I get this question a lot.
Everybody wants to know how many CFL bulbs are needed per plant.
But how can you answer that without a little more info?
I mean, how large is the plant?
What is the wattage on the bulbs?
In order to accurately tell you how many CFLs for one plant, I need to know those two variables at least.
That said, I can still help, even without them.
You’ll need to do a little (very simple) math on your own, but together, we can figure out exactly how many CFL bulbs you need to light your garden.
Let’s begin by figuring out what you’ll need to light one plant, since everyone always asks about the lighting needs per plant.
How Many CFLs For One Plant
As mentioned above, plants vary greatly in size.
For that reason, we need to use a standard measurement to determine lighting needs. The easiest to use is the square foot.
When using CFL bulbs to grow a plant with a high light requirement (like cannabis), you want a minimum of 60 watts per square foot.
Ideally, you want 100 watts per square foot.
And it doesn’t hurt to go higher (though there’s not much point going over 150w). You really can’t give your plants too much CFL light.
Those numbers are for the flowering stage. For the vegging stage (or for seedlings or clones), you could cut that in half. I’d shoot for around 60 watts though. The same goes for plants with a lower light requirement.
Actual Watts Or Equivalent Watts?
Compact fluorescent lamps usually have two wattages listed on the packaging: one is the actual wattage that the bulb consumes and the other is the incandescent equivalent.
For example, a CFL that is equivalent to a 100 watt incandescent bulb will actually use around 23 to 28 watts of power.
When I say you want to aim for 100 watts per square foot, I am referring to the actual wattage.
So, to continue with the bulb from the example: you would want 4 of these bulbs, not one of them. The actual wattage counts.
If you have two larger plants that each take up 3 square feet of space, you would want 300 watts per plant, or 600 watts total. That would mean 24 of those 23-28 watt bulbs from the example.
Note: once you have more than a plant or two, it makes more sense to get a T5 fixture or a cheap LED light. CFLs are great for a small grow, but once you get up to 10 (or even 5, in my opinion) bulbs, it just becomes too much work to keep readjusting the lights, due to the rapid plant growth.
Where Do I Put The Compact Fluorescent Bulbs?
Ideally you want to surround your plant(s) with CFL bulbs, so that they get light from every side. Most people asking these questions are looking to grow marijuana, so let’s focus on that plant for a bit.
First make sure you plant grows the right way
You can choose how your plant grows. Weed plants grow upward into a Christmas tree shape naturally, but that is not good for us. This results in one large bud at the top and several smaller buds below.
You want to force your plant to grow short and fat. That way they are more flat on top and can grow multiple large buds. You can get them to grow this way by bending and tying down various stems to get them to grow outward.
If you force the main stem to grow horizontally from the beginning, other stems will grow upward from it, creating a shorter and fatter plant naturally. You’ll find much more on training your plants to grow the way you want here.
Then put your bulbs in the right spots
If you have a short and fat plant, there is no need to hang bulbs below the canopy. Hang them all above, with some off to the side and a bit lower, so that they are next to the top of the plant.
Anywhere you see a shadow area, put a bulb there. If you do not have enough bulbs to fill all the shadows, that means you need more bulbs.
CFL bulbs can generally hang fairly close to your plants, but this does depend on their wattage. A 20 to 30 watt bulb can hang within a few inches.
You can always use the “back of the hand test” to see if your bulb is too close. Hold you hand where the plant is, with the back of your hand facing the bulb. Keep it there for 30 seconds. If your hand gets too hot, then it will also be too hot for the plant. Back the bulb off a bit.
Weed plants grow rapidly, so keep a close watch to make sure they don’t grow into the bulbs and get burned. They can grow several inches in a day, so if you won’t be around to keep an eye on them, make sure to position the bulbs a bit further away.
If you have a plant that is growing tall, you may want to get some light on it from beneath as well, or at least from the sides. This will depend on the plant, however. If you are growing cannabis, you will get a much better yield by not growing tall plants, so I highly recommend keeping them short and fat.
Do I Need Special Grow Bulbs?
No, you do not.
In fact, I would encourage you not to buy special CFL grow bulbs. They are generally much more powerful, so you would only have one or two above the plant, ideally in a reflector.
The problem is that fluorescent bulbs do not have great penetration. That is why you hang them so close to the pants. With fewer powerful bulbs, you have to hang them further away and are more likely to get shadows. More bulbs that are less powerful allow you to hang them close and to completely bathe your plant in light.
In addition, bulbs that are specifically labeled as grow lights are almost always overpriced. Regular old fluorescent bulbs are every bit as good and do exactly the same thing, but they cost far less.
The main thing to look out for is the color temperature. There are cool white bulbs that have a color temperature around 6000K to 7000K (‘K’ stands for Kelvin), warm white bulbs with a color temperature around 2000K to 3000K, and neutral bulbs with a color temperature in the middle, around 3500K to 5500K.
The cool bulbs (with a higher color temperature) are better for vegging, since they contain more blue light in their spectrum and the warmer bulbs are better for flowering, since they contain more red.
