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Compact fluorescent (CFL) grow lights

In the old days growing marijuana under fluorescent (CFL) grow lights was pretty useless as they had low output for a large size. However, the new CFL (compact fluorescent lights) grow lights that between them cover the full visible light spectrum have made them a viable alternative.

Jump to the bottom of this article to see how CFLs are losing to LED grow lights.

CFL compact fluorescent grow lights are becoming increasingly common among small-scale, closet and tent marijuana growers for use in their grow rooms. They are energy efficient and produce very little heat so you can place them as close to your plants as a few inches away. This means you can grow your plants easily in a small space.

Efficiency at the high end ranges from 75 to 98 lumens per Watt consumed. Bulbs in the 2700k to 3000k (Kelvin) range provide higher yellow / red spectrum which promotes flowering. CFL grow lights in the 5,000k to 6500k range spectrum offer lots of blue light which promotes plant growth.

As CFL grow lights don’t pose any heat extraction issues ventilating your grow room on account of heat from sodium lamps and high intensity discharge (HID) lights can be very costly which makes them very popular for small marijuana grows. (Note that for commercial spaces, HID lights are more suited as they penetrate the plants further to produce tighter buds at a faster pace.)

If you are interesting in growing your own marijuana with compact florescent lighting, there are some things you should know.

Above is a picture of a CFL 300 Watt which is self ballasted, cool running grow light and you can get it in a choice of light spectrums with high lumen output. Often CFL grow lights cover one part of the growing range, either vegging or flowering and often do not cover the entire light spectrum required for high yield marijuana growing.

One product on the market is Eco-Lights which claim to be a ‘high powered compact fluorescent grow lamps (CFL)’ and usually are very good as a supplementary light to your main grow light. Like many fluorescent grow lights they are available in both the blue and red spectrums for vegetative and flowering growth respectively. These are not as high intensity lights as HPS but are cheaper to buy and cheaper to run. They do not require a separate ballast as they simply screw into a standard E40 fitting (normal grow light lamp holder) and run directly through a mains plug. We here at HowtoGrowMarijuana.com however like the full spectrum LED grow lights. They run for longer, take up less space and deliver a lot more intense light equivalent to a HPS but about half the electricity and like the CFL’s do not require a ballast.

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How to choose a CFL grow light

CFLs are cheaper than HID lights and they’re mobile and flexible. Your objective when buying CFLs should be to purchase the Wattage that is the highest possible for your grow area which usually gives you the largest amount of usable light. You may even use combinations of several bulbs depending on your grow space.

First know what’s available. The smallest powered CFLS come in 15 and 26 Watts which are really of no use for growing weed. Then there are bulbs of 42, 65, 68 and 85 Watts of which you can use several without significant heat output. High Watt outputs are available at 125, 250 and 300 Watts.

The 125 Watt lights above with Euro reflector, 480mm length, 400mm width and 120mm depth. It comes in both red 2,700k and also blue 6,400k and both put out approximately 5000 lumens.

This CFL grow light offers around 12,500 lumens, more than twice the 125W and is ideal for propagation or supplementary lighting. Again you have to buy either the blue for vegging at 6,400k or the 2,700k for flowering.

Spiral type CFLs are better suited to emitting light in a sphere than tube type CFLs, so they are usually preferred and compact florescent lighting will offer around 10,000 hours before requiring a change.

If you’re wondering what brands to buy, you can check out the T5 and T8 CFL tube systems that offer the full light range of the visible spectrum. Eco-lights is a brand that offers ‘high powered’ fluorescent compact CFL grow lamps that are good supplements to main grow lamps. They are available in blue and red spectrums. You don’t need a ballast to run them – they’re screwed to a socket and run by the mains.

The light in the image above is a Senua 200w CFL Hydroponics Grow Light, with alternative RED 2700K & BLUE 6400K filaments. This is ideal for propagation, side lighting and mother plants. It is used with an E40 hanger or CFL reflector- so you can just plug in and grow. Blue light for growth and red light for flowering.

