Sources of artificial lighting

Incandescent lighting sources

Incandescent bulbs are not designed for plant growing, because they produce low levels of light. Over 70% of the energy produced is converted into heat and only a small percentage is transformed into light. When placed too near the plants, there is a danger they will burn the foliage and dry out the soil. If placed far enough away to avoid these problems, they do not supply enough light to the plants. On the other hand, incandescent bulbs provide large amounts of red light, but very little blue, so they can be used to increase the photoperiod to encourage blooming or for starting seeds. Otherwise, they may be used as supplementary lighting to create special effects.

Fluorescent lighting sources

Fluorescent lamps are much better for growing indoor plants. They are entirely adequate for seedlings, cuttings and flowering plants. They are more efficient than incandescent bulbs, since they convert only 13 to 20% of the energy into heat. They come in a wide range of sizes and colours. A lamp with two 120 cm tubes, fitted with reflectors, is generally used for growing indoor plants. Several types of tubes, each emitting light from different portions of the spectrum, are available. For instance, Cool White and Daylight lamps emit plenty of blue but little red, while Warm White and Deluxe lamps emit a lot of red but little blue. One way of obtaining the entire required spectrum would be to combine these two types of lamps. This type of fixture would be well suited to African violets and foliage plants.

For even better performance, you may wish to use lamps with a spectrum specially d esigned for growing plants, such as Gro-Lux, Vita-lite, Verilux and Powertwist, for instance. These lamps will produce compact plants with much more intense colouring than will ordinary fluorescent lamps. Although they are expensive, these lamps are the easiest to use because they can be used on their own. Remember that light intensity from fluorescents decreases toward the ends of the tube. This means that foliage plants, cuttings and seedlings should be placed toward the ends, while plants that need the most light should be placed in the centre.

High-intensity sources

Other lighting sources, such as metal halide (MH) and high-pressure sodium lamps, are much more powerful than the previous two varieties. They emit light that is much closer to the spectrum of natural light. These two types of lamps are compact and very popular in greenhouses because they do not interfere with daylight. They have a long life and provide uniform lighting. Unfortunately, they are expensive, making them less accessible to amateurs.