Using CFLs to grow autoflowers

groing autoflowers under cfls


CFL or Compact Fluorescent Lights are bulbs that use the fluorescent (daylight) tube technology but can be placed in regular household E27 sockets. The way that these bulbs work is by fluorescence and that means that there is a gas inside a glass tube that gets excited when electricity is passed through it. When the gas gets excited it produces ultraviolet waves that passes through the glass wall that is usually coated with phosphor from the inside. As this ultraviolet light passes through this coating the Phosphor starts to fluoresce and that in fact produces the visible light waves.

This technology is nothing new and it has been used by fluorescent T12, T8 and T5 tubes for decades but because those lights need ballasts to sustain a continuous current they can’t be made smaller. Here comes in the CFL lights that have a ballast in the base of the light just above the socket contact. This ballast is usually the hottest part of the light because it needs to transform electricity from your regular E27 socket output to one usable by that folded fluorescent tube. In fact if you have seen a T5 or T4 fluorescent tube light then you can imagine how that long tube is folded in various curves and fit together to form a compact bulb like shape.



How CFL lights work on autoflowers

Many growers think that because CFL lights are less efficient in their lumen output per watt that they would grow smaller plants but that is not the case because fluorescent lights have a greater photo synthetically active radiation index than HID grow lights. This means that light produced by CFL lights is more usable by plants and there is less light that is created and simply lost because plants cannot use that light. There are various spots on the spectral scale where cannabis uses light more and less and you can see that light usage scale at the image behind this paragraph. but for example the green light is entirely unusable and also yellow light is used in low amounts and if we think that HPS lamps create most of their light in the yellow spectrum but CFL lights produce Blue or Red light depending on the bulb type then we can imagine how less efficient light can produce the same and maybe bigger yields. 


Basically if you use a proper grow guide and a lot of CFL lights you can grow better plants than HID bulbs could produce!

cfl autoflowering

CFL variations and efficiency

Compact fluorescent tubes come in different variations and shapes and they also come in various Wattages and lumen output and not all of them have the same efficiency ratings.

Basically when looking at various different shapes you will find that all of them can be divided in two categories the Spiral and Tubed shapes and usually the tubed ones are longer but thinner and the spiral bulbs are wider but shorter but they can go both ways so just pick the bulb that suits your need.

As for the efficiency ratings the shape doesn’t matter and almost always the wattage is the main thing to look and and calculate the lumen per watt ratios. Here are some sample efficiency ratings of CFL bulbs. There may not be completely accurate and you should calculate them before buying any light bulb.

  • 13 W = 825 lumen => 63 lumen/watt
  • 26 W = 1610 lumen => 61 lumen/watt
  • 42 W = 2700 lumen => 64 lumen/watt
  • 45 W = 2800 lumen => 62 lumen/watt
  • 85 W = 4250 lumen => 50 lumen/watt
  • 125 W = 7,000 lumen => 56 lumen/watt
  • 200 W = 9500 lumen => 47 lumen/watt
  • 300 W = 13,975 lumen => 46 lumen/watt


From these ratios you can see that the most efficient CFL bulbs are at the start of the range from 12 W to 45 W but 13 W bulbs are too weak and you would need a lot of them to grow cannabis so the best CFL lightbulb for growing marijuana is the 42 W one because it is the most efficient and will grow the biggest plants. Because that 42 W bulb emits around 2700 lumens you will need a couple of them to truly utilize the light and get maximum amount of bud production because cannabis plants need from 5000 to 10000 lumens per square feet of growth so you should measure your growing area and calculate the total volume and if for example it is 10 square feet you would need at least 20 CFL bulbs to support the plants that are growing there but you can place them all around those plants and get the best light penetration and growth.



CFL color temperatures

As you may know by now Cannabis plants need different kind of light for various growth stages and it is best to supply more Blue light at the seedling and vegetative stages but give more red light at the flowering stage but basically these plants use almost all color spectrum light except green and CFL light come in different kind of spectral outputs that are measured in Kelvin (K). There are the two main categories like the 6500K for seedling and vegetative growth and 2700K for Flowering but because you will need a lot of bulbs to grow great buds you can supply both spectrum bulbs and even the middle range 3000K and 4000K ones because they will have different spectral outputs and your plants will receive more diverse light that will be more usable to them. I usually give my plants the majority of 6500K light at the vegetative stage and a majority of 2700K or 2500K light at the flowering stage but also have a few opposite spectrum lights and one or two middle range ones. 


