The most fundamental difference lies in their botanical classification and physical structure. Herbaceous peonies (Paeonia lactiflora and others) are non-woody perennials. Their stems and foliage die back completely to the ground level each winter, with new growth emerging from their crown (buds at the base of the plant) in spring. Tree peonies (Paeonia suffruticosa) are deciduous, woody shrubs. They develop a permanent woody framework of stems that remains above ground year-round, shedding only their leaves in autumn. Itoh peonies, or Intersectional peonies, are man-made hybrids resulting from a cross between a herbaceous peony and a tree peony. They exhibit a blend of traits: their stems may start off woody at the base but largely behave like herbaceous peonies, dying back significantly in winter while retaining a small, low woody structure.
The appearance of the plant when not in flower is distinctly different. Herbaceous peonies have soft, green stems that are produced anew each growing season. Their foliage is typically deep green and often has a pointed, lance-shaped form. Tree peonies possess sturdy, brown, woody stems that branch out, giving the plant a shrub-like appearance that provides winter interest. Their foliage is also more ornamental; it is often larger, deeply lobed, and can have a blue-green or sometimes purple tint. Itoh peonies inherit the exceptional foliage of their tree peony parent. Their leaves are large, deeply dissected, and maintain a dark green, healthy appearance throughout the growing season, turning shades of copper, gold, or red in the fall, unlike most herbaceous types.
All three types produce the large, lavish blooms peonies are famous for, but there are nuances. Herbaceous peonies produce their often-fragrant flowers at the tips of their soft stems. Because the stems are not always strong enough to support the heavy blooms, staking is frequently required to prevent them from flopping over after rain. Tree peony flowers emerge from the scaly buds on their woody stems. They are renowned for their immense size, incredible color range (including true yellows and corals not found in herbaceous types), and often intricate flower forms with dramatic flares of color at the center. Itoh peonies combine the best of both: they produce enormous, tree peony-style flowers in unique colors, but these blooms are held upright on strong, sturdy stems inherited from the herbaceous parent, eliminating the need for staking.
These groups differ significantly in their lifespan and speed of establishment. Herbaceous peonies are extremely long-lived perennials, often thriving in the same spot for 50 years or more with minimal care. They establish and begin flowering relatively quickly, usually within 2-3 years. Tree peonies are even more long-lived, with specimens in Asia known to be over 100 years old. However, they are notoriously slow-growing and can take 5-7 years to reach maturity and flower prolifically from a newly planted specimen. Itoh peonies share the longevity of their parents but have a growth rate more similar to herbaceous peonies, typically flowering within 1-3 years of planting.
