Indoor Lighting Principles 

Without sufficient light, plants are unable to produce green chlorophyll during photosynthesis. While they will never produce as much light as the sun, High Intensity Discharge (HID) lighting systems produce enough light of the proper wavelengths to encourage healthy growth even in the most sun loving plants. Since the sun also produces much light that plants do not use, modern garden lighting has been designed to provide only those portions of the spectrum that plants need, this allows for the most efficient use of electricity and ensures healthy growth in all stages of plant development.

The Basics

 Plants combine light energy with carbon dioxide (CO2), water, and nutrients to form green chlorophyll and carbohydrates, releasing oxygen as a byproduct. Light energy is measured in wavelengths which are divided into tiny units called nanometers (1 nm = .000001 meter). The most important colors of the spectrum for maximum chlorophyll production and photosynthetic response are blue (445 nm) and red (650 nm). Phototropism is the reaction of a plant to illumination. Foliage moves toward light (positive tropism) or away from it (negative tropism) depending on the spectrum and intensity emitted. Positive tropism occurs with most species in the blue end of the spectrum around 450 nm. At this optimum level, plants lean toward the light, spreading their leaves out horizontally to absorb as much light as possible.

Photoperiod

 The relationship between periods of light and darkness is called the photoperiod, and it affects the life cycles of all plants. Most house plants, such as gesneriads, oxalis, Exacum affine, and many begonias are "night neutral," meaning they will grow and bloom well with 6 to 8 hours of darkness. Chrysanthemums, poinsettias, Christmas cacti, and related plants bloom only if given a longer night of 14 to 16 hours of darkness. Summer blooming plants, such as Geraniums, annuals, tuberous begonias, most vegetables, and seedlings of virtually any plant require 18 hours of light per day. Even African violets will double their blossom production if given 18 instead of 12 hours of light a day. Plants such as rex begonias, aroids, palms, marantas, ferns, and others grown for their green foliage can adjust to varying photoperiods without much effect to their growth.

 Short-day plants remain in a vegetative growth stage while 18 to 24 hours of light per day is maintained. Flowering is induced most efficiently in short-day plants with 10 to 12 hours of light with a corresponding period of uninterrupted darkness. It is possible to gradually decrease the daylight hours while increasing the dark period to simulate a natural photoperiod, but this is not neccessary and usually delays flowering by several weeks.

 After short-day plants are 2 to 12 months old, altering the photoperiod to 12 hours each of light and darkness can produce an optimum flowering potential in 2 to 4 weeks, especially if the photoperiod includes high levels of intense light from the red end of the spectrum. Most plants fall into this 12-hour category as it replicates a photoperiod normally experienced during the longest and sunniest days of outdoor gardening season around spring equinox in June. More than 12 hours of light will prolong flowering, but a shortened or interrupted period of darkness through the use of dim lighting, inadvertent ambient light pollution, or even by suddenly opening the door to your indoor garden, can confuse the plants and cause them to try to revert to a vegetative growth stage. Flowering in this case can take forever.

Coverage Area

 Most gardeners recommend at least 25 watts per square foot of garden space. You may need less than this if you are supplementing natural sunlight, or if you are growing plants, such as lettuce, that do not require as much direct light. Conversely, some gardeners try to achieve faster growth rates by significantly exceeding the recommended wattage. They must pay close attention to the temperature in the garden since HIDs can emit an intense amount of heat. While there's no such thing as too much light, using a large lamp in a small space can result in high temperatures and increased humidity that can be difficult to control and encourage pest infestations. Ventilation becomes increasingly important as your indoor garden is lit with more and more HID light systems. To reduce temperature control problems in your greenhouse or solarium, supplemental HID lighting may only be used during dark periods or on cloudy days when it will be most effective.

 Differentreflector designs will spread light differently, and the height of the reflector above the plants is also a factor. Remember that light is most intense in the center of the garden area, and becomes less intense as it radiates away from the bulb. The average distance from plant to bulb is 18 to 24 inches. Lower wattage bulbs may be placed closer while higher wattage bulbs may need to be placed farther away. Note that the higher the lamp is placed, light will spread to a larger area and be less intense when it hits your plants' leaves, lamps placed closer to plants provide more intense light but to a smaller garden area.

 HID lights will increase your electricity bill from $5 to $50 a month depending on the size of your system and the number of hours in operation, see details here. Remember, though, that most grow lights are extremely efficient, especially when used and maintained properly, and that you are getting a lot of energy for your money.