The genus Paeonia is divided into three sections, Paeonia, Moutan and Onaepia. Paeonia is the largest section of the genus and is comprised of approximately 28 herbaceous species native to China and/or Europe. Onaepia is the smallest section containing only two herbaceous species that are native to the western United States. Section Moutan contains approximately 8 species, all of which are woody shrubs that are generally referred to as tree peonies. Hybrid peonies can also be divided into three main catagories; 1) Herbaceous hybrids, 2) Tree peony or shrub peony hybrids and 3) Intersectional hybrids (ITOH).
[From Peonies, by Allan Rogers, p. 11-12:] The name of the genus itself, Paeonia is a Latinized rendering of the Greek word Paionia, which commemorates Paion, physician to the gods and mythological discoverer of the medicinal applications of many herbs... peony or paeony in English, pivoine in French, p