Sage

Sage is a plant well-known to both muggles and magical folk as having many useful properties. It is grown easily in home gardens and there are both small and shrubby varieties.

Dementors

A larger, branchier variety of sage is a natural magical deterrent to Dementors, who avoid the plant.

Sage was used by Herbology Professor Branwen Strangely in the 2010 to bolster Hogwarts' defense against Dementors, Dementors Dementing. Sage was grown and then gathered into bundles and secured along Hogwarts' walls in vulnerable locations.

"Woody evergreen, flowers violet. Muggles know it and use it well. A cultivar called Extrakta is used by we magic folk as a traditional deterrent for Dementors. They don't like it. No one is sure why. Could be the smell, could be it's naturally uplifting properties. While a great deal is known of sage, very little is known of Dementors. It's nowhere near the kind of ward that garlic is to vampires, but it's something, isn't it? And that's what we need - a whole bundle of something. Thus: my humble contribution to the national defense. A shrub." -- Branwen Strangely, [1]