Fundamentally, the primary distinction lies in their botanical classification and physical structure. Herbaceous peonies (Paeonia lactiflora and hybrids) are soft-stemmed perennials. They die back completely to the ground each winter, with new growth emerging from their crown (a collection of buds at the soil line) in spring. In contrast, Tree Peonies (Paeonia suffruticosa and hybrids) are deciduous, woody shrubs. They possess a permanent woody framework of stems that remains above ground year-round. Their growth is additive; each year, the plant grows taller and the stems thicken, much like any other shrub. This structural difference is the most critical factor for landscape design and winter interest.
The foliage and overall form of these plants contribute significantly to their role in the landscape. Herbaceous peonies typically form a rounded, mound-shaped clump of deeply divided, lush green foliage that serves as an excellent textural foil for other plants. Their presence is seasonal, filling a space from spring until fall. Tree Peonies offer a more architectural presence. Their woody, often gnarled stems provide structure and interest even in the dormant winter months. The foliage is also different, often being larger, more deeply dissected, and sometimes with a bluish or grayish tint. The plant's form is more open and airy, growing 3-7 feet tall and wide, allowing for underplanting with shade-tolerant perennials or spring bulbs.
While both are celebrated for their flowers, the blooms differ in key aspects. Herbaceous peonies produce their enormous, often fragrant flowers on new, green growth in late spring. Their bloom season, while spectacular, is relatively brief. A major consideration is that their heavy flowers frequently require staking to prevent them from falling over after a rain. Tree Peonies bloom earlier in the spring, on the old wood from the previous season. Their flowers are even larger than those of their herbaceous cousins and are held more securely above the foliage on sturdy woody stems, rarely requiring support. The bloom texture is often remarkable, ranging from delicate, tissue-paper-like singles to incredibly dense, massive doubles. The color range in tree peonies is also broader, encompassing true yellows and deep maroons that are rare in the herbaceous types.
Both types demand well-drained soil to prevent rot, but their placement needs differ. Herbaceous peonies thrive in full sun (at least 6 hours) and require a period of winter chill (vernalization) to bloom well, making them ideal for most American climates. They are incredibly long-lived perennials. Tree Peonies prefer a site with high, filtered shade or morning sun with afternoon protection. Their delicate flower petals scorch easily in hot, direct afternoon sun. They are also exceptionally long-lived plants, but they are slower to establish and are considered a permanent landscape investment. A crucial planting difference is the graft union; many tree peonies are grafted onto an herbaceous rootstock and must be planted with the graft union 4-6 inches below the soil surface to encourage the tree peony to form its own roots.
