Plants have been the basis for medical treatments throughout all of recorded human history.
Herbalism focuses on recommending specific plant medicines for healing specific health issues. While the study of nutrition overlaps with herbalism, herbal medicine focuses on the use of concentrated medicinal plants that may not be a common food plant.
Much of herbal medicine comes from traditional wisdom and, because it does not offer the same kind of corporate profit as pharmaceutical drug research, has comparatively little funding for large-scale clinical trials. This can make it challenging to scientifically determine or prove its effectiveness.