The genus Paeonia, which includes the beloved peony flower, is a perennial known for its large, often fragrant blooms. These plants are native to temperate regions across Asia, Europe, and Western North America, requiring a period of winter cold to thrive. While there are numerous species and thousands of specific cultivars, garden peonies are organized into three distinct horticultural categories based primarily on their growth habit and stem structure. Understanding these groups helps in selecting the right plant for a specific garden setting.
Herbaceous Peonies
Herbaceous peonies are the most traditional and widely recognized type, often called the common garden peony. Their defining characteristic is the complete dieback of all above-ground growth in the fall, disappearing entirely to ground level. New stems emerge directly from the crown, a structure just below the soil surface, each spring. These plants typically reach a mature height between 18 and 36 inches, forming a dense, bushy clump of foliage.
The majority of these classic varieties are derived from Paeonia lactiflora, the Chinese Peony, noted for its sweet fragrance and suitability as a cut flower. Herbaceous peonies display a wide range of flower forms, including single, semi-double, Japanese, anemone, bomb, and full double. Cultivars like ‘Duchesse de Nemours’ (double white) and ‘Karl Rosenfield’ (double pink) are classic garden staples. They require a period of winter chilling to set flower buds, performing best in cooler climates, generally USDA Hardiness Zones 3 through 7.
Tree Peonies
Tree peonies are deciduous shrubs with permanent, woody stems that remain above ground all year. Unlike herbaceous types, these plants do not die back in winter, retaining their structure and setting buds on old wood. This woody structure allows them to achieve greater height, often growing between four and seven feet tall, giving them a shrub-like appearance.
Their blooms are frequently larger and more dramatic than those of the common peony, sometimes reaching up to 10 inches in diameter, and they typically flower earlier in the season. Tree peonies are known for their slow growth rate and can take a decade or more to reach their full mature size. They are developed from species like Paeonia suffruticosa and its hybrids, which introduced a broader color palette, including true yellows and unique blends. They are generally reliable in USDA Zones 4 through 8.
Intersectional Peonies
Intersectional peonies, commonly known as Itoh hybrids, combine traits from both parent groups. They resulted from a successful cross between a woody tree peony and an herbaceous garden peony, first achieved in 1948 by Japanese hybridizer Toichi Itoh. The plant inherits the tree peony’s exotic-looking flowers, finely cut foliage, and strong, self-supporting stems. Like the herbaceous peony, their stems die back completely to the ground in the fall, allowing them to survive harsh winters.
This hybrid vigor results in a robust, rounded bush form, typically growing two to three feet tall. The most notable contribution from the tree peony parentage is the introduction of colors previously unseen in herbaceous varieties, such as vibrant yellows, copper, and mahogany. Intersectional varieties also offer an extended bloom period, producing multiple flowers that can last for three to four weeks, often bridging the gap between the tree and herbaceous blooming seasons.
Comparing the Three Types
The classification of peonies is fundamentally determined by the fate of their stems as the season ends. Herbaceous peonies have non-woody stems that die back completely to the ground each autumn. Tree peonies are woody shrubs with persistent stems that remain intact above the soil line throughout the winter. Intersectional peonies display a unique blend, possessing the sturdy stems of the tree peony but losing all above-ground structure in the winter.
In terms of height and habit, herbaceous types are the shortest, forming a dense clump up to three feet. Tree peonies are the tallest, developing into a sizable shrub that can reach seven feet or more. Intersectional hybrids generally fall in the middle, creating a strong, bushy plant that typically grows between two and four feet tall. The general sequence of bloom sees the tree peonies flowering first, followed by the herbaceous varieties, with the intersectional hybrids often blooming last and providing the longest overall flowering window.