CFLs offer three real-world advantages:
1. Low startup cost: The bulbs only cost a few dollars each, fit into standard cheap sockets, don't require special ballasts, and are available literally everywhere.
2. High flexibility: You can easily position them around plants, or even inside plants if you like. You can easily turn off any individual bulb by simply unscrewing it, limiting unnecessary energy use early in grows, or easily add more bulbs to increase lighting later, etc.
3. Low contact temperature: CFLs can be brought close to plants without burning them. Note that on a watt for watt basis, CFLs don't put out any less total heat than LEDs, HPS, or Metal halide lights. All of these lighting systems are relatively inefficient, and in fact convert most of the energy input into waste heat. Just by nature of the large bulb surface area, CFLs spread their heat over a greater area so the temperature at any one single point is lower.
Offsetting some of these advantages, CFLs aren't nearly as efficient for growing as you'd think just by looking at the numbers. If you just look at output as lumens per watt, CFLs "seem" to be about 75% as efficient as HPS, but the reality is that they're nowhere near that good.
First of all, lumen measurements are not entirely standardized, and you can't always trust what the bulb makers claim. More important, what matters to plants isn't lumens, which are designed to measure what the human eye likes to see, but rather photosynthetically active radiation or "PAR", which is the portion of the spectrum of light that plants need to grow and flower. Not only do HPS lights put out more light per watt energy used, the overall spectrum of HPS light is typically better for growing, explaining why they are still the gold standard for artificial horticultural lighting.
Next, and a major drawback, is that conventional CFL bulbs are specifically designed to throw light in a spherical shape. That's good for lighting a room (which is what CFLs are for), but lousy for lighting a single plant or flat plant canopy.
Not only is a good bit of the bulbs light wasted by being trapped inside the bulb spiral itself, but more important, if you simply hang a CFL bulb, most of the light it gives off will be directed away from your plant canopy. That's why fluorescent bulbs actually DESIGNED for growing plants are all in a simple tube/linear arrangement like T5s. That sort of arrangement makes it easy to capture virtually all of the bulb's light with simple linear/parabolic reflectors.
Now, the loss of light with CFLs can be improved a great deal with specifically designed GOOD quality reflectors, but how many people actually use polished dimpled batwing/parabolic reflectors over their CFLs? There aren't even many reflectors like that on the market, and the few I've seen aren't particularly cheap nor compact. At $20+ each, using them also reduces many of the cost and flexibility advantages of CFLs that people like. Yes, its possible to construct your own reflector, if you are careful and know what you're doing, but again, how many people actually do it?
Bottom line is that CFLs are a good way to get started growing cheaply and quickly, and they're a good way to go if you want to light a really small space (eg a computer case, a small trunk, etc). They're also adequate for seedlings or cloning. But once you get into "serious" gardens involving multiple square feet, you're going to find T5 fluorescent or HID lighting to be quite a bit more efficient and that's why none of the pros use CFLs in their grow rooms.
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