Propagate butterfly weed by seed, according to Michigan State University, or by division in spring. Pre-chilled seed will readily germinate at 70 to 75 degrees within 3 to 4 weeks of planting. Butterfly weed has a long taproot, which needs to remain intact for successful transplanting.
Choose a site in full sun where the butterfly weed can spread. According to the University of Florida Extension, Asclepias tuberosa tolerates most soils, but doesn’t like being disturbed once planted.
Space plants 24 inches apart, being careful not to damage the taproot. Mark the planting location because plants are late to appear in spring.