March 2013: Linnaeus’ Floral Clock

Spring is in the air, and we can see early spring flowers opening to attract dozy bumble-bees and other insects.

Wood Anemone (Anemone nemorosa), for instance, definitely gets a head-start before the rest of the vegetation catches up – its beautiful, star-like white flowers attract flies and beetles that eat its pollen.

And although we are all familiar with the concept of certain flowers blooming at certain times of the year, flowers have an even more sophisticated inner clock. So sophisticated, in fact, that we could use them to tell what time it is.

Carl Linnaeus observed over a number of years that certain plants consistently opened and closed their flowers at particular times of the day, these times varying from species to species. He concluded that one could deduce the approximate time of day according to which species had opened or closed their flowers. Arranged in sequence of flowering over the day they constituted a kind of floral clock or “horologium florae”, as Linnaeus called it in his Philosophia Botanica (1751, pages 274-276).

Here, Linnaeus differentiated two groups of flowers:

• Flowers that vary their opening and closing times, either in response to the weather (Meteorici) or depending on the length of the days (Tropici) 
• Flowers that have fixed times for opening and closing (Aequinoctales)

Naturally, only the latter are useful for a flower clock.

Linnaeus probably did not plant a flower clock himself, but, as usual, made accurate observations on many different plants in different habitats.

As many of the indicator plants described by Linnaeus were wildflowers of his native Sweden, and the opening/closing times depend on latitude, the complexities of planting a floral clock make it a difficult undertaking.

A detailed account of the Floral Clock in English can be found in F.W. Oliver's translation of Anton Kerner's The Natural History of Plants (1895, vol.2, pages 215-218).

The BBC has provided a list of plants for people keen to try planting a floral clock in their own garden. One flower was apparently even called the “Four o’clock plant” by the Victorians (the “Marvel of Peru”, a Mirabilis species). 

Various attempts have been made to illustrate the concept, mostly in the form of a clock-face decorated with an image of the flower for the time of day, see the Plant Physiology website for a beautiful example.

An article about Linnaeus' Floral Clock was published in The Linnean in 1987 and re-published for the Linnean Tercentenary in 2007.

So, time to properly wake up to spring (what about a floral alarm clock?) and ponder the complexities within that underpin the beauty of flowers we all enjoy so much after a long winter.

Unfortunately, the Linnean Society does not have a copyright-free depiction of Linnaeus’ Floral Clock in its collections. Images can be found, for instance, in Martinsson, Karin, Linnés blomsterur (Prisma, 2004).

Image of 'Marvel of Peru': Garden World Images