Light is electromagnetic radiation that comes in waves, the basic units of which are called photons. Plants use light to turn carbon dioxide into sugars necessary for growth, a process called photosynthesis (putting together with light). Sunlight is a full spectrum of different wave lengths of light. Seen through a prism, this spectrum turns into bands of color: red, orange, yellow, green, blue, indigo and violet. The color of light measures its quality.
Light and Plants
Plants absorb light that they use to grow and to produce flowers, fruits and seeds. Plants look green to us because they reflect green light. They don't absorb it. So plants are unaffected by green light. Plants use other colors of light to fulfill basic needs of their life cycle. Plants are most affected by blue light and red light.
Sunlight gives off all colors of light, but the sunlight of the lengthening days of spring and the long days of summer is predominately blue. Plants that were dormant in the winter begin growing in the spring. Seeds begin to germinate. Therefore plants need blue light for the growth of stems, stalks and leaves. Germinating seeds, seedlings and transplants need blue light.
As the days shorten with the ending of summer, the blue light that dominated the spectrum of sunlight is slowly overtaken by light in the red band. Ripening fruits and vegetables are ordinarily harvested in late summer or early fall. Red light is needed for flowering and the production of fruit.
The color of light is a major consideration when you grow plants indoors. Fluorescent light that contains lots of blue light is good for starting seeds indoors. Full spectrum fluorescent light will encourage growth and flowering. The light produced by a metal halide lamp is intense blue, which is good for growing plants. Some greenhouses use high-sodium grow lights that yield abundant red light, to induce flowering and fruiting.