Ivy belongs to Him because it blooms in the autumn, when the grapes are harvested and the Child was snatched from Semele’s womb, and it bears fruit in the spring, when the fermentation is complete and the God is married. Its time of glory is the winter months, when Dionysos rules at Delphi, His epiphany. When Zeus blasted Semele, the ivy entwining her waist protected the Child by its cool nature. The duality of Dionysos is expressed by the ivy and the vine, for ivy belongs to the cool element of moisture as vine belongs to the hot element of fire. (So we wear ivy, as Lord Dionysos instructed us, to temper the fiery effects of wine.) The vine shows the God in the world of the living as the ivy shows Him in the world of the dead (which is why it is used to decorate graves). He is alternately lord of moist warm creation and lord of the moist cold creation.
– Apollonius Sophistes, Dionysian Meditations: THE LENAIA