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Growing Indoors with Fluorescent Lights

By Sunny Datko

Fluorescent lights work by placing an anode and a cathode at opposite ends of a glass tube. Inside the tube are a partial vacuum and a small amount of mercury vapor. When energized, the mercury vapor is ionized and emits ultraviolet radiation. The inside of the tube is coated with a phosphor – a powder that “fluoresces” (gives off light) when stimulated by ultraviolet radiation, thus producing visible light. The chemical composition of the phosphor determines the spectrum or color of the emitted light.

Although fluorescent lights are very energy efficient, there is a phenomenon known as “cathode decay” that causes, over time, less energy to be transferred through the mercury vapor. The net effect is that the tube will emit less and less light as it gets older. To all appearances, the tube will put out the same amount of light until it suddenly stops dead one day, (which can take years), but for all practical purposes, because the drop off in light output is an exponential decay, the tube should optimally be replaced at least once a year.

Fluorescent lamps are labeled according to the diameter, for example T5, T8, or T12. The number following the T refers to the diameter in one-eighth inch increments: 
T5 = 5/8″ diameter, 
T8 = 1″ diameter, and 
T12 = 1 1/2″ diameter.  Figure 2 shows diagrams of lamp ends of T5, T8, and T12 lamps. The bulbs typically come in 2-foot and 4-foot lengths. The 2-foot length works well if space is limited.

Normal output, also known as standard fluorescent lights (T8 & T12), are generally suitable for seedlings but will not provide enough photosynthetically active radiation to support robust and vigorous growth. For high-energy plants, you will want to consider using a high output T5 during the vegetative stage of growth. While plants can flower under a fluorescent light, the results will not be as impressive.  However if the grower has no other option, they may want to swap their cool spectrum fluorescent tubes for warm spectrum during flowering.

There are also compact fluorescent lights (CFL) available, which take the cool burning fluorescent technology and pack it into a focused, high output “bulb”, that you can use not only for propagation, but for growing larger plants as well. Compact fluorescents work in specially designed reflectors that efficiently direct light to the plants, much like high intensity discharge lamps. However they have relatively poor light penetration and lower overall efficiency as compared to other types of lighting.

Type of Bulb Power Length Life Average Lumens Lumens per Watt
Standard T12 30 watt 35.78″ 18,000 hours 1870 62.3
Standard T8 25 watt 35.78″ 20,000 hours 1845 73.8
T5 High Output 39 watt 34” 20,000 hours 2803 71.9
Compact Fluorescent 36 watt 16.6” 12,000 hours 2494 69.3

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