Many plants have a biological clock, which regulates the time of day that their flowers open and close. For example, the flowers of catmint (Nepeta cataria) - also known as catnip - open between 6am and 7am; orange hawkweed follows between 7am and 8am; field marigolds open at 9am and varieties of Helichrysum1 wake up for 10am. Other varieties follow, with Convolvulus opening at noon.
By making observations of the times when flowers open and close during the day, Carolus Linnaeus (the 18th-Century Swedish botanist, recognised as the father of taxonomy), conceived the idea of arranging certain plants in an order of flowering, so that they constituted a kind of floral clock. This was described in Linnaeus's Philosophia Botanica (1751) in which he referred to it as an horologium florae (floral clock). Apparently, Linnaeus was able to use his clock to determine the time accurately to within half an hour.
In Philosophia Botanica Linnaeus described three groups of flowers:
Meteorici - flowers which change their opening and closing times according to the weather conditions.
Tropici - flowers which change their times for opening and closing according to the length of the day.
Aequinoctales - flowers which have fixed times for opening and closing. (Note that these are unaffected by the weather conditions.)
Only Aequinoctales are suitable for use in a flower clock.
It is thought that Linnaeus probably never planted a 'flower clock' but rather made accurate observations on plants growing at various localities. This is partly because his son, Carl Linnaeus fil., began to write a thesis on Horologium Plantarum which was never completed. It contains only a few examples of Aequinoctales and does not contain a description of a flower clock as such.
Following Linnaeus's observations, it became popular for 19th-Century gardeners to plant up 'flower clocks' in which the flowerbeds were laid out in a circle or an annulus to form the clock face. The 'clock face' was divided into 12 segments, each of which contained flowers that either opened or closed in that one-hour time period.
What flowers can I use?
|0200||Night blooming cereus closes|
|0500||Morning glories, wild roses|
|0600||Spotted cat's ear, catmint|
|0700||African marigold, orange hawkweed, dandelions|
|0800||Mouse-ear hawkweed, African daisies|
|0900||Field marigold, gentians, prickly sowthistle closes|
|1000||Helichrysum, Californium poppy, common nipplewort closes|
|1100||Star of Bethlehem|
|1200||Passion flower, goatsbeard, morning glory closes|
|1300||Chiding pink closes|
|1400||Scarlet pimpernel closes|
|1600||'Four o'clock' plant2 opens, small bindweed closes, Californian poppy closes|
|1700||White waterlily closes|
|1800||Evening primrose, moonflower|
|2000||Daylilies and dandelions close|
|2200||Night blooming cereus|
Unless stated otherwise, these times are the approximate times that the flowers open. The exact opening times vary according to the latitude and climate of the garden.
Jean Françaix - L'horloge de flore
Linnaeus's idea for a collection of flowers that opened or closed at a particular time of day was taken up by the French composer Jean Françaix in his composition L'horloge de flore (The Flower Clock), a concerto for solo oboe and orchestra. The following list gives the hour of the day, the French names, the English and the botanical names of the plants he chose to represent in this piece:
|Time||French name||English name||Botanical name|
|0300||Galant de jour||Day jessamine||Cestrum diurnum|
|0500||Cupidone bleue||Cupid's dart||Catananche caerulea|
|1000||Cierge a grande fleurs||Night-blooming Cereus||Selenicereus grandiflorus|
|1200||Nyctanthe du Malabar||Night-flowering jasmine||Nyctanthus arbor-tristis|
|1700||Belle de nuit||Moonflower||Ipomaea bona-nox|
|1900||Geranium triste||Geranium||Pelargonium triste|
|2100||Silene noctiflore||Night flowering catchfly||Silene noctiflora|
The Edinburgh Floral Clock
Unfortunately, a flower clock which is both attractive and useful is an impractical proposition. This is because many of the flowers look very similar, being yellow weeds; and many also occur in particular ecological niches. To overcome this, various attempts have been made to illustrate the concept, usually in the form of a clock-face decorated with the image of the flower that is open at that time of the day.
Other so-called 'flower clocks' don't even attempt to illustrate the concept, but consist of a clock-shaped flower bed planted up with attractive flowers. A particularly fine example can be seen in Edinburgh's Princes Street. Here, the floral clock has hands operated by a clockwork mechanism. The clock face is made up of thousands of small plants, mainly sempervivens, and the clock hands are also covered in plants. The Edinburgh Floral Clock is used to publicise organisations and special events.
...and another thing
A floral clock features in the fictional city of Quirm, in Soul Music, one of the books in Terry Pratchett's Discworld series.