It is common knowledge that plants need light to grow, and that natural sunlight tends to work best for growing plants. This is because the sun produces many different kinds of light, or wavelengths of light, all of which produce different responses in plants. In the visible spectrum of light, these wavelengths are seen as colors and known as photosynthetically active radiation (PAR).
Light travels in waves. Ultraviolet light has the highest frequency, though it is not visible to humans as color. Far red light has the lowest, and it is also invisible. The visible light spectrum can be broadly classified into three major colors for determining a plant's response: blue, green and red. Blue has the highest frequency, followed by green, then finally red. Plant chlorophyll absorbs and responds more readily to the blue and red spectrum, with little to no response in the green spectrum.
Blue light, responsible for producing vegetative growth, is most helpful while germinating and cultivating young plants or vegetables such as spinach or lettuce. When the seasons change and the sun is lower in the sky, there is more red spectrum light available to plants. This change is detected by photoreceptors, which are light-detecting mechanisms in plants. The change in light color triggers flowering and fruiting.
In experiments where a colored film is placed over a light source, plant species tend to respond differently. In her article Photomorphogenesis, Kathleen Yeomans explains that blue light can make petunias grow tall, but four-o'-clocks and tomatoes stay short and sturdy. Conversely, red light produced average-sized tomatoes, tall and gangly four-o'-clocks, and the petunias stayed short. These results indicate that certain light conditions are overall ideal for one species of plant, but less than ideal or even detrimental to others.
Normal fluorescent or incandescent lights produce either light of low intensity or too much heat to grow plants by themselves. Grow lights, on the other hand, have been engineered to produce intense blue light for vegetative growth or a combination of intense blue and red to provide ideal light color for plant growth. These lights produce less heat than incandescent bulbs, so plants do not burn.
Research is underway to produce greenhouse light filters that convert ultraviolet and green light into red and blue light, while allowing natural blue and red light to reach plants unhindered. This would provide more usable light for the plants inside while blocking the kinds of ultraviolet rays that cause genetic and vegetative damage to plants. These advances along with artificial light supplements help increase the quality and growing capabilities of crops and ornamental plants.