Tree peonies, or Paeonia suffruticosa, are part of the Paeoniaceae family, which has only one genus, Paeonia. The plants are shrubs that can grow as tall as 6 feet, with thick twigs and deeply dissected leaves. The flowers can sometimes be as large as 10 inches across--generally larger than those of herbaceious peonies. Paeonia suffruticosa is native to China and arrived in Japan in the 17th century. After that time, Japanese breeders favored cultivars with single or semi-double flowers and little or no scent. Chinese breeders developed varieties with a range of shapes. Some are also fragrant.
Chinese tree peonies are called "moutan" or "hua wang" (king of flowers). They are iconic in China and have been cultivated since the fourth century. Cultivation of Paeonia suffruticosa was centered on Sichuan by the 11th century, at which time yellow varieties were developed. The modern "moutan" mecca is the Chinese city of Heze, which has an annual spring tree peony festival. The tree peony arrived in England in the late 18th century, but specimens did not survive and flourish until the early 19th century. It was introduced in New York in 1822.
Development in Japan
Japanese tree peonies breeders produced specimens with lighter, less complex flowerheads. They also developed an expertise in grafting less hardy varieties onto hardy root stocks. The tree peony industry has flourished in Japan to the point that the country is a major supplier to the world market. In 1948, Japanese breeder Toichi Itoh successfully interbred the tree peony, Paeonia suffruticosa, with the herbaceous peony, Paeonia lactiflora. The resulting "intersectional hybrids" have expanded the range of peony colors and forms.
Traditional Chinese and Japanese tree peonies often have evocative names. A deep red variety has a name that translates to 'Black Dragon Holds a Splendid Flower.' A maroon specimen with a large central cluster of golden stamens is called 'Gold Sand in a Black Ocean.' A Japanese-bred, white, semi-double specimen is named 'Renkaku,' or 'Flight of Cranes.'
Peonies bought bare-root should be planted in the fall and are generally hardy in USDA hardiness zones 4-8. Tree peonies require full sun to high, light shade and well-drained soil. The crowns will rot if the plants are subjected to standing water or extremely moisture-retentive soil. Herbaceous peonies are planted close to the soil surface, but tree peonies should be planted deeper, according to the grower's or retailer's specifications. Mature tree peonies are drought-tolerant, but mulching, especially in spring, is useful.
Tree Peonies and Medicine
The Chinese have traditionally used tree peony roots in medicinal preparations. "Mu Dan Pi" is a remedy that is reputed to have antiseptic properties and to "cool the blood" (probably producing a sedative or calming effect). Tree peonies are still grown and harvested for the roots' medicinal properties, and the compounds contained in them are the subject of pharmacological research.