Answer: The seeds of asclepias tubrosa need to be cold stratified or chilled prior to germination. You could either stratify them naturally outside by planting them in the fall where you want the plants to grow or outside in pots protected in a cold frame. Indoors, you can plant them in a small pot of barely damp sand or damp soilless seed starter mix, enclose the pot in a small plastic bag to maintain the humidity, and place that in your refrigerator for about a month. Then bring the seeds into normal room temperature to germinate. It can also help to have fresh seed. Good luck with round number four!
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