The hydrangea and mock orange family
Hydrangeaceae is a small family of shrubs built around one clever idea: spending a few flowers on advertising so the rest do not have to. Everything distinctive about a hydrangea, including why it lasts the way it does, follows from that.
around 17 genera
roughly 220 species
Split between eastern Asia and the Americas
Everything in this guide that belongs to Hydrangeaceae
A wild hydrangea head is a flat plate of tiny fertile flowers, dull and hard to see, ringed by a handful of large flat showy ones. The big ones around the edge are sterile. They have no working pollen and no ovary. Their entire job is to be visible from a distance and steer an insect into the middle, where the real flowers are.
That is the lacecap, and it is the original. The mophead everybody pictures is what happens when that arrangement is pushed to its limit by breeding, so that nearly every flower in the head has become a large sterile one and the fertile flowers have all but vanished. A mophead hydrangea is, in the plainest sense, a plant that has been bred into being almost entirely advertisement.
Look closely at what those showy flowers are made of and they are not petals either. They are sepals, four of them, in tissue much thicker and tougher than a petal.
The soil story everyone knows is half right and the missing half is the interesting part. The pigment in a hydrangea is the same molecule whether the flower is pink or blue. What changes is what that molecule is holding on to. To turn blue it has to bind an aluminum ion, and aluminum is present in most soils regardless.
What acidity actually does is make the aluminum soluble enough for the roots to take it up. Acid soil frees the aluminum, the plant gets it into the flower, the pigment complexes with it and the color shifts blue. In limey soil the aluminum stays locked up and unavailable, so the same pigment stays pink. The pH is a lever, not the cause.
Which is why a white hydrangea will never change color no matter what goes on the soil. It has none of the pigment in the first place, so there is nothing there for the aluminum to work on, and anybody promising otherwise is selling soil treatment.
An ordinary flower is on a schedule. Once it is pollinated it has finished its job, the plant stops supporting it and it goes over quickly, and that clock is the reason most cut flowers fail on a timetable.
A sterile hydrangea flower has no such clock. It cannot be pollinated, so nothing ever tells it to stop, and it stays open for as long as the tissue physically lasts. Combine that with sepals rather than petals, which are structurally closer to a leaf than to the fragile tissue of a rose, and you have a flower head built to persist.
That is also why hydrangeas behave so strangely as they age. Rather than collapsing they stiffen, the color fades toward antique greens and russets, and the head stays whole. Most flowers rot. This family has the structure to dry instead.
The family has a curiously broken distribution. Its members are concentrated in eastern Asia and in the Americas, with the Atlantic and most of Europe empty in between. This is a well known pattern shared with magnolias and several other groups, and it is the signature of a family that was once continuous across the northern hemisphere and was cut in half by cooling climate.
The relative worth knowing is mock orange, a hardy shrub whose white flowers throw a heavy orange blossom scent across a whole garden in early summer. Given that hydrangeas have essentially no fragrance, a close cousin being one of the most powerfully scented shrubs in cultivation is a fair surprise. Deutzia and the climbing hydrangeas fill out the rest, and one Asian member of the family supplied a traditional antimalarial that medicinal chemists are still working on.