Why use CFL lights?

As you may know CFL light can be as efficient in growing marijuana plants as HPS bulbs but you need a lot of them but that can be a very helpful thing because you can place them around the grow room and around your plants to maximize the light usage. This can be done because CFL light emit low amount of heat and your plant can be within 2 inches (5 cm) of the lamp and won’t get burned and it is better to have multiple light sources than one big one because you decrease shadows and more leafs get light and that means faster growth!




  • Cheap bulbs
  • Easy to replace
  • Will fit into regular E27 bulb sockets
  • Come in various color temperatures
  • High PAR efficiency
  • Stay cooler than HID lamps
  • Can be easily placed around plants in different angles



  • Higher wattage bulbs get less efficient
  • You will need many bulbs to supply adequate light amount
  • CFL’s have shorter lifespans than LED lights


As you can see CFL lights are good for growing cannabis and you just need a lot of them to grow dense buds and get big yields but they are cheap, easy to use and don’t get as hot as High Discharge Lights!

This Cannabis CFL growing post was written by Midowo from Autoflowering cannabis Blog

Many thanks to him!! 

35 thoughts on “Using CFLs to grow autoflowers

    • maurice mckinley says:

      professional greenhouses including the indoor greenhouses at major chain stores like walmart,home depot,and lowes use 6500k T5s in their indoor greenhouses. if you look at the action spectrum for terrestrial plants you will see that most of the known pigments use blue light. so all the evidence supports the use of 6500k for vegging and adding 2700k for flowering altho many have gotten successful grows with just 6500k bulbs. only the LED crowd believes that cannabis is radically different from any other plant and requires a lot of red light and very little blue light. there is no evidence to support this view.

      • weed widow says:

        i ran a led light with what used to be called multi-spectrum – it had only 3 of the 15 leds as red. plants looked great in veg but buds didnt flower properly.

        • maurice mckinley says:

          poor spectrum is just one of the problems with leds. the low output (37.5 lumens per watt) and high cost are two other considerations.lights are just one of many factors that affect plant performance. many people get great yields and quality with just 6500 or 5000k bulbs. and cheap led lights like the one you describe only provide one wavelength of blue light. usually around 460-470 nm. a look at the known pigments and the wavelength needed to stimulate them will show that these many of these pigments are not stimulated by led lights including pigments that produce chemicals that protect the plant from insect and disease as well as regulating growth. bud development requires some UV light as well. fluorescents and full spectrum hid lights provide the full spectrum of light that plants need. most led grow lights do not. TRW has just introduced a white led that is broad spectrum and has a very high uses a UV led to drive three different phospours just like flourescents. the efficiency is only 61 lumens per watt and the cost is high but in time the price will come down and the efficiency will go up. someday these new leds will make led grow lights practical and cost effective.for now I will stick with fluorescents. where I live heat is too much of a problem with HIDs.

    • weed widow says:

      There are two schools of thought on hours of light given to autos. Some people like to give them 18 hours a day light (or 20) and then a rest period of 6 hours (or 4) on the basis that this combines lots of light hours for maximum growth & flower with a natural rest period. Others just keep the light on 24 hours a day. I’ve run both schedules and haven’t noticed any difference in end yield.
      This goes for whatever light (cfl., hps, led etc) that you’re giving your plants

    • weed widow says:

      Its a bit complicated to put into a few sentences Saki but I’ll try! During growing period your plants use more blue light than red. So if you have only a flowering lamp, they won’t have as much blue available to them and potential growth rate is reduced. (flowering lamps have some blue light in them) So whilst its not impossible to just use flower bulbs its not ideal

  1. maurice says:

    a mix of 6500k and 3000/2700k works best. the 3000/2700k will work for vegging but your plants will grow more slowly so you will need to veg longer to get the same yield.with autos their life cycle is set by genetics so your yield will be lower with just flowering lamps since you can not adjust the veg time to compensate for the reduce blue with photos you will need to find strains that grow well under fluorescents but this is true with any light source.

