|"Is this all real? Or has this been happening inside my head?"
The topic of this article is of a real-life subject that has been mentioned "in-universe" in a canon source. The Harry Potter Wiki is written from the perspective that all information presented in canon is true (e.g., Hogwarts really exists), and, as such, details contained in this article may differ from real world facts.
Horseradish (binomial name Armoracia rusticana) is a perennial plant of the Brassicaceae family (which also includes mustard, wasabi, broccoli, and cabbage). Its large white, tapered root is used, when grated, as a spice with a distinctive pungent aroma spice.