poplar, it is said, grows on the Acheron, and it was a wreath of poplar twigs that Heracles put on after he had conquered Cerberus.
Burkert establishes the poplar as a chthonic tree sacred to the underworld, whose ritual use by Heracles simultaneously expresses bondage to death and its conquest, anchoring the symbol within Bacchic mystery theology.
, Greek Religion: Archaic and Classical, 1977thesis