Efficiency of CFLs

Your aim with the CFLs will be to provide the maximum ‘amount’ of light to your plants while not compromising on power drawn. A 42 watt CFL provides about 2700 lumens or 64 lumens for every Watt. Therefore four such bulbs will draw 168 Watts and provide a total of 10,800 lumens to your plants.

Which is the most efficient bulb in terms of light/watt ratio? The 150 W CFL is rated 7,500 lumens, so l/w ratio is 7500/150 = 50. The 200 W is least efficient emitting 9,250 lumens, with an l/w ratio of 46.25. The highest ratio (and therefore the most efficient) may be the 26 W bulbs each of which emits 1,700 lumens with an l/w ratio of 65.38.

How much light you need to provide your plants will depend on various issues such as your growing area and how big you want the plant to grow etc. We’ve taken a look at that in a section below.

Considering color temperatures of CFLs

For CFLs it is not enough to consider power alone. Color temperature is important (which is not the visible color of the bulb). For growing marijuana, you will need bulbs that release blue UV light in the growing phase and red-yellow UV light in the blooming phase. The purpose of this is to emulate sunlight as much as possible. In the spring, sunlight is largely composed of UV rays in the blue end of the spectrum and in summer, those towards the red end. These are the lights that the plants absorb and use the most, not the yellow, green and orange lights that the sun (and CFLs) also emits.

You’ll find CFLs also labelled in Kelvin which is a measure of color temperature. Bulbs are available with ratings of 2700k, 3000k, 4100k, 5100k and 6500k. The lower rated ones (warm white) are for flowering or blooming stages and the higher rated ones for growing stages (cool white).

Ideally, you should aim for a balanced spectrum with a combination of low rated, high rated and mid range (4100 Kelvin) bulbs for the best growth.

Light distribution across the grow area and height of lights

Now that you know about types of bulbs, you will have to decide how many to buy. Note that hanging your lights securely with chains is a good way to give yourself the flexibility to raise and lower them to suit the height of your growing plants. You can safely place your 42, 65 and 85 W CFLs about an inch and a half near your plants without burning them. For 125 W and 250 W lamps, the plants should be about 3 inches away. That should give you an idea of the height of the lights.

The size of your growing area and how big you want your plants to grow will determine how much light you need.

Small plants require about 3000 lumens per square foot, which is the minimum amount of light you will need. But since a lot of light is lost with distance and reflectivity, it is safer to aim for higher light. Ideally, try to achieve about 7000 to 10,000 lumens per square foot for medium plants. Compare with the sun, which emits 10,000 lumens per square foot on a sunny day in peak summer. That is the luminous flux you should try to achieve.

Using a reflector

In order to make the maximum use of the light from your grow lamps and reduce wastage you will need to use a reflector with your grow lamps. Reflectors will direct light towards the plants. They will reduce light escaping upwards and out towards dark walls. They will focus the light on your plants. Many grow lights come with their own reflectors. Otherwise they are readily available at lighting stores such as Walmart (whose reflectors are cheap but not fragile), Home Depot or Lowes (who offer heavy duty reflectors for a few more dollars).

Also cover the walls of your greenhouse with inexpensive Mylar sheeting so light can be reflected back into the room from dark walls. This is an essential step. Mylar sheeting can efficiently reflect back as much as 95 to 98 percent of the light that falls on it. Alternatively, you can paint your walls a white matte color. Never use silver foil, which can create hot spots that end up absorbing light.

A Grower’s tip

Many home growers who don’t have to think about budget suggest that in the last week or two of budding, it can help to install a metal halide (MH) lamp which produces the richest buds with more smoke. It doesn’t affect the yield but the quality of the plant.

CFL Vs LED grow lights

The compact fluorescent lights (CFL) are a fantastic way to grow in small stealth locations where heat emissions are very important to control and also they are cheap to buy and run but they are generally limited on power. If you want some more power but still maintain a small heat signature, albeit more than CFL you might want to start looking into LEDs – especially the full spectrum LEDs. We have a good article here that outlines what you might expect from LEDs there days. More on LED grow lights.