    • weed widow says:

      Thanks Maurice for the technical answer to Saki’s question! In short, really he needs to get a grow spectrum CFL for the first few weeks of his auto’s life cycle … wouldn’t you agree?

  2. maurice says:

    I am not sure what you mean by “grow spectrum” but any 6500k bulb will work and should be used during the first few weeks.I prefer to leave them in for the whole grow and just add 3000k bulbs (I use PL-L55 style bulbs and they are not available in 2700k) as the plant gets larger and needs more light but yes a blue spectrum bulb will dramatically increase growth rate during the veg period and a bigger plant going into flowering means bigger buds at harvest.but names like “full spectrum” or “grow spectrum” are just titles some growshops attach to regular 6500k bulbs along with a higher price tag.home depot or walmart bubs are the same thing for a lower price.and as the article pointed out 23 or 42 watt bulbs are much more efficient (more light and less heat) than the high power (125-300 watt) grow lights. a 40 watt 6500k bulb and clamp on reflector seems from walmart or home depot seems like the cheapest way to add blue spectrum bulbs and address his lighting needs

  3. Saki says:

    Now I have dual spectrum 125 w CFL bulb and the growing is much better. I have a question, my older plants had been “poisined” by too much fertilizer, they are in flowering phase and I wonder if they will flowering any way or they will be death soon. I read a lot on the net but I couldn’t find any answers about that.

    • jay says:

      Try putting corn seeds spinach broccoli in the soil to extract the nitrogen for 2 weeks then kill the seedlings.. they are nitrogen horny so put them in there.. they can also be used to pretreat the soil to get them fertilizers out..

  4. maurice says:

    SAKI you could try flushing your plant to remove excess fertilizer from the soil by running several gallons of water thru the soil and going back to 18/6 lighting to try and put them back in veg for a while to give them time to recover but the damage that is already done is permanent. if the leaves are not to badly damaged they will flower but you need to flush the excess nitrogen out of the soil

  5. Saki says:

    there is no more leaves and buds in starting level with no white spot on it and there is no smell like it was when they were healthy and hairs still white and no change in buds. I think that autoflower couldn’t be done back in veg.

    • weed widow says:

      Hey there – if its an indica based variety it’ll finish flowering in 7-8 weeks from start of flower; if its sativa based it could be anything up to 16 weeks (very rare this) – more likely 10-12 weeks. Watch out for signs of males and hermies when using bagweed. Happy tokin!

  6. Leke (@horsebones) says:

    They make plant CFLs now. That’s right, no Kelvin rating on them. They are made to output around the main red and blue spectrum and produce the lilac type of colour the LEDs have. I’m a noob, so I wouldn’t trust their performance based on my results, but I would love to see the results an experienced grower would produce.

  7. maurice mckinley says:

    fluorescent grow spectrum lights have been around for a long time. I have tried the T5 HO plant growth lights and did not see any noticeable difference from the standard 6000k/3000k mix. these bulbs are a lot more expensive and I do not believe they are worth the extra cost.the belief that plants need a lot of red light is a myth started by LED grow light makers to cover up a short coming with led is not supported by any peer reviewed studies. plants can use red light for starch production in the absents of broad spectrum blue lights but with florescent they do not need a lot of red light. standard bulbs produce enough. greater intensity would do more good than specialty bulbs

  8. courtney jefferson says:

    Is to OK to use both red and blue spectrums at the sametime to grow autos since they enter the flowering stage based on their genetics?

  9. G-Raw says:

    I have a 300 watt 6500k light that sits over an Abbaponics closet monster hydroponic grow system. How far should the lights be from the plants? what should my temp and Humidity be? Also what nutrients do you recommend that do not have a lot of salt? I’m currently using General Hydroponics Flora Duo system & their Cal-Mag. I only use 1/4 teaspoon per 2 gallons of water. What is your advice?

  10. Serhat says:

    Hi I have 4 auto seeds and I have 2×200 watt cfl dual spectrum My grow is it correct? Enough for light flowers? Thanks..

  11. Carol says:

    Howdy, first time grower, need a little guidance please.. how many CFL 250w can I use for 7 Dutch passion ultimate auto ,, would appreciate any advice x carol

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