Following from the previous post.
Theoretically, a window is the best place for indoor plants that like it bright: it gets the most direct sunlight, which even in winter is stronger than most indoor illumination. But you have to think about duration as well. In the northern hemisphere, a south-facing window gets the most sun. But even then, unless the window is very broad, or a bay window, the sun in winter won’t shine through long enough to satisfy most sun-loving plants. And of course, that assumes that there is sun – around here, it’s not unknown for the sky to stay constantly overcast for one or even two weeks at a time in winter. In the old days, plants that naturally grew in shady conditions tended to make some of the best (i.e. easiest) houseplants.
Nowadays, plant lights come to the rescue. Actually, in my experience, regular (modern) incandescent or fluorescent bulbs work just as well as so-called grow lights of similar wattage, but let’s pretend for a moment I didn’t say that. These days we can get all sorts of bulbs designed to optimise plant growth in terms of intensity and wavelength, and many of those are more energy-efficient than incandescent bulbs.
You’ll notice I brought up wavelength again. At different times of year, the sun strikes the temperate zones at different angles, and this makes certain wavelengths dominant at different seasons. This phenomenon works along with intensity and sometimes photoperiodism to affect plant growth, and can in fact be the determining factor in whether or not your plant grows steadily, blooms, or simply sits there. In general, red-dominant (warm) or blue-dominant (cool) light has the most effect, but unless you know exactly what you’re doing, don’t be fussed about it. Oh, and it has little to do with what colour the bulb or light appear to be (remember what I said about plants perceiving light differently?). Full-spectrum bulbs are the safest bet if you’re dead set on using actual, purpose-made plant lights. In general, though, ordinary fluorescent bulbs (most of which are blue-dominant) will do just fine, especially as a supplement to sunlight. There’s nothing wrong with using different types of light; a couple hours of artificial light after the sun has gone down can make all the difference. As for brightness, aim for at least 60 watts, preferably more.
I’ll repeat some of the usual advice about placing houseplants in the window: turn your plant as regularly as possible so it won’t develop a significant bend in one direction. The same applies if your artificial light is placed to the side of the plant rather than above it. If a plant is growing thin and “leggy” (long distances between nodes/leaves) and especially if the growth is pale, that is a reliable sign of insufficient light. Place plants close under (or to) plant lights if you’re using them, but watch that the heat given off by the bulb and/or fixtures doesn’t burn the plant (and don’t let the plant touch the bulb). Figure out (research) how much light a plant needs, how much you can provide, and then act accordingly.
Finally, if you want to start seedlings indoors, this is one time I would recommend actual grow lights, and the brighter the better. It’s not worth starting them in a window in my opinion; this usually results in weak, leggy seedlings, although you might just get away with it in a south(west)-facing bay window. Seedlings must be no more than six inches from the light bulb, and preferably closer (as long as they don’t get burned). Also, unlike mature plants, seedlings don’t need to “sleep”, i.e. they don’t need a period of dark or night. For at least the first two weeks after sprouting, you can keep seedlings under constant light 24-7. If you do this, they can grow much more quickly (depends on the plant), so watch that they don’t get too close to the bulb and make sure they have enough water.