Floral clocks and flowering thyme

While preparing for last weekend's chat about 'time' on ABC RN's Blueprint for Living, I got diverted on Carl Linnaeus's horologium florae, his floral clock. An eccentric and impossible folly, but what fun!

Here in Melbourne we are proud of our floral clock, a nineteenth-century sensation reaching Australia in the 1960s. It consists of a buried clock, Swiss made, with giant hands sweeping across a carefully manicured garden bed. At times long-lived box hedges have added structure, but mostly annuals such as begonias and marigolds contribute bling to the timepiece. Beautiful – at least to some eyes – and reasonably functional.

The original concept of a Floral Clock was quite different, and even barmier. By the mid eighteenth century we were starting to get a grip on the variety and apparent vagaries of plant reproduction. Different plants not only flower at different times of the year, but at different times of the day. Some species are quite specific as to when they open and shut their flowers, to take advantage for example of obligate morning or evening pollinators. This led Swedish biologist Carl Linnaeus*, creator of what we call our binomial system of naming (e.g. Homo sapiens, for us, humans), to postulate a horologium florae.

With judicious planting, one can imagine a garden where the open flowers tell you the time day. In the Swedish city of Uppsala, where this idea first took root**, you might bounce out of bed when the Dandelion (Taraxacum officinale) flower-head unfurls at 6 am, rush to work when it shuts between 8 and 10 am, grab a sandwich at noon when the Field Marigold (Calendula arvensis) flower-heads close, leave work at 6 pm when the Sad Geranium (Pelargonium triste) finally opens its dull yellow flowers and, if you like, party until the Queen of the Night cactus (Selenicereus grandiflorus) blooms at midnight.

Let’s get something clear up front though. This is like picking your dream sports team of all time or forming the world’s best super band. It’s a paper or mental exercise. A true Floral Clock can’t be created in the real world. These plants won’t grow together in the same place, they won’t all flower on the same day of the year (each flower of the Queen of the Night cactus, for example, opens for one night only), you would have to re-calibrate for every degree of latitude or altitude, and depending on the weather some might not open at all.

While a full functioning Floral Clock is a folly, you could conceivably create a coarse local variant that might work in spring or thereabouts. You could start with two Victorian species of flax lily (Dianella), one of which (Dianella amoena) opens early to mid-morning, the other (Dianella tarda) early to mid-afternoon. But then you know when it’s mid-morning or mid-afternoon don’t you? Perhaps it is better to spend your time wondering how it is that plants spend their own.

Now, plants set their own clocks from the amount and quality of the daylight they receive. They have a system for tracking seasonal changes in day length, triggering when they flower or renew growth. The variations within a single day are controlled by light, temperature and even humidity, plus an inbuilt circadian rhythm like animals. Such precision is the result of evolution, with successful plants protecting their assets (flowers) until a suitable pollinator is active. For the night-flowering cactus it’s a moth. For the two flax lilies, it may be the same or different species of native bees.

Some plants will open and close their flowers on a daily cycle, by growing new cells every day in the inside (to open) or outside (to close), or by expanding and contracting existing cells in the petals. It’s worth the effort to protect delicate parts from wind, rain and dew, and to keep pollen dry and ready for action. Plants growing in tough environments are more likely to have these diurnal cycles. A good example is Pigface (Carpobrotus glaucescens), a succulent adapted to dry sand dunes, which has daisy-like flowers that close up every night. The Sensitive Plant (Mimosa pudica) folds up its leaves each night to protect that equally important, and in this case sensitive, part of the plant.

The flowering head of a Sunflower (Helianthus annuus) takes it all a step further, arcing during the day in response to its own internal clock. Young sunflower plants (with their flower-heads unopened) track the sun from it rising in the east to its setting in the west, then get themselves sorted overnight so they can do the same the next day. Once the giant yellow flower-head opens, it stops moving about and faces east.

The sunflower has a circadian mechanism behind its solar tracking, and then some internal signalling to position its mature flower-heads eastward. The solar tracking is caused by different growth rates on the sunny and shady side of the stem. During the day the cells on the east side grow a little faster than those on the west, and then vice versa at night. This all stops when the elongation of the stem stops and the best orientation for an open flower is facing the morning sun - warm flowers attract more bees.

Plants do keep time in other ways. Flowers can last a few hours (our night-flowering cactus) or a few months (moth orchids), and the entire life cycle can be a matter of a few months (desert annuals) to thousands of years (the Bristlecone Pine, Pinus longaeva). Annuals typically flower once and die, all within 12 months, but other plants, such as the Century Plant (Agave americana) and several palms, will live for a few decades then flower once and die. For most woody plants and trees, though, they flower each year and continue growing until like us they senesce.

There are plenty of seasonal cycles of course, allowing plants to survive and thrive in their particular nook and cranny of the world. We can manipulate these cycles, as we do with the Christmas Poinsettia. In mid-winter in Australia, a garden Poinsettia produces red or pink leaf-like bracts around its tiny flowers. The amount of colour change in these bracts depends on their exposure to sunlight, or rather lack of exposure. They need long 'dark periods' for about three months before they flower. Northern Hemisphere gardeners report that interrupting the ‘long night’ by even flashing with a torch during October through to December may stop the leaves changing colour.

To get our Poinsettias flowering, and ‘bracting’, in nurseries around Christmas we manipulate their environment. Glasshouses are set up to provide artificially long nights and the Poinsettia thinks its winter. Although a plant growing in my office building (hello Frank Udovicic!) persists in flowering close to Christmas without any overt manipulation, perhaps due to day length variations on its windowsill. Plant life is seldom simple and it seems you can’t really set your watch, or your Christmas shopping schedule, from a flower.

*Carl’s father devised their surname, Linnaeus, so he could register at university (before then he, like many others of the time, had no need of a surname). The Swedish word ‘linn’ is used for what we call the Linden or Lime Tree (Tilia) and the freshly minted name honoured an impressive specimen of Linden growing beside their family home. So it is no wonder Carl had such a passion for plants. Perhaps too much. His classification system for plants was based largely on their sexual organs, the flowers, and considered by some to be a little too explicit – loathsome harlotry according to one contemporary commentator.

**My source for the plants Linnaeus proposed for his horologium florae was a copy of The Natural History of Plants by Anton Kerner von Marilaun, published in 1896, and part of the State Botanical Collection library at Royal Botanic Gardens Victoria.

Images: Flowering Thyme, of course. In the Royal Botanic Gardens Victoria, Melbourne Gardens, a few months ago. And just above. a much reproduced image of Carl Linnaeus, this one from a sign in the Jardin Botanico Atlantico, Gijon, Spain