Artificial lighting is crucial for successful indoor gardening, but not just any indoor garden lights will do. Inadequate lighting will result in tall spindly plants, yellowing leaves and failure to bloom or set fruit. Different types of lights are best suited to different kinds of indoor gardening, so select the right kind of indoor garden lights for your plants' requirements.
Plants use sunlight to convert carbon dioxide into sugars for growth. While houseplants usually require low amounts of sunlight to thrive, garden plants--flowers, herbs, or vegetables--generally need six hours of direct sunlight per day to successfully bloom or set fruit. It is difficult to get this much direct sun through a window: Building orientation, overhanging trees and window screens block significant percentages of available light. Artificial indoor garden lights function as a sunlight substitute.
High-intensity discharge lights containing metal halide or sodium vapor produce powerful artificial lighting outputs, albeit with an expensive price tag. Regular incandescent reading-light bulbs produce too little power and too much heat to be effectively alone for indoor gardening, but their red light-wave emissions can be useful. Florescent light tubes come in many different color wavelength outputs that can be combined to produce sufficient indoor garden light at an efficient price.
Artificial lighting produces significantly less candle power than the sun, so indoor garden plants require a considerable period of time each day under the lights. Plants require a period of darkness daily, too, so do not leave lights on around the clock. The University of Vermont Extension office recommends 16 to 18 hours of artificial lighting per day for seedlings and plants with high light requirements.
Plants primarily use the red and blue wavelengths of the spectrum of visible light. Cool florescent light tubes produce ample blue light for most plants' needs, but can fall short on red wavelengths. Full-spectrum florescent light tubes developed specifically for plant growth are available but more expensive than ordinary cool-white tubes. David Trinklein of the University of Missouri Extension Service recommends supplementing cool-white florescent bulbs with a few incandescent bulbs of approximately one-third the wattage to add needed red light waves to the indoor garden environment.
Sodium vapor lights provide a good balance of red and blue light, but they tend to produce a distinctly orange-to-pinkish effect in their visible light, which can make them unattractive for in-home use. Metal halide lights are closer to white in appearance, but, like sodium vapor lights, require a large fixture that has the effect of making its use impractical for indoor gardening on most home shelves or windowsills.
Sodium vapor and metal halide lights are used by some commercial hydroponic growers, and might be necessary in order to grow the most sun-loving vegetables or garden flowers indoors for consumption or market. Florescent bulbs can simply not produce enough light to grow some garden vegetables successfully indoors. Tomatoes, for example, require 3500 candle power, but according to Dr. Perry at the University of Vermont, two common florescent tubes placed 6 inches above a plant generate only about 700 candle power. Even four florescent bulbs directly above the tops of tomato plants might not bring sun-loving garden plants to fruit. Leafy green vegetables and flowers that can tolerate partial shade in the garden can thrive under florescent garden lights.