Lighting is especially important to bonsai trees because typically bonsai are larger plants by species that are restricted to a small pot of soil. This is not how they are genetically intended to grow. So it becomes more important to offer more light so that the leaves can use the energy and produce the nutrients which the tree needs to grow. Normal home lighting is definitely not enough for bonsai trees.
Should I use natural sunlight?
Natural sunlight is by far the best light source for a bonsai tree. Whenever temperature and humility are within acceptable ranges the best place for your bonsai tree is outside. Unfortunately that is not always the case.
What about next to a window inside?
Inside on a windowsill the bonsai will get sunlight at least part of the day. A good plan is to make a list of windows in your home where you could place your bonsai and track how many hours a day each receives direct sunlight. Remember that as seasons change the angle of the path of the sun will change as well and the best window for your bonsai might change. Place your bonsai tree in the location that is currently getting the most hours of direct sunlight. Try rotating the bonsai tree periodically so tree does not start to grow toward the sun in a way you did not intend. But understand that this method almost certainly will not give enough light.
If the sun is not giving my bonsai enough light, what do I do?
In almost all cases you are going to want to supplement natural lighting with artificial light. Any grow light that uses non-incandescent bulbs will work. Position your lighting so that the light bulb is within 2-4 inches of your bonsai. Set up a timer to turn on when the natural light dissipates and turn off when the total combined time of sunlight and artificial light reaches 14-16 hours. So if your bonsai is getting nine hours of sunlight a day, set the timer to add an additional 5-7 hours of artificial light.
Why are incandescent bulbs bad for bonsai trees?
There are a few reason why you should avoid incandescent bulbs. First, the light from an incandescent bulb is of poor quality for bonsai and plants in general. There are better options. Second, incandescent bulbs radiate a good deal of heat and placing them too close to your bonsai can damage your plant. Finally, incandescent bulbs are more expensive. You can get a florescent bulb that fits into a traditional incandescent socket that will provide better light and cost less long-term since they last much longer.
Why do I need to rotate my bonsai tree?
You do not need to rotate your bonsai tree. But, with any lighting system you are using if you notice that your bonsai is growing towards the light in a direction you did not want it to grow you should make changes. You can rotate the bonsai, change the lighting position or both. You want your bonsai growing how you planned, not toward a light source.
What if I am not using any natural sunlight for my bonsai tree?
If you are not using natural light at all, set up a grow light 2-4 inches from your bonsai and set a timer so that your bonsai receives 14-16 hours of light per day.
Do I need a special grow light?
Absolutely not. When I refer to a grow light I mean any light you are using to give light to your tree. In fact, standard two bulb office style four foot fluorescent lights placed on blocks just above your bonsai tree with 40 watts white light bulbs offer exceptional lighting.
What about metal halide lamps for bonsai trees?
Metal halide lamps produce a very intense light that work very well for bonsai trees. If you can afford the expense then metal halide is a good choice. In fact, if you are growing multiple bonsai in an enclosed area like a grow room or greenhouse, metal halide can actually be slightly cheaper than fluorescent lights. But for one or two bonsai fluorescent lights offer a much more cost effective option and provide more than ample lighting for your bonsai tree.
As always, check with bonsai enthusiasts in your area for tips on how to best optimize your lighting for your region. And trust your own results more than any advice you get here or anywhere else. Experimenting and finding out what works best for you is going to yield better results than meticulously following the advice of someone else.