The most-promising thread I’ve pulled at for this after a few searches is that this all (hedge wizard, hedge witch) derives from “hedge school”, with either “hedge witch” or “hedge wizard” being the first of this form, and later uses of those and related terms (like Martin’s “hedge knight”) deriving from these derivatives, incorporating meaning and context not necessarily intended by the original formulation, for reason of not realizing the specific original reason for labeling them “hedge” (the association with unofficial, unorthodox, and maybe illegal schooling)

https://en.m.wikipedia.org/wiki/Hedge_school