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A Flower Clock Garden

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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.

The University of Uppsala in Sweden, where Linnaeus was a professor, has a public flower clock, and details of the flora can be viewed by clicking on the link (this link also has a flower clock which operates on 'Javascript'). To make a flower clock one must be prepared to replace species at the end of flowering.

What flowers can I use?

Approx timeFlower
0200Night blooming cereus closes
0500Morning glories, wild roses
0600Spotted cat's ear, catmint
0700African marigold, orange hawkweed, dandelions
0800Mouse-ear hawkweed, African daisies
0900Field marigold, gentians, prickly sowthistle closes
1000Helichrysum, Californium poppy, common nipplewort closes
1100Star of Bethlehem
1200Passion flower, goatsbeard, morning glory closes
1300Chiding pink closes
1400Scarlet pimpernel closes
1500Hawkbit closes
1600'Four o'clock' plant2 opens, small bindweed closes, Californian poppy closes
1700White waterlily closes
1800Evening primrose, moonflower
2000Daylilies and dandelions close
2100Flowering tobacco
2200Night 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:

TimeFrench nameEnglish nameBotanical name
0300Galant de jourDay jessamineCestrum diurnum
0500Cupidone bleueCupid's dartCatananche caerulea
1000Cierge a grande fleursNight-blooming CereusSelenicereus grandiflorus
1200Nyctanthe du MalabarNight-flowering jasmineNyctanthus arbor-tristis
1700Belle de nuitMoonflowerIpomaea bona-nox
1900Geranium tristeGeraniumPelargonium triste
2100Silene noctifloreNight flowering catchflySilene 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.

1The name Helichrysum is derived from the Greek word Helios meaning sun and chrysos meaning gold, which refers to the bright yellow colour of their flower heads. The Helichrysum belong to the daisy family.2The 'Marvel of Peru', Mirabilis species. This was a great Victorian favourite, especially in flower clocks, but seldom grown now.

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