Tree peony and herbaceous peony are sold under one English name and under two entirely different ones in Chinese, which is where most of the confusion begins. They are related, they flower within a fortnight of each other, and a garden that has the wrong one planted where the other was wanted will not show it until the second winter.
The difference that decides everything
Tree peony is a deciduous shrub. It builds permanent wood and can reach two metres. Herbaceous peony has no standing stems at all: the top grows from the root each spring, and in winter all of it collapses, with the plant topping out around a metre. That single fact accounts for how the two are pruned, how long an old plant lasts, and why one can be trained into a specimen while the other resets itself annually.
Winter is therefore the simplest test of all. Walk the border in January: where a tree peony still shows its old branches above the ground, a herbaceous peony shows nothing whatever.
Leaf, and how to read it in summer
The leaves separate them when there is no flower to go by. On a tree peony the leaf is broader and pale green on the upper surface, carrying a fine whitish bloom that gives it a matte finish. On a herbaceous peony the leaf is narrower, darker green, glossy, and built as a compound leaf of two or three divisions. In July, colour and shine alone will identify either one.
Flower and flowering time
Tree peony carries its flowers singly at the ends of the season’s wood, and the largest reach about 20 cm across. Herbaceous peony flowers in clusters at the tops of its green shoots, on blooms typically nearer 15 cm. The forms differ as well. Centuries of breeding on the tree type produced the lotus, chrysanthemum, rose, crown and hydrangea shapes of flower, while the herbaceous group offers far fewer of them, and its colour range is narrower too.
Timing is the easiest field test there is. Tree peony is in flower from mid to late April, herbaceous peony around mid-May, with roughly a fortnight between them. That gap is the origin of the old saying that one belongs to spring and the other to summer, and it also means a garden holding both has peony colour across six weeks rather than three.
What the difference costs in practice
It also decides what you are buying. A herbaceous peony changes hands as a root division or a bare crown, and it will never become a shrub however old the plant gets, because it has no capacity to build wood. A tree peony sold as a named cultivar is usually grafted, and the graft line matters: growth from below it belongs to the rootstock and will not match the label. Plant either one in a permanent place, because neither is a plant that recovers well from being lifted a second time.
Neither is a plant to move on a whim. Both want deep ground that drains properly and both hate a wet root run, so the drainage discipline that matters for rosemary in a pot matters considerably more for a peony in the border, as the rosemary pot article argues from the other direction. The pruning consequence of permanent wood is the same one set out for purple-leaf plum: cut hard in winter and you have removed the wood this season’s flowers were going to sit on.