You best bet would be to have a ratio of 2 cool to 1 warm for vegging, then add a few more warm ones for flowering. That way you also add more wattage for flowering and plants need more wattage then than they do during vegging anyway.
Where do I get CFL bulbs?
You can buy CFL bulbs pretty much anywhere. If you want to get them online, Amazon has some good options. Their value packs are especially good deals. You can get all the bulbs you need in one pack.
Here is a good value pack for warm white bulbs (flowering).
And here is one for cool white bulbs (vegging).
If you don’t have light fixtures for these bulbs (they fit in any standard lamp fixture), you can simply buy cords with a socket on the end like these.
They are extremely flexible and let you mount the lamps anywhere you like and in any position you like. Alternatively, you could put them in a reflector (or just rig up your own), to direct more of the light in the direction you want, i.e. at your plants.
How Many CFLs Per Plant: Final Thoughts
Hopefully this article has helped you figure out exactly how many CFL bulbs you need for your plant or plants. If you’re not exactly sure, err on the side of getting too many bulbs.
Since fluorescents are not all that powerful, getting more bulbs than you need won’t harm your plants and will actually help them a bit. On the other hand, if you do not have enough bulbs, your plants will suffer.
Mike says
GREAT TIPS!! thanks – for my grow I have about 125W per plant, each having around 1 square foot on the plus side. 2 plants side by side
Steven says
Sounds like a good setup.
Shaun says
How many lights is that?
Aidan says
This is by far the BEST information page out there! Forget all of those bloody youtubers being paid to say 1 thing is better than the other. Here are straight facts that tell me what lights I need, how many of them and why they work. This has been the most helpful website for me personally (after info searching for several months). Thank you so much
Steven says
I’m glad to hear that, thank you for the kind words!
Ben says
Me too!
Thanks!
Caderbavah Muhammad Siddick says
Thank u all for these very educative and useful information….
ron smith says
how many cfl 2700k ‘s are need for flowering one plant can you tell me
thank you
Steven says
I wrote this entire article to answer this question. Did you read it?
ron smith says
never mid i found it that help me big time thank you so much ill have to save yourt page for more info
Bluntmankillier says
Lol, i guess to busy smokin..
Bluntmankillier says
Great info, i was already doing what you described, with the cfl,s. You just proved my instincts right..thanks
Arrif says
Great advice cost saving. Tried and test here in S.A tops man salute to your great words. Thank you
Jennifer says
How longs do I keep the lights on the plants and how long do I keep them off
Steven says
It really depends what you’re growing. If it’s cannabis, then 18 on/6 off during vegging and 12/12 during bloom.
Emma Wilson says
Thanks, to be honest I was totally going to ask about this same question as well. For your personal preference, would you recommend LED lights over CFL, are they more effective?
Steven says
Yes, they are far more efficient.
Genie says
LED is more effective than CFL? (Sorry for jumping in)
Steven says
Yes, good ones are.
John Gunner says
First off, congrats on the article, it helped a lot understanding this matter.
I was looking on the website, and I didn’t find something like this for LED. If I want to do the same but with LED bulbs it works? And I still have to use the principles of actual wattage, kelvins, etc?
What I mean is, simply put, can I use like 3 LED bulbs (i.e. ratio of 2 cool to 1 warm for vegging)? And I still need to use the same amount of wattage on the LEDs? Because sometimes we see something like “125W LED substitutes 250W.”
Thank you in advance
Steven says
This article covers how to figure out how much light you need with LEDs: http://growlightinfo.com/what-size-grow-light-do-i-need/
Husky says
What an insightful article. I’m planning to build a space bin for 1 plant. The plan is to hang 4 to 6 cfl bulbs down from the cover and lift it higher as the plant grows. During vegging i’m thinking of using 3 x 24W Cool CFL + 1 x 26W Warm CFL (It’s not exactly you 2:1 recommendation but hopefully this is fine). During flowering, I’m thinking of adding in 2 more warm 26W Warm CFLs. Also am planning to line the sides on the bin with LED strips.
I have a few questions.
1. Would the setup above be too much, too little or ok for just 1 plant?
2. For the LED strips, they come in cool and warm whites. Should I get a mixture or just stick to cool since they’re be on the side walls and I’m guessing would be most useful during vegging.
3. I am finding it harder to find CFL bulbs nowadays. Most bulbs have been replaced by LED bulbs (household LED bulbs, not grow lights) which are supposed to be more energy efficient while being brighter. Would cool and warm LED bulbs have the same effect as cool and warm CFL bulbs?
