Light is only one of the necessary elements that affect plant growth. Nutrients, soil and water have a big impact as well, but light is different in that it is a necessity, and also facilitates the use of the other necessities. In order to grow the strongest, healthiest plants, a grower needs to understand the impact of lighting on plants and how to regulate the proper light levels for their plants.
Growth and Leaves
The natural spectrum of color in a ray of light includes all the colors of the rainbow, but only two are ultimately necessary for plant growth--red and blue. They are the colors a plant will separate out of the spectrum, and each has a different affect on plant growth. Red light encourages flowering. Plants that do not flower need less red than they do blue. Blue light promotes leafy vegetation and is necessary for all plants. Plants absorb most light, but filter out the color they do not need. Plants appear green to us because they reflect that color rather than absorb it.
Plants require light in order to produce the proper nutrients for their survival. They use light to produce vital elements such as sugars and starches for growth.
Natural light produces heat. Heat is necessary for many plants to grow properly. Other plants require a definite "cool" period at regular intervals approximating a winter solstice in order to hibernate. Indoor lighting does not provide the same warmth as natural lighting so growers must provide an extra source for warmth.
The amount of light is as important as the color of light. Whether you grow outside or inside, you must know how much light your plants are getting, and regulate it according to their specific needs. Human eyes do not have the ability to properly gauge light levels because they adjust to low-light levels, according to Ray Rothenberger of the Department of Horticulture at the University of Missouri-Columbia. A proper light meter will give an accurate reading of the light level.
Even when you are certain of the level of light, the plants are the best indication of proper lighting. Watch the growth of the plants, and the development of the leaves and stems. Poor lighting produces long, leggy stems and poor leaf development. A lack of light can cause a plant to lose leaves and/or develop poorly colored, pale leaves that are few in quantity. While it is nearly impossible to give a plant too much light, if the light source is too close, it could burn, so watch the leaves for signs of browning, curling, or shriveling.
Nature is hard to beat, and natural sunlight provides the original "natural full spectrum lighting." Even under the best circumstances it isn't always possible to get enough natural sunlight to your plants, so the next best option is artificial lighting. The science of artificial plant lighting has come a long way in the last decade and natural full spectrum (NFS) lighting from a bulb is an effective means of producing indoor light. The NFS bulbs make it possible for growers to make plants that aren't native to their area grow and thrive, they even allow food production during the winter months where outdoor environments would prohibit it. Choose bulbs with care. Always choose bulbs designed for plant use. Regular incandescent bulbs are too hot, and do not provide proper spectrum color. They are an adequate source of red light, but provide very little blue, according to Rothenberger.