To propagate butterbur, the most effective method is by dividing the rhizomes in early spring or late autumn; carefully dig up the plant, separate the rhizomes ensuring each section has at least one growth bud, and replant them in suitable moist soil conditions to establish new plants. 

Key points about propagating butterbur:

Propagation method: Rhizome division is the primary way to propagate butterbur. 

Best time to divide: Early spring or late autumn when the plant is actively growing. 

Important consideration: Butterbur can be invasive due to its aggressive spreading nature, so ensure you plant it in a contained area with barriers to prevent uncontrolled growth.