A “foot-candle” sounds like a gadget that might have been advertised in an early edition of The Old Farmer’s Almanac, but actually it is a measure of light, specifically the amount of light cast by a candle over a one-square-foot area from the distance of one foot away. To put things in perspective, note that on a sunny summer day an exposed garden gets 10,000 to 15,000 foot-candles of light. A fluorescent lamp (two 40-watt bulbs) can provide 800 foot-candles if it is practically sitting on top of a plant, but move it back even a foot and the number drops to under 300.
In order to conduct photosynthesis—the process of converting raw nutrients to carbohydrates, thus enabling the plant to grow and bear fruit—plants require both quality and quantity of light. Windowsills and greenhouses can supply plants with most of the light quality and energy sent from the Sun, but certain wavelengths, such as ultraviolet, are altered when they pass through even one thickness of glass. This is why you can’t get a suntan by sitting in front of a picture window.
While many houseplants and seedlings will do quite well over the winter in a window location, most benefit from a supplement of artificial light and, when weather permits, a few hours basking on a sunny porch. Variegated leaves, for example, will be more pronounced with improved lighting. Seedlings have especially high energy requirements and do best indoors under grow lights.
Duration of light, called the photoperiod, is critical. For most plants this period is at least eight hours daily, regardless of how many foot-candles are being produced. Conversely, plants also respond to darkness. Short-day plants, such as poinsettias, chrysanthemums, and Christmas cacti, can actually be prevented from coming into flower by lengthening their days with artificial light. Lengthening the hours of darkness by putting these same plants in darkness by the end of the afternoon will encourage bloom. Most fall-blooming perennials are short-day plants.
You may want to set up a “light station” where you can rotate your plants that seem to want a little luminary boost. You might also consider setting up your seed-starting area a bit earlier for use as a light nursery for winter-weary houseplants.