Paul M. says
If you plan on using only CFL bulbs for your entire grow from seedling to flower, it would be best not to build the space too big. 2’x2′ or 3’x3′ wide maximum, but give yourself enough height to work with as well, to be able to get in there and adjust things as necessary. And you will still need room above for a carbon filter and extraction fan. IMO, CFL’s work great in smaller spaces because the penetration into the canopy isn’t much more than 6-8 inches. In a smaller area you can hang them around the plant, completely surrounding it. You can overlap them and the lumens become much more intense with reflective material along the walls. IMO the CFL bulbs that they sell at hydroponic stores are over priced and you are better off sourcing them yourself online, Take your time and look around because you can still find the larger CFL bulbs online at stores like Home Depot.com, they sell 65 watt 2700k bulbs for the flowering stage and even larger 100+ watt 6500k bulbs for veg. You can also find 80-125 watt 6500k CFL photography bulbs on Amazon as well. (They work great and give off almost no heat) The wattage I speak of is TRUE WATTAGE, NOT THE EQUIVALENT. You could hang four 65 watt bulbs over the top of a plant using a Y splitter and then add 2 more Y splitters to that creating a 4 way chandelier over the top of the plant. You could then hang 8 medium sized 40 watt bulbs on Y splitters (2 bulbs on Y splitters around each side of the plant) about 1/3 of the way down from the top of the canopy. That would be 580 true watts of light, with way less heat output than a 400 watt HPS. Just remember watt for watt, the electricity will still cost the same, also if the larger wattage CFL bulbs are placed too close, the plant can suffer from light stress. so you will have to adjust the distance from time to time. You really have to be creative on how to hang them and please be careful and don’t overload any power strips and make sure to have everything securely hung. I have found that topping the plant for 4 colas and using low stress training to open up the center works best with CFL’s. I have grown countless plants from start to finish using nothing but CFL’s. It is important to use the correct kelvin temperature for each stage. As long as you use a decent extraction fan, you should not have problems with heat. I have since moved on to HPS/metal halide lighting for the sake of simplicity, but I now have to struggle with heat. So in the summer I still grow with CFL bulbs in my small 2’x2’x5′ tent and in the winter I use my larger 3’x3’tent with the HPS and metal halide systems.Good luck!
Jennifer Cole says
I just want to share my appreciation for the clarity offered here. For the added tips and where to for bulbs. It’s so thoughtful, this piece is so well written and I’m grateful.
Ricky Mack says
Well hey there. Got one girl my friends tell me to flip already. I’m holding off for a few more days. Have over a dozen spots that I imagine will turn into flowers. Very intense relationship has been created. It’s funny. First time grow. I have 12 23 w cfl 1 72 watt flood 4 10 watt led grow bulbs and 1 26 wat job site lite over her in a 2’x2′ closet. Been doing the shuffle in and out with a minimum of 7 hours of full beach sun and then back she goes and has for another indoor bath 10 hours. More will be revealed.
Oucheeeeee says
Hi, trying to work out what I’ll need for my 1.20m x 1.20m x 2.00m Secret jardin grow tent. I’ve dug it out after 10 years in the loft as Covid turned prices ridic (and I lost my income so can’t afford even the rubbish stuff). I have 6 x 150w CFL BLUE lights (showing 6700K) and 6 x 150w CFL RED lights showing 2700K but to be fair, I figure a couple of bulbs probably broken so will figure on using only 4 at a time (In splitters). Will use BLUE for veg, then switch to RED for Flowering.
Trying to get short/Fat girls – maybe 6 in 3 x twin pots or 8 in 4 x twins.
Any chance I’d get away with 2 bulbs for veg to keep leccy costs down or will I need all 4. Maybe 2 for first couple of weeks then add 2 more. Def use all 4 RED for flowering.
Whaddayathink?
Any comments apprciated as 10 years since last grow.
Thx
Oucheeeeee
Paul M. says
a 4×4 tent is a big area to be using only CFL’s but It definitely can be done and it sounds like you are on the right track with the bulbs that you have picked out. ARE THOSE THE ACTUAL WATTAGES OF THE BULBS, OR ARE THEY THE EQUIVALENTS?? If they are actual wattages then you should be ok. I would top 1 plant for 4 main colas and then spread the center of the plant wide open. I would use the Y splitters to form a 4 bulb “chandelier” and hang that directly over the top of the plant and then hang 2 bulbs on Y splitters on all 4 sides of the plant about 1/3 the way down from the top of the canopy. I have grown many plants using only CFL bulbs from start to finish, but did so in more confined areas like 2×2 or 3×3. You will need all 12 of those bulbs as I described to flower 1 plant and get any kind of decent yield. If you intend to grow more than 1 plant, you may want to start thinking about adding some high output T5 units in there. Good luck!
Rob says
I used 6 x 125W CFLs in 3 Reflectors (2 bulbs per reflector) in a 1.2m x 1.2m x 2m Grow tent. I even switched from Blue (6700K) to Red (2100K) bulbs when switching to flowering.
Must say, whilst the quality of bud was always good, the yield was pathetic (Avg 100g per plant). I’ve switched to a 600W HPS and current grow looks 5-10 times better.
But Good luck – Hope you do better than I did (and my last CFL grow was 10 years ago using Envirolights – Maybe CFL technology has moved on but I do still see the lights I used in most current adverts)