forget to label the bag with the date and type of seed. Store the mixture in a refrigerator (33–38°F) for the recommended time on your seed packet. For Butterfly Milkweed (Asclepias tuberosa) Prairie Moon Nursery recommends 30 days of cold stratification. Try not to disturb your seeds at all during this stage and if the seeds start to sprout in the bag, plant immediately!
Milkweed species that need cold stratification and their recommended period of exposure* Common Milkweed (Asclepias syriaca) 30 days Swamp Milkweed (Asclepias incarnata) 30 days Whorled Milkweed (Asclepias verticillata) 30 days Prairie Milkweed (Asclepias sullivantii) 30 days Western Sand Milkweed (Asclepias arenaria) 60 days Butterfly Milkweed (Asclepias tuberosa) 30 days Showy Milkweed (Asclepias speciosa) 30 days * According to Prairie Moon Nursery. Always check the back of your seed packet for specific instructions.
Step 2: How to plant your seeds After 30-60 days, it’s time to plant your Milkweed seeds. You can plant them in seed trays, peat pots, small pots or any other container that allows for water drainage. Once the soil is damp, place 1-2 seeds into each container. Sow seeds lightly on the top of the soil. You may press in gently but they need light to germinate so please don’t bury them!
Step 3: How to water your seeds Be gentle when watering the new seedlings because they are very delicate. It is recommended to water from the bottom up by adding a half-inch of water to the bottom of a tray and placing the pots inside this tray. Some people prefer to use a spray mister to water their seedlings. You can check the soil moisture simply by touching it. Take care not to overwater your seeds which can lead to fungus growth. It is helpful to have a fan nearby to provide airflow and ventilation. Step 4: Your seeds need light! Make sure your milkweed has enough light to grow whether that be under an artificial grow light or near a sunny window. If your seedlings start to become "leggy" expose them to more light. The sturdier the seedling, the better!
Step 5: Plant outdoors Milkweed seeds can be sown outdoors after the danger of frost has passed and the seedlings have grown around 1-5 inches high. To make the transition from indoors to outdoors less stressful, start by putting the seedlings outdoors for a few hours a day for up to a week before planting them outdoors.
Resources The American Meadows Blog: How to Germinate and Grow Milkweed Seed. https://www.americanmeadows.com/blog/2015/06/11/how-to-germinate-and-grow-milkweed-seed/ Prairie Moon Nursery: How to Germinate Native Seeds. https://www.prairiemoon.com/blog/how-to-germinate-native-seeds Henrikson, Bob. Gardening with Prairie Plants. Nebraska Statewide Arboretum. https://plantnebraska.org/plants/bloom-box/resources-for-bloom-box-gardeners.html