The mountains of central and western China are the original home of tree peonies (Paeonia suffruticosa and related species). Like their Japanese cousins, Chinese tree peonies are widely adapted and thrive throughout the U.S.
Chinese tree peonies (mudan hua) were first cultivated in the mid-seventh century in the gardens of the Chinese emperors; they soon gained such popularity among the upper classes that they were, and still are, recognized as China's national flower. The plants have long held a place of honor in the gardens of China, Japan, and Korea.
The reasons for this veneration are found in the characteristics of the flowers themselves. Colors range from white through shades of pink, magenta, red, and yellow, as well as variable hues. But descriptions alone don't do justice to the way they look. Each petal has numerous shades that give the flower a look of shimmering silk.
Flower sizes range from 6 to 12 inches across. Some of the largest blooms are so heavy that they hang or droop on the plant, and this has led to a misconception that droopy stems are a feature of all Chinese tree peonies. This is true of only a few kinds.
Flower shapes vary from daisylike singles through numerous semidouble forms -- all with stamens and petal-like carpels (where the seed grows) showing -- to complex doubles. The double shapes remind the Chinese of strings of coins, and the flowers are symbols of prosperity and wealth.
Many plants display both semidouble and double flowers at the same time. Very heavy double flowers are likely to face downward or to the side, while single and semidouble flowers face upward. Similarly, varieties with the largest, most complex flowers won't produce fully developed flowers until they are sufficiently mature. You may have to wait two to three years after planting to see the "thousand-petal" form of 'Zhao's Pink'. A few kinds, such as 'Pea Green', might take 10 years.
Fragrance ranges from woodsy through spicy to sweet, and its intensity from delicate to dense. Blossoms with a dense spicy or sweet fragrance will fill the space around them with an aroma unique to these plants.
From an eighth-century document, we find this testimony to tree peonies' tremendous appeal: "In front of the Audience Hall of the Emperor, there are planted many thousand-petaled tree peonies. When the flowers first opened, the fragrance of their perfume was perceived by everyone. Each flower blossom has a thousand petals, is large, and deep red. Every time His Majesty gazed upon the sweet-scented luxuriance, he would say, 'Surely such a flower never existed before among humans.'"
In China, tree peonies grow from the edge of the Gobi Desert to the very warm areas south of the Yangtze River. Translated to USDA Hardiness Zones, that means the plants are hardy from zone 4 (southern Minnesota) through zone 9 (parts of southern California). Our experience tells us that the plants need a three-week dormancy period in the fall, with temperatures of 35 to 40°F.
Planting success has two key elements: plant at the right time -- fall -- and start with plants that are old enough and strong enough to survive transplanting. We suggest you buy from growers who sell only plants that are at least three years old. These will cost more, but younger plants, even though less expensive initially, more often fail to survive transplanting.
Buy from a reputable nursery that ships plants in fall, and plant as soon as the dormant bare-root plants arrive, a moment that will vary according to where you live. Plant in September if you live in zone 4, November in zone 9.
Don't plant too early, or plants will produce leaves that die as soon as cold weather arrives. Plant too late (after soil temperature is below 40°F), and the plant won't have time to produce the root hairs it needs to support growth in spring. Avoid planting in spring, when plants will put out top growth at the expense of developing a good root system.
Choose a location that receives no more than four hours of direct sunlight with afternoon shade, or day-long dappled shade. Soil should be moist, fertile, and humus-rich; amend it if necessary. The plants will grow rapidly in full sun; however, under those conditions the flowers will go through their life cycle in just a day or two. Planted in semishade, flowers last 10 to 14 days (the bloom cycle lasts about 31/2 weeks).
In China, tree peonies will grow in acidic soil with a pH of 5.8. They also do well in Tasmania, Australia, where the soil pH is a slightly alkaline 7.5. However, experience tells us that tree peonies thrive best in soils with a pH of 6.5 to 7.0.
In a ninth-century gardening text, Chinese growers advised siting the plants where water drains quickly. This instruction means that tree peonies must be planted in a raised bed, on a slope, or in a garden bed that has excellent drainage.
To save time, dig planting holes before plants arrive. Make holes 2 feet deep, 2 feet wide, and no closer together than 5 feet. Set the plants deep enough that all pink or white buds at the base are at least 2 inches below the surface. Spread the roots, backfill about one-third, and "mud in" the plants with a mix of water and commercial seaweed with fish fertilizer. Make sure all air spaces are filled. Add the rest of the soil and mud in again. Fertilize with seaweed or fish fertilizer every three to four weeks after spring flowering has finished.
In warmer climates, force dormancy in November by trimming off green leaves and withholding water in November and December. New leaves will appear in January, with blooms in February and March.
Pests are at a minimum on tree peonies. In the Northeast, rose borers (the same insects that bore into cut rose canes) sometimes afflict older plants, and in spring, carpenter bees act as one of many pollinators. You can detect them from spring to fall by a 1/16-inch hole in the woody stems. If you find such a hole, poke a wire inside to kill the larvae, and seal the hole with clay or white glue. In other parts of the country, thrips and nematodes can be pests. Trap or hand-pick thrips and destroy all plants with nematode galls (visible on the roots).
Tree peonies are sometimes infected with the botrytis fungus. It occurs during wet, cold weather in spring and fall. Cut below the infection and remove and destroy affected parts. In the fall, remove all leaves, infected or not, to prevent any botrytis from overwintering underneath the plant.
Tree peonies have many wonderful qualities: ease of cultivation, relative freedom from pests and diseases, widespread hardiness, and flowers acclaimed for hundreds of years for their beauty. We invite you to visit Cricket Hill Garden (in Thomaston, Connecticut) in late May, when you'll see more than a hundred varieties of mudan hua in bloom. You'll learn that paintings and photographs are only pale reflections of the real thing.
Besides Chinese tree peonies, three other types have been in cultivation for many years in the West. The Japanese types are common, European and American hybrids are rare, and Itoh hybrids are extremely rare.
Japanese tree peonies are descendants of Chinese plants brought to Japan in the early 17th century. The Japanese, with their own aesthetic principles, chose to grow and cultivate plants with only certain characteristics: single or semidouble flowers, plants that stay under 6 feet, and most important, flowers with little or no fragrance.
Hybrid (European and American) tree peonies were first devised about a hundred years ago, first in France, and then in the United States. The parents were species from China, yellow-flowered Paeonia lutea (of Tibetan origin), and some unnamed Chinese or Japanese cultivated varieties. Thus, yellow is the characteristic undertone color in many of these flowers. In the second generation, many less desirable characteristics of the original hybrids were eliminated (such as small or down-facing flowers), and flowers of great beauty were bred. From P. lutea, all of these, even darker colors, have an almost translucent undertone of yellow and a fragrance of lemon vanilla.
Itoh or Intersectional hybrids. There is some evidence that Chinese tree peonies and herbaceous peonies (P. lactiflora) are closely related. About 40 years ago, a Japanese breeder, Itoh, was able to hybridize a few cultivated varieties of each kind. The resulting plants had the leaf and flower characteristics of a semidouble tree peony but died to the ground each fall -- a major characteristic of herbaceous peonies. In recent years, American breeders Anderson and Hollingsworth created flowers with a whole range of colors (deep yellow and violet) not seen before in tree peonies.
David and Kasha Furman are the owners of Cricket Hill Garden -- peony heaven -- in Thomaston, Connecticut.