|"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 existed), and, as such, details contained in this article may differ from real world facts.
Black Walnut (binomial name Juglans nigra) also known as the Eastern Black walnut is a specific type of walnut that is native to eastern North America. It grows mostly in riparian zones, from southern Ontario, west to southeast South Dakota, south to Georgia, northern Florida and southwest to central Texas. Isolated wild trees in the Upper Ottawa Valley may be an isolated native population or may have derived from planted trees.
Less common than the standard walnut wand, that of black walnut sought a master of good instincts and powerful insight. Black walnut was a very handsome wood, but not the easiest to master. It had one pronounced quirk, which was that it was abnormally attuned to inner conflict, and lost power dramatically if its possessor practised any form of self-deception.
If the witch or wizard was unable or unwilling to be honest with themselves or others, the wand often failed to perform adequately and must be matched with a new owner if it was to regain its former prowess. Paired with a sincere, self-aware owner, however, it became one of the most loyal and impressive wands of all, with a particular flair in all kinds of charmwork.