Wikipedia
Tree peony is the vernacular name for the section Moutan of the plant genus Paeonia, or one of the species or cultivars belonging to this section. It consists of shrubs that have perennial aereal woody stems. Other peonies do not have perennial woody stems, but their stems die back after the growing season, to emerge again from buds just below the surface early in the following year. Tree peonies have been in culture in China for millennia, and it is likely that hybrids came into being in gardens, where different wild tree peony species were planted closely together. They are used in China both for medicine and as an ornamental, particularly the hybrids called Paeonia suffruticosa. These hybrids in particular, but other tree peonies too are called 牡丹 (mu dan) in China.