In general, the most appropriate type of light to use for growing all varieties of indoor plants, including seedlings and bulbs, is a cool-white fluorescent tube light. Used in conjunction with other equipment, a bright plant light is essential for germinating strong, healthy seedlings. Many flower bulbs, on the other hand, are not suitable for growing indoors at any time, though the handful of bulbs marketed for indoor-only culture can benefit from strong artificial light during its growing season.
Characteristics of Plant Lights
Plants generally require only two colors of the white-light spectrum: blue and red. Blue light encourages healthy growth of leaves and stems, and red light combined with blue is necessary for the plant to produce flowers. Though sunlight provides the proper wavelengths of light, most indoor plants receive inadequate amounts of this light. Inexpensive and widely available, artificial fluorescent lighting is the best type of supplemental lighting to help indoor plants flourish and grow.
Seedlings and Sunlight
Though plants with mature root systems and healthy foliage may flourish in a sunny, south-facing window, the amount of sunlight entering through a window is almost never strong enough for healthy seedling development. Seedlings grown even in the sunniest windows tend to grow long and leggy, with pale stems that are weak and susceptible to damping off. For optimal seedling growth, young plants almost always require an additional source of light.
Benefits of Plant Lights for Seedlings
To ensure that germinated seeds develop into seedlings with thick, stocky stems, they must be exposed to a source of very bright light. A fluorescent tube light placed no more than 4 inches above seedlings encourages sturdy stem formation and rapid foliage growth. By raising the light as needed as the seedlings grow, young plants will be healthier and more prepared for outdoor transplanting than their window-grown counterparts. Seeds will also germinate faster at higher temperatures, so using an electric seedling heat mat helps seeds sprout quickly.
Plant Lights for Bulbs
Most potted flowering bulbs sold for indoor displays, such as paperwhites, hyacinth, crocuses and tulips, have been artificially forced to bloom before their season. After blooming, these bulbs typically do not ever bloom again in an interior setting, but will perform wonderfully when transplanted outdoors. Reblooming bulbs such as amaryllis benefit from fluorescent lighting after blooming is finished and the foliage has emerged, helping the bulbs re-form for next year's blossom. The bulb requires a cool-temperature dormancy period in the fall, usually two or three months at around 50 degrees.
The shamrock plant (Oxalis species) is another indoor bulb that enters dormancy on its own each year. After breaking dormancy, placing the plant under fluorescent lights helps it quickly regrow a lush crop of new foliage.
Problems with Incandescent Lights
Whether for bulbs or for seedlings, never use incandescent bulbs to provide additional light for your indoor plants. Incandescent, or traditional filament-type light bulbs, not only emit damaging levels of heat but also radiate wavelengths of light in the yellow, red and orange spectra. Though the yellow color is pleasing to the human eye, this type of light is useless to plants. Bulb-type flowers and especially seedlings will quickly overheat, wither and die when placed under strong incandescent lighting.