You probably know the lighting needs of most of your houseplants. Your cactus, for example, requires bright light while your dracaena prefers medium light and will take low light if necessary, etc. Great, but how can you tell if the spot where you want to put the plant offers bright, medium, or low light?
After all, you can’t trust your eyes: they adjust too readily to dim light and give a false impression of bright light as long as there is something dark nearby for contrast.
Of course, it’s possible to buy a light meter (hydroponic shops offer them), but at what price? Especially when there is a really simple and totally free way of determining the light level in your home: the shadow test.
How to Do a Shadow Test
On a sunny day, place a sheet of white paper on the spot where you grow or intend to grow your plant. Now hold your hand about l foot (30 cm) above the paper. What do you see?
If you can see a clearly defined shadow, the spot receives bright light.
If the shadow is fuzzy, but nonetheless recognizable as a hand, the spot receives medium light.
If you see only a faintly discernable shadow and you really can’t pick out the different fingers, that spot only receives low light… and the choice of plants that will survive in that spot is limited.
Finally, if you can’t make out a shadow, you know that spot is basically equivalent to deep shade, and is not a good place to keep plants other than on a strictly temporary basis.
Note that the shadow test applies not only to natural lighting, but also to artificial lighting. The less distinctly you see the shadow of your hand under a fluorescent lamp, for example, the lower the light. The shadow test can, for example, help determine when the tubes have lost so much intensity that it’s time to change them.
Light Intensity Changes Throughout the Year
It’s not enough to carry out a shadow test just once and assume you’ve got a handle on the situation. Outside of the tropics, indoor light intensity changes enormously depending on the season. For example, a location profiting from bright light in the summer may only receive low light in the winter. That’s why it’s worth repeating the shadow test at different times of the year and adjusting where you place your houseplants according to the results.
In general, all houseplants will prefer a location close to a sunny window during the winter, at least in northern regions, because natural light intensities decrease dramatically at that time of year.