Myths, folklore, and symbolism
Vervain: Pieces of vervain root, or a sachet of its dried leaves, were hung round the neck to cure scrofula, prevent nightmares, and make one immune to snakebite; it was also said to staunch blood, because it grew at the foot of Christ's cross. One Jacobean poet says it 'hindreth witches of their will'...this probably alludes to hag-riding, since it is nowhere mentioned as a house protection. (Oxford Dictionary of English Folklore)
Do you know of vervain folklore from other countries?