If you don't have a window with a southern exposure and strong enough sunlight to produce good seedlings, you can use a fluorescent light. Even with good southern exposure, if your plants are in the windowsill, a long spell of cloudy weather could interfere with good growth. Most of the time it's necessary to provide additional lighting so seedlings receive enough light for healthy plant growth. Although artificial light does not duplicate sunlight, there are lights that produce the right combination of waves to stimulate good growth in plants. Incandescent bulbs, while high in red light, are low in the blue and violet ranges. Incandescent bulbs can bum hot enough to destroy young seedlings if placed too close, and we don't recommend using them.
Ordinary fluorescent lights are best. They produce almost no heat and the seedlings can be set close to the lights so the plants receive strong, steady light. Results using fluorescent light are very good. The seedlings should be kept 4 to 6 inches beneath the lights for 12 to 18 hours per day. You can use a timer to turn them off at night and on again the next morning. Remember: Provide good air circulation and never let the planting mix dry out completely.
It is not true that seedlings need a period of darkness each day. At Burpee, some gardeners grow their seeds under lights twenty-four hours a day when they want faster growth. The low-intensity lights give the plants only as much light as they can absorb. The only danger with using lights 'round the clock is that the seedlings need to be watched more closely; they will need more water, and more frequently will have to be moved to maintain the right distance (4 to 6 inches) between the light and the top of the plants.
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