Of Light and Lamps
Knowing the "right" light for your plants
Plants need light for photosynthesis. If a plant doesn't get enough illumination it will die of starvation because of a lack of nutrients. For indoor plants, especially during the winter, light intensity is crucial. If there is not enough daylight in your rooms, you'll have to install additional sources of light (see here).
The sun emits "white" light (at 5600 Kelvin) that can be separated into long-wave red, short-wave blue, and green light with a prism. Plants absorb the red and blue parts of this light while they reflect the green ones, so they appear green, usually. Plant growth depends on the intensity of light and on the proportion of red and blue light waves. The less intensive light is or the bigger the fraction of red light is, the more the plant grows - unfortunately only with longitudinal growth, so the plant gets weak and runs the risk of snapping off. A blue light only illumination would result in almost no longitudinal growth. That means, if you want your plant grow healthily, it should get light that is close to normal daylight.
There are many possible lamp solutions that may fulfill your needs. You will have to consider that a lamp shouldn't emit too much heat, that its light temperature should be rather cold (> 5500 K) and that it won't charge your electricity bill too much. Perfect candidates are fluorescent tubes or energy-saving lamps of type "cold white" or "daylight". You should find such lamps in any building centre / DIY store near you.
Besides unexpensive fluorescent tubes, you may find special "plant lamps". There are three types:
- lamps similar to normal light bulbs: these may have slightly violet coloured glas that reduces the effectiveness of this lamp type even further because they absorb more red light than normal. Don't use them.
- special fluorescent tubes for plant illumination: they are relatively expensive and since they only emit blue and red light the resulting magenta may not please you. So there is no advantage for these lamps in comparison to normal fluorescent tubes.
- metal halide lamps: very effective light sources that you might consider to buy. Their only disadvantage to fluorescent tubes is that they are quite expensive.
Not only the correct light but light intensity is important for your plants. (You may ignore deciduous plants during the winter - just put them in a cold and dry place.) A role of thumb recommends 9 hrs of illumination / day at > 700 lx (Lux) and between 5 to 10°C (41 to 50°F). The higher the temperature, the more light is needed, but leave them some hours for sleeping (at least 6 hrs). [5]
The Luxmeter
A luxmeter is a device for measuring the intensity of illumination. It displays in lux the amount of incoming light φ per unit of area. Usually, a silicium photodiode is used for measuring. There are analog and digital devices, the latter are much more precise. Unfortunately, digital luxmeters are quite expensive. If you're a photographer you may use your photometer because this also is sort of a luxmeter. Ferns will feel well usually at 200 - 600 lx, some species (Phlebodium, Boston Fern, Asplenium) like bright light at 1000 lx and more. Just avoid direct sunlight (this will have > 100000 lx).