[Jove's beard]. Grown on house-roofs, from the notion that it warded off lightning. Charlemagne made an edict that every one of his subjects should have house-leek on his house-roof. The words are, “Et habet quisque supra domum suum Jovis barbam.” It was thought, to ward off all evil spirits. Fevers as well as lightning were at one time supposed to be due to evil spirits.
“If the herb house-leek or syngreen do grow on the house-top, the same house is never stricken with lightning or thunder” —Thomas Hill Natural and Artf. Conclusion.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894