There seems to be a lot of misconceptions about CFLs out there.
There are basically two general spectrum of light that you need to consider.
The first, generally referred to as 'blue light' is used in the vegetative state of plant growth. This type of light promotes growth of the roots, stems and leaves. A lamp can truly be said to be in the blue spectrum if it has a color temperature of >6000K. CFLs are readily available in this range (1), although they tend to be somewhat expensive.
There seems to be the general opinion here that if you have to pick between 'blue' light and 'red' light, then you should go with 'red' light, because it promotes bud density - which is true - but one must also keep in mind that buds cannot properly be produced at all without an adequate root system, healthy leaves, and stems capable of efficiently allocating nutrients throughout the plant. Therefore, adequate levels of 'blue' light are just as important to the plant as 'red' light (particularly in the early vegetative stage of growth).
On a side note, metal halide lamps tend to be in the 'warm' to 'cool' white range - which is about 3500 - 5000K. Although they are available in the >6000K range (2), these tend to be extremely expensive 'special application' (aka grow light) bulbs. 'Cool' white metal halide lamps may be used - to great results - in the vegetative stage of growth, but the light that they produce isn't quite as useful to the plant as higher color temperature (>6000K) lamps, such as fluorescent, CFL, or specialty metal halide. The main advantages to metal halide lamps is their intensity and efficiency. Despite popular perception, metal halide lamps are actually quite a bit more energy efficient than fluorescent or CFL lamps - metal halides get around 80 - 90 lumens / watt of energy used, while CFLs get around 60 - 75 lumens / watt (3) (4). Beyond that, light from metal halide lamps travels farther distances and has more penetrative power - transferring more light to lower leaves.
The second type of light is 'red' light, which as already noted, promotes bud growth and density. A lamp can truly be said to be in the 'red' spectrum if it has a color temperature of <2200K - although for the purposes of plant growth, it is not necessary to have light lower than 2200K. The lowest commonly available color temperature for CFLs is 2700K (5) - which isn't really in the 'red' spectrum at all. Therefore, CFLs are not ideal for the flowering stage of growth - although they can be used. High pressure sodium lamps, which typically have a color temperature from 2000 - 2200K, are much more effective in the flowering stage.
Well, that's about all I have to say, except for a couple minor notes:
High pressure sodium lamps are by far the best, having a high light output intensity, an ideal color temperature for their particular application, and the best energy efficiency (>100 L/W). In this respect, specialty metal halide (with a color temperature of >6500K) are also highly effective for their respective application, although in many cases they are considerably overkill - sometimes to the point of literally killing young plants or clones by either heat or light intensity.
Color temperature, although useful as a general guideline, is not an entirely accurate way of measuring a lamp's usefulness to plant growth. In order to get the best idea of a lamps usefulness in any given stage of growth, the color temperature must be considered in combination with a lamp's PAR rating - which is a measure of the photosynthetically active radiation that the lamp produces. Unlike color temperature ratings - which are sometimes best to be in the 2000 - 2200K range, and sometimes in the >6500K range, depending on the stage of growth - PAR should always be high in grow lighting applications. Unfortunately PAR rating are very rarely listed on lamp packaging, so they must looked up elsewhere. One major perk involved with the use of CFLs as opposed to high intensity discharge lamps (high pressure sodium or metal halide), is that CFLs (and fluorescent lighting in general) has a higher PAR than standard high intensity discharge lamps, although high PAR versions of both types of high intensity discharge lamps are available - at respectively high prices.
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(1)
http://www.elightbulbs.com/catalog_...&list_source=&portal_type=&search_type=&ansi=
(2)
http://www.elightbulbs.com/catalog_...&list_source=&portal_type=&search_type=&ansi=
(3)
http://www.elightbulbs.com/GE-43828-MVR400-U-Metal-Halide-Light-Bulb
(4)
http://www.elightbulbs.com/TCP-01836-28027M65K-Twist-Screw-Base-Compact-Fluorescent-Light-Bulb
(5)
http://www.elightbulbs.com/catalog_...&list_source=&portal_type=&search_type=&ansi=
just built from Aus:
