The most significant difference from a gardener's perspective lies in the fundamental growth habit of these plants. Paeonia lactiflora, commonly known as the herbaceous or garden peony, is a perennial plant. This means that its soft, herbaceous stems and foliage die back completely to the ground each autumn. The plant survives the winter through its underground storage organs—a clump of thick roots and buds (often called "eyes") at the base of the stems. In spring, new growth emerges from these subterranean buds.
In contrast, tree peonies (a group primarily consisting of Paeonia suffruticosa and its hybrids) are deciduous woody shrubs. They develop a permanent, woody framework of stems and branches that remain standing above ground throughout the year. Each spring, new growth emerges from the buds on these existing woody stems. Unlike their herbaceous cousins, they do not die back. This structural difference dictates their form, pruning needs, and long-term presence in the garden landscape.
This difference in growth habit directly impacts the plant's size, form, and role in garden design. A mature Paeonia lactiflora typically forms a rounded, bushy mound of foliage, generally reaching 2 to 3 feet in height and spread. Its seasonal nature means it occupies space in the herbaceous border or perennial bed, often providing a lush, green presence from spring until its foliage declines in autumn.
Tree peonies are significantly larger and more architectural plants. Over many years, they can grow into substantial shrubs, often reaching 4 to 7 feet in height and width, with some venerable specimens growing even larger. Their woody skeleton provides winter interest and a permanent structure in the garden. They are best treated as small flowering shrubs and placed accordingly, perhaps as a specimen plant or within a mixed shrub border, where their form can be appreciated year-round.
While both are celebrated for their magnificent flowers, there are subtle distinctions. Paeonia lactiflora blooms are often held atop the herbaceous stems and can be so heavy that they require staking to prevent them from flopping over after rain. The flower forms are incredibly diverse, from single to double, bomb, and anemone forms, and they come in a wide range of whites, pinks, reds, and corals. They are renowned for their intoxicating fragrance.
Tree peony flowers are typically even larger and are borne on the woody stems from the previous year's growth. The blooms often appear more delicate, with papery, sometimes crinkled petals, and they famously come in a wider array of unusual colors, including true yellows, deep purples, and vibrant magentas that are rare in the herbaceous world. Crucially for gardeners, tree peonies bloom about two to three weeks earlier than most herbaceous peonies, effectively extending the peony season in the garden.
The planting requirements for both are similar in one critical aspect: the depth of the graft union or eyes. Paeonia lactiflora must be planted with its "eyes" (the pink or red buds on the crown) no more than 1-2 inches below the soil surface. Planting too deep is a primary reason for failure to bloom.
Most tree peonies are grafted onto herbaceous peony rootstock. The graft union (a noticeable bulge on the main stem) must be planted 4 to 6 inches below the soil level. This encourages the tree peony to eventually form its own roots and ensures the rootstock doesn't sucker. Pruning is another key difference. Herbaceous peonies require only a simple cutback to the ground in autumn after frost. Tree peonies require very little pruning, limited to the removal of dead wood or weak branches in spring, and careful deadheading after flowering.
