There are three main types of peonies: herbaceous cultivars, which die back to the ground each autumn, tree peony types, small shrubs with woody stems that lose their leaves in the autumn, and Itoh hybrid peonies.
Itoh peonies are a cross (or technically an intersectional) between herbaceous and tree peonies. For years, peony growers had tried to cross herbaceous peonies with tree peonies, in order to obtain an herbaceous plant with the rich yellow flower displays found in the tree peony, but without success, because they are not closely related within the genus. In 1948, after a monumental effort (it is said that he tried more than 20,000 crosses), Japanese grower Toichi Itoh finally succeeded in making an intersectional cross between a hybrid tree peony and a white-flowered herbaceous peony. In 1964 Itoh’s first crosses began to bloom and several were the first to have deep yellow, double flowers. An American horticulturist named Louis Smirnow discovered them some years later, and obtained permission from Itoh’s widow to patent four of the plants in the late 1960s. He imported them into the United States and named them ‘Yellow Crown’, ‘Yellow Dream’, ‘Yellow Emperor’ and ‘Yellow Heaven’.
Peonies require winter chill—herbaceous the most, with over 400 hours at 40 degrees and below. Itoh and tree peonies require correspondingly less winter cold. All produce the buds for next year's flowers in the fall. On tree peonies (Paeonia suffruticosa), these buds form mostly on the stems above the ground. In herbaceous peonies, the buds are formed underground at the crown of the plant.
Most varieties flower for about a week in late spring to early summer, but the period of bloom can be lengthened for some weeks by intermixing early, mid- and late-season cultivars.
Flower colors range from white and yellow to shades of red, pink and mauve - any color except blue. As for scent, most double flowering types are more fragrant than singles, and pinks tend to be more fragrant than reds.
Peonies grow two to four feet tall and thrive in sunny flowerbeds and well-drained soils. They are fairly heavy feeders, compost or well-rotted manure are the best sources of nutrients. They tolerate a wide range of soil types, but the best success with come with good soil rich in organic matter. Top dressing around peonies in the fall with compost will improve the soil structure as well as its nutritional content.
The peony is among the longest-used flowers in ornamental culture and a traditional floral symbol for nobility and value in China. The peony became popular in the imperial palaces during the Sui and Tang dynasties, and earned the title of the "king of flowers." A symbol of spring, it is also used as a metaphor for female beauty, and is often used as metaphor for those forbidden or unspeakable pleasures of the flesh. Pictured in full bloom, the peony symbolizes peace. Tree peonies, particularly a gold colored variety, were considered the sole pleasure of the emperor and possession by others could merit the death penalty.