While plants need light for survival, flowering is one of the points in its life’s where light or its absence gains additional significance.
Some plants’ need for the certain length of darkness and light to flower is called photoperiodism (1). This is important not just in the production of ornamental flowers, but also for growing vegetables and fruits.
- Long-day plants need longer days with 14 to 16 hours of daylight and short nights to flower. These are the plants that will flower in summer or grow in regions where there are 12 and more hours of daylight. Common long-day flowers are asters, Asiatic lilies, and coneflowers; among vegetables potatoes and beetroots are known to be long-day plants (1, 2).
- Short-day plants need less daylight -8 to 10 hours- and long nights or 14 to 16 continuous hours of darkness; chrysanthemums, poinsettias, and soybeans are some examples (1, 2).
- Day-neutral plants can flower regardless of the length of darkness they are exposed to, as in the case of roses, sunflower, tomatoes, cucumbers, corn, and some varieties of strawberries and peas (1, 2).
It is not just the duration of light that is important in commercial flower production, but also the colour and intensity of light.
Duration of Light
In temperate countries, where greenhouses are used to extend growing time, the shorter lengths of winter days can be a problem, when growing long-day flowering plants.
Light has to be provided to prolong day length for optimum plant growth. Asiatic Lilies that are forced to flower for the holiday season or for Easter have to bloom in winter outside their natural flowering time. These plants need 14 hours of light to form buds and for them to continue their development into flowers (3).
In the case of cannabis, the type of light source is important; it was found that LEDs promoted quicker flowering compared to other light sources (4).
Colour of Light
Flowering can be triggered also by using the right color of light. Narrow-width band lights like LEDs have an advantage over traditional lights here, since specific colors and hues can be used.
Far-red and red initiate flowering in Asiatic Lilies a long-day plant, while rose that is day-neutral doesn’t need any specific color to flower (5). The right light color can increase the number of flowers in some species. For example, a combination of white, red, blue, and orange LEDs can increase the number of flowers in Asiatic Lilies (3).
Intensity of Light
Flowering and photoperiodism can be manipulated by the amount of light or intensity, which is measured in photosynthetic photon flux density or PPFD (6).
Many plants need high light intensity to flower, such as sweet peppers (7). In Asiatic lilies, bud and flower development will suffer without sufficient light intensity (8).
However, more is not always better. In the case of rose, the yield of flowers will decrease if temperature increases due to the use of more light (9). In this case, LEDs are the better choice as they produce less heat due to radiation compared to traditional lights (10).
LEDs are the Better Choice
Since growing flowers indoors or in greenhouses requires more light, LEDs which use 60% less energy than traditional lamps are more efficient, and therefore more economically viable and environmentally sustainable. Moreover, the ability to supply a targeted spectrum of light to improve yield and shorten growing time makes LEDs the optimum choice.
- Singh D, Basu C, Meinhardt-Wollweber M, and B Roth. LEDs for Energy Efficient Greenhouse Lighting. Hannover Centre for Optical Technologies, Nienburger.