The best light for seedlings is an approximation of natural, outdoor sunlight. When available, seedling containers may be placed in the direct sunlight provided by a southern facing window inside the home. This type of light source may be recreated using a combination of different types of fluorescent bulbs. These bulbs are generally energy efficient and have a lower heat output than other light sources, allowing them to be placed as close as possible to the germinating seeds.
Natural outdoor sunshine provides the widest range of light for seedlings in the full color spectrum that is necessary to nurture strong and healthy roots and leaves. This type of light is typically only available for maximum periods of time through a large, south-facing window in the northern hemisphere or a north-facing window in the southern. Windows facing other directions in the home tend to create lengthy periods of shade which do not allow the seedlings to sprout adequately. Growing containers need to be placed as close as possible to the window to absorb a maximum amount of sunlight. Seedlings placed in this type of location can be rotated regularly throughout the day to prevent top heavy growth on only one side of the plant that can topple thin sprout stalks.
When such a window is not available, natural sunshine can also be mimicked using man-made means available at most local home improvement stores. Young plants tend to respond well to long exposure to fluorescent bulbs. Best results may be achieved by combining one warm white bulb, and one cool white bulb together in a workshop light type assembly. Shop lights are typically designed using metal housing, and provide a reflective surface behind the bulbs that amplifies the light.
Any type of light for seedlings, whether fluorescent or high-powered grow lights, should be placed relatively close to the growing containers; 2 inches (5.08 centimeters) is a maximum height at which the lighting fixtures should be placed from the plants. This provides the level of intensity young seeds need to sprout and create thick, healthy stems. The heat output of fluorescent lights tends to be extremely low, and this close distance to the plants does not risk burning the tender leaves and stalks.
Some gardeners may prefer the use of high intensity discharge grow light for seedlings. These bulbs, depending on their type, tend to favor one end of the color spectrum over another. Bulbs which shine brightest in the red and orange spectrum can encourage fast flowering, while bulbs that favor the blue and green wavelengths produce heartier stalks and leaves. Gardeners who wish to cultivate a fast harvest may hang these lights above their seedlings, and should maintain a distance up to 3 feet (approximately 1 meter) from the plants to prevent burning, which can occur with these higher heat output light sources.
The light source can be attached to an automatic timer inserted in the electrical outlet. This timer can be set to turn the lights above the growing containers on and off at set times throughout the day. Exposing the seedlings to periods of light and darkness simulates the night and day cycles they are naturally accustomed to outdoors, and provides much needed periods of darkness necessary to encourage flowering.