The peony family
Paeoniaceae is a family containing exactly one genus, and that genus is the peony. Nothing else belongs to it. For a plant this familiar to sit alone in its own family is unusual, and the reason it does is a genuinely interesting piece of botanical history.
one genus, Paeonia
around 33 species
Temperate Eurasia, with two species in western North America
Everything in this guide that belongs to Paeoniaceae
Peonies spent a long time filed with the buttercups. The resemblance is obvious enough: many separate petals, a mass of stamens in the middle, several separate seed cases rather than one. On appearance it is a reasonable call.
The details would not sit still. Peony stamens develop from the center of the flower outward, while buttercup stamens develop the other way around; the peony has a fleshy disc at the base of the flower that buttercups lack; and the way the embryo forms in the seed is unlike anything in that family. Enough differences accumulated that the peony was moved out on its own, and molecular work later confirmed it is not closely related to the buttercups at all but sits near the saxifrages and the currants.
The name goes back to Paeon, physician to the gods in Greek myth, who is said to have used the root as a medicine. It is one of the older plant names still in use and the root really was a standard drug in both European and Chinese medicine for centuries.
Herbaceous peonies die back to the ground every winter and send up new growth in spring. They are the ones grown for cutting, and a well established clump will outlive the person who planted it; peonies are recorded flowering on the same spot after a hundred years.
Tree peonies are woody and keep a permanent framework of stems above ground. They flower earlier, carry much larger blooms, and were the imperial flower of China, bred there for well over a thousand years before Europe had seen one.
The third kind is recent. Crossing the two was tried for decades and mostly failed until Toichi Itoh succeeded in Japan in the 1940s, producing plants with the woody peony's flower on a herbaceous plant that dies back each year. He did not live to see them bloom. They are still called Itoh or intersectional peonies and they remain expensive, because they can only be increased by division.
Peonies do something almost nothing else in the trade will tolerate. Cut in tight bud, wrapped dry with no water at all and held just above freezing, they can be stored for weeks and in commercial practice for months, then taken out, recut and stood in water, and they will open normally.
This is why a flower with a natural season of a few weeks is available for a great deal longer than that. It is also a technique worth knowing on a small scale: a florist can hold peonies for a date rather than buying them into it.
The bud stage decides everything. Cut too tight and it never opens at all; cut open and it is already spending its vase life. The stage wanted is the one where the bud has softened and yields slightly to a squeeze, with color showing. Blind buds, which are the usual complaint about peonies, are almost always buds that were cut a few days too early.
A peony bud is sticky, and the reason is a feature this family carries that most flowers do not. Nectar is normally made inside a flower, where it buys a pollinator. Peonies also make it outside, from glands on the surface of the sepals, before the flower has opened and where no pollinator can reach it. Botanists call these extrafloral nectaries, and they turn up across a scattering of otherwise unrelated plants.
What they are for is a reasonable argument rather than a settled fact. The strongest case is that they are paying for protection: the sugar brings ants, the ants patrol the bud, and insects that would chew it get driven off. That trade is documented in other plants and it fits what happens on a peony. What is certainly not true is the old belief that the flower cannot open without them.
One more thing worth knowing about this family in a directory covering all fifty states: a serious share of the peonies sold in the American summer are grown in Alaska. The season there runs when nowhere else in the northern hemisphere has them, which turned a short lived local crop into a real industry.