The bulbous plant Allium Sativum, grown for centuries as a flavouring agent for foodstuffs, is also said to be efficacious in warding off Vampires.
The mere mention of the name certainly wards off Mr Aimsbury, the head chef who works for the Chairman of the Royal Bank of Ankh-Morpork. It causes him to go rigidly still for five seconds, which allows anyone in the line of sight to get out of the way, as he will then hurl a very large sharp knife and rant in fluent Quirmian until he gets it out of his system.
Garlic was one of the three main abominations of the god Nuggan, and his believers were forbidden to eat it along with chocolate and mushrooms. In Borogravia the entire country was under Nuggan's law, and the priests made everyone get up early in the morning to stamp on the mushrooms, the chocolate makers had all moved to Ankh-Morpork and no garlic was allowed to be planted. Given Borogravia's proximity to Überwald, this seems unwise, (not that any of Nuggan's commandments were wise) and several vampires were known to live in Borogravia including Maladict.