Light is measured in foot-candles — the amount of light cast on a square foot area by a candle at 1 foot distance — and the sun on a clear summer day can bathe us in 10,000 foot-candles. Contrast that with the paltry 500 foot-candles dribbling down on an overcast winter day.
Houseplants' problems are further compounded by windows, which cut sunlight by another 10 per cent. No wonder these plants, if they are growing at all, stretch for light this time of year.
There are ways you can help them.
One is to clean your windows. Any dirt on the glass cuts down light. While you're at it, dust or spritz off your plants' leaves; dust has the same ill effect.
Fortunately, many houseplants hail from the shade of tropical jungles. Still, flowering and fruiting take energy, which comes ultimately from the sun, so if you want flowers or fruits from such plants as Jerusalem cherry, flowering maple, citrus and miniature roses, you have to arrange for abundant light. Otherwise these plants will just stay alive, might even grow, but will not flower and fruit.
HOW BRIGHT IS YOUR WINDOW?
How much light is enough? Most flowering and fruiting plants need 1,000 or more foot-candles, although some, such as African violet, rex begonia, flowering maple, zebra plant and crown-of-thorns, will provide colorful displays even at about 500 foot-candles. Below that level, stick strictly to foliage plants such as cast iron plant, Swiss cheese plant, baby's-tears, parlour palm, pothos and ferns.
You can translate those needed foot-candles into light measured by a digital SLR camera. Set it on aperture priority with the aperture at f/8 and the ISO at 100. Hold a white sheet of paper so that whatever light you're measuring falls directly on it, and measure the shutter speed reading that the camera gives you (without a flash) for a good picture from about a foot away. Multiply the shutter speed times 4 for the approximate foot-candles. (Shutter speed is usually expressed as a fraction of a second, so a speed of "500" is really 1/500th of a second; for foot-candles, you'd multiply 500 times 4 for 2,000 foot-candles. If light is very dim, the shutter speed might be more than a second; no need to measure, in that case, because in such light any plant will barely stay alive.)
Take measurements at various locations and times of day; you'll probably be surprised at how little light falls near even a bright window.
If such exactitude is not your style, just gauge light by window direction. A south-facing window is brightest, followed by east and west-facing windows, with north windows being the darkest. Any obstruction — even a leafless tree — will reduce light levels, as will moving a plant back from a window.
ARTIFICIAL LIGHT CAN HELP, MAYBE
Natural light can be augmented with artificial light. Don't expect too much from artificial light, though, especially for large plants, the bulk of whose leaves cannot get close to the light source. A fluorescent light, for example, casts as much as 900 foot-candles of light, but that's only within 6 inches of the light bulb. Light levels drop dramatically as you retreat from the bulb — with the square of the distance, so double the distance and you have only one-fourth of the light, triple it and you have only one-ninth, etc.
In addition to intensity, the spectral distribution of the light can impact plant growth. Flowering requires more light at the red end of the spectrum; fluorescent light tends toward the blue end of the spectrum.
Bulbs other than fluorescents have their own advantages and disadvantages. Incandescent lights convert much of their energy into heat, so you can't put a plant close enough for a dramatic effect on growth without scorching leaves. Special high intensity lights, such as mercury vapour and sodium lights, can dramatically increase growth, but the intensity and spectrum of the light will make your living room look more like a hospital operating room. LED (light-emitting diode) bulbs also have potential for indoor plant growing. They are efficient, and the spectral output can be tailored to plant needs.
My own plan is to wait out the sun. It's reassuring to watch plants naturally gather steam as the days grow longer.