Light Spectrum and Plant Growth

Light has profound effects on the development of plants. The light-mediated changes in plant growth and development are called "photomorphogenesis". The most striking effects of light are observed when a germinating seedling emerges from the soil and is exposed to light for the first time. Light plays a vital role in photosynthesis. The amount of light a plant receives per day, the intensity of light and the wavelength or light spectrum all affect plant growth and development. The light spectrum emitted by the sun is most visible in rainbows as bands of red, orange, yellow, green, blue, indigo and violet. Of all these wavelengths, only the colors at the far ends of the spectrum--blue and red--have any notable effects on plant growth.

Chlorophyll
Chlorophyll is a green pigment found in cyanobacteria and the chloroplasts of algae and plants. It is vital for photosynthesis, which allows plants to absorb energy from light. Chlorophyll plays a crucial role in a plant's ability to absorb sunlight. Sunlight from the violet-blue and reddish-orange-red spectrums is mostly absorbed through chlorophyll. Plants such as grass that have a lot of chlorophyll reflect sunlight from the green and yellow spectrums, thus allowing us to see these plants as different shades of green.

Photosynthesis
Photosynthesis is a process used by plants and other organisms to convert light energy, normally from the sun, into chemical energy that can be used to fuel the organisms' activities.  Photosynthesis uses a series of chemical reactions to convert the light captured by chlorophyll into the essential elements of plant growth such as cell walls. This conversion creates the food that plants need to survive and thrive. The waste product of the photosynthesis process is oxygen, which is essential for the survival of all living things.

Blue Light
The shorter wavelengths of blue light, also known as cool light, trigger chlorophyll production, which ends plant dormancy and starts the development process. Plants initially grown under blue light will have compact growth with thick leaves and strong stems. Outdoors, this light is strongest in spring and summer. Inside, home gardeners often use the blue light spectrum in the form of fluorescent lights to start seedlings.

Red Light
Combining the longer wavelengths of the red light spectrum with the shorter blue light spectrum wavelengths will begin the next phase of plant growth. This phase encompasses germination; development of the plant's underground network of roots, tubers or bulbs; and finally blooming and the development of fruit. Red or warm light is more prevalent as days shorten in the fall and winter. Eventually, this will cause the onset of dormancy in some plants. Red light is the most effective kind for photosynthesis.

Far-Red Light
The wavelengths included in the far-red region are those that are in the range of 650 to 800nm, although some authorities put the cut-off point closer to 740nm than 800. The low end is the important part. The red wavelengths are in the range of slightly less than 600 to 650 nm. And far-red light is mostly used by the plant to detect competing plants.

Green and Yellow Light
Plants reflect green and yellow light, which are in the middle of the color spectrum. Thus, these two wavelengths have little effect on plant growth.