Winter may have just begun, but we’ve already had the shortest day of the year. It’s all up hill from here. The winter solstice fell this year on June 22. The days are gradually getting longer now as the sun rises a little earlier and sets a little later. Mind you, it’s still very dark here in Tassie even at 7.30am.
While a bit of extra daylight may brighten a winter’s day and make us feel more positive, it can have a dramatic affect on flowering plants. Many plants are triggered to flower in response to changing day length – a process called photoperiodism.
Photoperiodism can also trigger seed germination, growth and fruiting along with formation of tubers or even dormancy.
Short and long day plants
Some plants ‘count’ the number of hours of darkness that they experience. This is done by the production of an enzyme that responds to light and dark. Plants that flower in autumn and early winter may be triggered to bloom by lengthening nights. These are known as short-day plants.
Poinsettias are great example of a plant that flowers in response to changes in the duration of day and night. Naturally winter flowering, it is pushed into bloom by lengthening autumn nights. An understanding of its flowering trigger has allowed huge industry to blossom based on the manipulation of poinsettias, which are timed to bloom for Christmas.
While this is only tweaking in the northern hemisphere, where Christmas falls in winter, in the southern hemisphere it requires turning night and day on its head as summer’s lengthening days are artificially drawn to a close with blackout curtains.
Chrysanthemums also flower in autumn in response to shorter days and longer nights.
The lengthening days now however will be encouraging ‘long-day’ plants to switch to flowering mode. Peas are long-day plants so that’s good news for gardeners hoping for crops on pea plants. Other popular crops that also respond to longer days include beetroot, radish, potatoes and spinach.
Not all plants bloom in response to day length. Many plants are day-neutral plants – roses for example. Flowering in day-neutral plants is triggered by other environmental conditions such as the plant’s development or in response to a period of chilling.
Plants that flower when their flowering stems reach the right stage of maturity can be manipulated into flower by pruning. When roses are in full growth for example they generally come into bloom around six weeks after pruning. Pruning also brings many perennials back into bloom after their first flowering or can be used to delay flowering by pruning in early spring.
This is good news for a gardener who is wants to have plants in bloom for a special occasion such as a garden wedding. Rose growers also use pruning to time a flush of red roses for harvest in the week leading up to Valentine’s Day (February 14).
Pruning can also be used to delay flowering. Pinching out growing tips to encourage a more compact plant (for example tip pruning fuchsias or petunias) delays flowering by several weeks.
Flowering in response to hours of chilling (a process called vernalisation) is particularly important for blossom and fruit trees. Apples, cherries and a host of other spring-flowering trees need to accumulate hundreds of hours below 7ºC (45ºF) before they’ll flower. This is a mechanism that helps the plant control flowering in cold zones so that its blooms are not produced when it is too cold for them to survive and be pollinated.
[This story first appeared in TasWeekend on June 20, 2015 (a weekend supplement to The Mercury)].