Plants actually use very little of the light they receive. Light can be broken into wavelengths that we see as colors. Red, on one end of the spectrum has the longest wavelength, while blue and violet, at the other end of the spectrum, have the shortest wavelengths. Plants absorb only the wavelengths that we see as red and blue. The middle green wavelength is reflected away, which is why plants appear green.
Blue light is essential for plant growth. It affects chlorophyll formation and is required for photosynthesis to take place. It is also responsible for vegetative growth. Plants grown under blue light tend to be shorter, bushy plants with thick stems. For this reason, blue light is preferred for growing seedlings and foliage plants. For flowering or fruiting to take place, plants also need red light.
Exposure to red wavelengths, which help control maturation, is necessary for the production of flowers, seeds and bulbs. It is also responsible for the elongation of stems. Red light alone will not produce healthy growth. Daylight contains a higher concentration of red wavelengths during the early morning and evening hours, which coincides with the highest rates of photosynthesis before daytime temperatures get too hot and plants shut down.
Artificial Light In Agriculture
The knowledge of the effects of light wavelengths, along with day length and temperature, has allowed the agricultural industry to manipulate plant growth in a controlled environment. Plants such as poinsettias or hydrangeas can be manipulated to bloom in time for a certain holiday, while the growth of seedlings can be manipulated to produce stocky, bushy plants early in the season.
Using Light at Home
The amateur gardener can also use light to manipulate the growth of plants. Specialized grow lamps can be purchased but are not necessary. Light bulbs, whether fluorescent or incandescent, are rated by color temperature. Bulbs with a rating of 4100 degrees Kelvin (4100K) will produce a bluer light. Bulbs rated between 2700K and 3000K will produce a longer, red wavelength. Full spectrum lamps will have a color temperature between 5000 and 5500K. By combining different bulbs, the home gardener can successfully grow plants under lights indoors.
Fluorescent or Incandescent
Color temperature isn't the only factor to consider when setting up an indoor lighting system. Heat and distance are also factors. Incandescent bulbs generally produce a redder light but are less efficient, and are designed to throw light over a wide area. They also produce a great deal of heat and can burn plants.
Fluorescent bulbs are much more efficient and burn cooler. By combining cool white (4100K) bulbs with warm white bulbs (2700K to 3000K) you can provide the necessary light to grow flowering and fruiting plants. Foliage plants require 15 watts per square foot of growing area, while flowering plants require 20 watts per square foot. A fixture that holds two 40-watt bulbs will provide 80 watts and can light an area of about 5 1/3 square feet of growing space. Doubling the number of bulbs to four will provide double the square footage. Fluorescent lights are generally suspended 12 to 15 inches above the growing area.
For larger plants that need only some supplemental lighting, spotlights can be installed. Full spectrum or mercury vapor lamps work best for flowering plants and can be suspended 6 feet or more above the plants.