The “divine weed,” vervain, said by the old Romans to cure the bites of all rabid animals to arrest the progress of venom, to cure the plague, to avert sorcery and witchcraft, to reconcile enemies, etc. So highly esteemed was it that feasts called Verbenalia were annually held in its honour. Heralds wore a wreath of vervain when they declared war; and the Druids held vervain in similar veneration.
Lift your boughs of vervain blue, Dipt in cold September dew; And dash the moisture, chaste and clear, O'er the ground, and through the air. Now the place is purged and pure.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894