SpectrumKing with SGL

The Tipping Point between CFL and LED has come and gone

Energy-efficient lighting industry professionals have seen a dramatic decrease in the use of CFL lighting in industrial applications with LEDs taking over in 2014, these days they are a great way to grow. In 2013, 26% of respondents to a survey of 5,000 professionals stated they would use LEDs and 68% percent said CFL, those figures for 2014 are now 49% LEDs and 46% CFLs so you can see a huge movement to LEDs. The same shift is occurring in the grow light arena with people moving away from CFL in smaller grow spaces to the LEDs. These days they are better bet. More on that article.


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84 thoughts on “Compact fluorescent (CFL) grow lights

  1. Thydopeone says:

    If I have one 150watt CFL light for one plant do I still need a fan

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    Rating: 0 (from 0 votes)
    • Buddy Buddy says:

      The fan will help circulate air. It also helps to have air moving around the plant to help keep stems strong and keep mold away. It’s not necessarily about keeping the bulb cool. Your 150W CFL shouldn’t generate too much heat.

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      Rating: +1 (from 1 vote)
  2. Jimmy says:

    what power cord to light socket adapter should be used for 4 42W CFLs? its 168 watts but cant find a single socket cord that high to put in 4 way splitter.

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    Rating: +2 (from 4 votes)
    • Doug says:

      The cord will be rated in amps. 120volts divided by 168 watts equals less than an amp. The common 15 amp cord will be more than sufficient.

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      Rating: +1 (from 7 votes)
  3. BoondockSaint says:

    I have 4 300 watt 2700k bulbs for a 3×3 grow space. So far taking them from outdoors to indoors I have seen nothing but happy ladies. I just used soil from walmart. Started out using 18-18-21, switched them to 10-40-10 at half strength over last 2 feeds in new containers. About week 4 and they are blossoming beautifully. May only get 100 grams of dried bud, but worth it for what i have. Next year I will add another 2 in there and 2 more 6400k bulbs at 200w.

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    Rating: +2 (from 2 votes)
  4. weedgod420 says:

    i have two plants and im using ecosmart 100w replacement cfl im using two of the cfls 1550 lumens 5000k daylight cfls bye the way there 3 days old my plants or .and well this be good for them im just wondering

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    Rating: 0 (from 2 votes)
  5. Wweedlover says:

    Bought a cfl 25w=100w 1600 lumens and daylight 5850 kelvin. Will that grow anything?

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    Rating: 0 (from 12 votes)
    • Drokkenian says:

      Here is some updated info I just finished my first grow. Here’s what I came up with:

      My grow space is a total of 3.11 sf

      I use four 42w (150w equiv) 2700k CFL’s and four 42w (150w equiv) 6500 CFL’s (picked up at home depot for $10 each, each bulb is 2800Lm)

      I also installed four 4′ fluorescent tubes (32w T8 bulbs – 4100k – 2700Lm each) around the perimeter of the grow space (ie one in each corner with DIY reflectors).

      Total Lumen in the cabinet = 33,200
      Total watts in the cabinet = 465

      This provides 10,675 lumen per square foot or 149.5 watts per square foot

      Total Heat, when unventilated, runs about 95f so I also use a total of three 120mm cpu fans (1 as intake at the bottom of the cabinet, 2 at the top near the lights as exhaust (which vents heat/smells into septic system piping). This keeps the temp at a steady 78F with about 40~ish% humidity.

      My first grow, single plant in soil, finished with just under an oz of decent clean bud on a 25″ tall plant.

      CFL’s are great for closet growing especially in regard to heat control and overall cost to set up.

      My next grow will be under the same light setup only this time I’m switching to hydro. Looking forward to seeing the results of 6 to 8 plants in 2″ net cups.

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      Rating: +5 (from 5 votes)

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