Horologium Florae · after Carl Linnaeus, 1751

The flower clock

A clock with no gears: species chosen because they open and shut at hours you could set your day by. Here is what Linnaeus proposed, why it only half works, and every plant named on Horae’s dial.

What Linnaeus proposed

Carl Linnaeus set out the Horologium Florae in Philosophia Botanica (1751) and planted a version of it in the botanical garden at Uppsala. It is not a clock face with flowers painted on it. It is a bed of living plants, chosen and arranged so that a visitor could walk past and tell the time from which species had opened and which had already shut for the day.

The idea rests on an observation anyone can repeat. Many flowers do not open at random: goat’s-beard closes around midday so reliably that English speakers named it Jack-go-to-bed-at-noon, and the four o’clock flower is named for the hour it wakes. What Linnaeus added was to treat that as data — to record the hours, sort the species, and lay them out in order around a bed.

Why some flowers keep time

Linnaeus sorted flowering behaviour into three classes. The Meteorici answer to the weather, opening wider on a bright day. The Tropici follow the length of daylight, so their hour drifts through the season. Only the third class, the Aequinoctales, opens and closes at a fixed hour whatever the weather or the season — and only those can honestly be used to tell the time.

Which is also why the flower clock has never quite worked. The hours were recorded at Uppsala, at sixty degrees north; carry the same planting south and the schedule slides. Add weather, altitude, the turning season and the plants’ own moods, and a bed that reads to the half-hour in one garden reads badly in the next. It survives as one of the loveliest things ever to fail as an instrument.

The twenty-four species on the dial

Horae’s flower face divides the day into twelve wedges, and each wedge carries two plants, inscribed in Latin around the rim in the manner of the engraved flower clocks that followed Linnaeus. Botany has been revised since the eighteenth century: where a name has changed, the currently accepted one is given beside it.

Three names recur around the rim — Calendula arvensis, Anagallis arvensis and Nymphaea alba. That is faithful to the old plates rather than an oversight: on a twelve-hour dial the same plant can be marked twice, once for the hour it opens and once for the hour it shuts.

How Horae draws it

The dial above is not a picture. It is the app’s own rendering code running in your browser, on your clock: four concentric rings, twenty-four binomials set letter by letter along their arcs, twelve hour numerals, a German motto through the middle ring in the manner of the flower-clock engravings of the 1780s, a sunflower at the centre, and tapered branch hands on the real time. Beside it stands the plate for the hour you are in.