When you read about the horticultural needs of plants, you may come across the terms 'day length', 'short day', 'long day' or the more technical term 'photoperiodism', and wonder what this is all about.
Plants can sense many aspects of their environment and can use changes in these as guides for growth. For some, especially arid zone plants, moisture from rain or floods may be sufficient to signal seeds to germinate and the resulting plants to grow, flower and reproduce. Seeds of other plants may require a certain number of 'chill-hours' to be met before starting to germinate. Others may depend primarily on 'day length' or various combinations of all of these factors.
Companies that supply flowers in bloom for cultural events - chrysanthemums for Mother's Day, Easter lilies for Easter, poinsettias for Christmas, etc. - have long manipulated the flowering season for their plants by controlling light exposure. This is usually done in greenhouses with shutters to keep out daylight, and overhead lights to produce light of the required duration and intensity at appropriate times.
Outside the greenhouse, the 'length' of the day or night is a result of the tilt of the earth, the season of the year and latitude. In equatorial regions, days and nights are of similar length all year round. As one moves farther from the equator, summer days are longer and winter days are shorter. At the poles, there can be months of darkness in the winter and continuous daylight through summer.
Plants vary in their dependence on day length. Some are strongly dependent. Coffea liberica is a short day plant and will only flower if the days are 13 hours or shorter. There are also long day plants and others that are called 'day neutral' and have no particular preference on this matter.
It was long thought that it was the length of daylight exposure that was the critical factor for photoperiod-sensitive plants. But it is now known that it is the amount of uninterrupted darkness that plants respond to, for example with flowering.
Plants produce a family of photosensitive proteins called phytochromes that can flip between two types depending on the intensity and duration of incident radiation. Their relative concentrations influence many plant phenomena, for example seed dormancy in autumn and satisfaction of requisite chill hours for seed germination, stimulation and inhibition of flowering in different species and increased chlorophyll formation in others.
Nurseries specialising in flower production used to leave the lights on in their greenhouses all night to delay flowering, until it was established that just a brief flash of light of sufficient intensity was sufficient to delay flowering in short day plants or promote it in long day plants. Control of the two phytochrome types can be achieved by artificial lights if their spectrum includes the important wavelengths (mainly red and far red).